2. Remove the current and pinch the end of the nerve, noting the result. With very fine wires, connect the battery directly to the ends of the muscle. Stimulate by making and breaking the current as before. In this experiment the muscle cells are stimulated by the direct action of the current and not by the current acting on the nerve.

3. With the wires attached to either the muscle or the nerve, make and break the current in rapid succession. This causes the muscle to enter into a second contraction before it has relaxed from the first, and if the shocks follow in rapid succession, to continue in the contracted state. This condition, which represents the method of contraction of the muscles in the body, is called tetanus.

NOTE.—In these experiments a twitching of the muscle is frequently observed when no stimulus is being applied. This is due to the drying out of the nerve and is prevented by keeping it wet with a physiological salt solution. (See footnote, page 38.)

*To show the Action of Levers.*—With a light but stiff wooden bar, a spring balance, and a wedge-shaped fulcrum, show:

1. The position of the weight, the fulcrum, and the power in the different classes of levers, and also the weight-arm and the power-arm in each case.

2. The direction moved by the power and the weight respectively in the use of the different classes of levers.

3. That when the power-arm and weight-arm are equal, the power equals the weight and moves through the same distance.

4. That when the power-arm is longer than the weight-arm, the weight is greater, but moves through a shorter distance than the power.

5. That when the weight-arm is longer than the power-arm, the power is greater and moves through a shorter distance than the weight.

*To show the Loss of Power in the Use of the Body Levers.*—Construct a frame similar to, but larger than, that shown in Fig. 120, (about 12 inches high), and hang a small spring balance (250 grams capacity) at the place where the muscle is attached. Fasten the end of a lever to the upright piece, at a point on a level with the end of the balance hook. (The nail or screw used for this purpose must pass loosely through the lever, and serve as a pivot upon which it can turn.) The lever should consist of a light piece of wood, and should have a length at least three times as great as the distance from the hook to the turning point. Connect the balance hook with the lever by a thread or string, and then hang upon it a small body of known weight. Note the amount of force exerted at the balance in order to support the weight at different places on the lever. At what point is the force just equal to the weight? Where is it twice as great? Where three times? Show that the force required to support the weight increases proportionally as the weight-arm and as the distance through which the weight may be moved by the lever. Apply to the action of the biceps muscle in lifting weights on the forearm.

*A Study of the Action of the Biceps Muscle.*—Place the fingers upon the tendon of the biceps where it connects with the radius of the forearm. With the forearm resting upon the table, note that the tendon is somewhat loose and flaccid, but that with the slightest effort to raise the forearm it quickly tightens. Now transfer the fingers to the body of the muscle, and sweep the forearm through two or three complete movements, noting the changes in the length and thickness of the muscle. Lay the forearm again on the table, back of hand down, and place a heavy weight (a flatiron or a hammer) upon the hand. Note the effort required to raise the weight, and then shift it along the arm. Observe that the nearer it approaches the elbow the lighter it seems. Account for the difference in the effort required to raise the weight at different places. Does the effort vary as the distance from the tendon?



CHAPTER XVI - THE SKIN

Protective coverings are found at all the exposed surfaces of the body. These vary considerably at different places, each being adapted to the conditions under which it serves. The most important ones are the skin, which covers the entire external surface of the body; the mucous membrane, which lines all the cavities that communicate by openings with the external surface; and the serous membrane, which, including the synovial membranes, lines all the closed cavities of the body. In addition to the protection which it affords, the skin is one of the means by which the body is brought into proper relations with its surroundings. It is because of this function that we take up the study of the skin at this time.

*The Skin* is one of the most complex structures of the body, and serves several distinct purposes. It is estimated to have an area of from 14 to 16 square feet, and to have a thickness which varies from less than one eighth to more than one fourth of an inch. It is thickest on the palms of the hands and the soles of the feet, the places where it is most subject to wear. It is made up of two distinct layers—an outer layer called the epidermis, or cuticle, and an inner layer called the dermis, or cutis vera (Fig. 121).

*The Dermis.*—This is the thicker and heavier of the two layers, and is made up chiefly of connective tissue. The network of tough fibers which this tissue supplies, forms the essential body of the dermis and gives to it its power of resistance. It is on account of the connective tissue that the skins of animals can be converted into leather by tanning. A variety of structures, including blood and lymph vessels, oil and perspiratory glands, hair follicles, and nerves, are found embedded in the connective tissue (Fig. 122). These aid in different ways in the work of the skin.

[Fig. 121]

Fig. 121—*Section of skin* magnified, a, b. Epidermis, b. Pigment layer. c. Papillae, d. Dermis. e. Fatty tissue. f, g, h. Sweat gland and duct. i, k. Hair and follicle. l. Oil gland.

On the outer surface of the dermis are numerous elevations, called papillae. These average about one one-hundredth of an inch in height, and one two hundred and fiftieth of an inch in diameter. They are most numerous on the palms of the hands, the soles of the feet, and the under surfaces of the fingers and toes. At these places they are larger than in other parts of the body, and are closely grouped, forming the parallel curved ridges which cover the surfaces. Each papilla contains a loop of capillaries and a small nerve, and many of them are crowned with touch corpuscles (page 342).

[Fig. 122]

Fig. 122—*Diagram* of section of skin showing its different structures.

*The Epidermis* is much thinner than the dermis. It is made up of several layers of cells which are flat and scale-like at the surface, but are rounded in form where the epidermis joins the dermis. The epidermis has the appearance of being moulded onto the dermis, filling up the depressions between the papillae and having corresponding irregularities (Fig. 121). No blood vessels are found in the epidermis, its nourishment being derived from the lymph which reaches it from the dermis. Only the part next to the dermis is made up of living cells. These are active, however, in the formation of new cells, which take the place of those that are worn off at the surface. Some of the cells belonging to the inner layer of epidermis contain pigment granules, which give the skin its color (Fig. 121). The epidermis contains no nerves and is therefore non-sensitive. The hair and the nails are important modifications of the epidermis.

*A Hair* is a slender cylinder, formed by the union of epidermal cells, which grows from a kind of pit in the dermis, called the hair follicle. The oval and somewhat enlarged part of the hair within the follicle is called the root, or bulb, and the uniform cylinder beyond the follicle is called the shaft. Connected with the sides of the follicles are the oil, or sebaceous, glands (Figs. 121 and 122). These secrete an oily liquid which keeps the hair and cuticle soft and pliable. Attached to the inner ends of the follicles are small, involuntary muscles whose contractions cause the roughened condition of the skin that occurs on exposure to cold.

*A Nail* is a tough and rather horny plate of epidermal tissue which grows from a depression in the dermis, called the matrix. The back part of the nail is known as the root, the middle convex portion as the body, and the front margin as the free edge (Fig. 123). Material for the growth of the nail is derived from the matrix, which is lined with active epidermal cells and is richly supplied with blood vessels. Cells added to the root cause the nail to grow in length (forward) and cells added to the under surface cause it to grow in thickness. The cuticle adheres to the nail around its entire circumference so that the covering over the dermis is complete.

[Fig. 123]

Fig. 123—*Section of end of finger* showing nail in position.

*Functions of the Skin.*—The chief function of the skin is that of protection. It is able to protect the body on account of the tough connective tissue in the dermis, the non-sensitive cells of the epidermis, and also by the touch corpuscles and their connecting nerve fibers. This protection is of at least three kinds, as follows:

1. From mechanical injuries such as might result from contact with hard, rough, or sharp objects. The main quality needed for resisting mechanical injuries is toughness, and this is supplied both by the epidermis and by the connective tissue of the dermis.

2. From chemical injuries caused by contact with various chemical agents, as acids, alkalies, and the oxygen of the air. The epidermis, being of such a nature as to resist to a considerable extent the action of chemical agents, affords protection from these substances. (89)

3. From disease germs which are everywhere present. The epidermis is the main protective agent against attacks of germs, but should the epidermis be broken, they meet with further resistance from the fluids of the dermis and the white corpuscles of the blood.

4. From an excessive evaporation of liquid from the surface of the body. In the performance of this function, the skin is an important means of keeping the tissues soft and the blood and lymph from becoming too concentrated.

*Other Functions of the Skin.*—Through the perspiratory glands the skin is an organ of excretion. While the secretion from a single gland is small, the waste that leaves the body through all of the perspiratory glands is considerable (90) (page 206). By means of the nerves terminating in the touch corpuscles, the skin serves as the organ of touch, or feeling (Chapter XX). To a slight extent also the skin may absorb liquid substances, these being taken up by the blood and lymph vessels, and perform a respiratory function, throwing off carbon dioxide. But the most important function of the skin, in addition to protection, is that of serving as

*An Organ of Adaptation.*—Forming, as it does, the boundary between the body and its physical environment, the skin is perhaps the most important agent through which the body is adapted to its immediate surroundings. Evidence of this is found in the great variety of influences which are able to affect the body through their action upon the nerves in the skin, and in the changes which the epidermis undergoes on exposure. The latter function is especially marked in the lower animals, the coverings of epidermal tissue (hair, scales, feathers, etc.) adapting each species to the physical conditions under which it lives. In man the most striking example of adaptation through the skin is seen in the variations in the quantity of blood circulating through it, corresponding to the changes in the temperature outside of the body. These variations are of great importance, having to do with the

*Maintenance of the Normal Temperature.*—It is necessary to the continuance of life that the temperature of the body be kept at a nearly uniform degree, called the normal temperature, which is about 98.6 deg. F. The maintenance of the normal temperature depends mainly upon four conditions: the chemical changes at the cells, the circulation of the blood, the nervous system, and the skin. The chemical changes produce the heat, the blood in its circulation distributes the heat over the body, and the nervous system controls the heat-producing and distributing processes (page 320). The skin is the chief means by which the body gets rid of an excess of heat and, by so doing, avoids overheating. (91)

*How the Skin cools the Body.*—The skin is a means of ridding the body of an excess of heat in at least two ways:

1. By the conduction and radiation of heat from its surface as from a stove. This goes on all the time, but varies with the amount of heat brought to the surface by the blood.

2. By the evaporation of the perspiration. It is a well-established and easily demonstrated principle that liquids in evaporating use up heat.(See Practical Work.) It is also a matter of everyday experience that the perspiration has a cooling effect upon the body and that its flow increases with the amount of heat to be gotten rid of. The quantity of perspiration secreted, and of heat disposed of through its evaporation, also varies with the amount of blood circulating through the skin.

*Temperature Regulation by the Skin.*—Variations in the quantity of blood circulating through the skin enable this organ to throw off just the right amount of heat for keeping the body at the normal temperature. If it is necessary for the body to rid itself of an excess of heat, the quantity of blood circulating in the skin is increased. This brings the blood near the surface, where more heat can be radiated and where it may cause an increase in the perspiration. On the other hand, if the body is in danger of losing too much heat, the circulation diminishes in the skin and increases in the internal organs. This stops the rapid loss of heat from the surface. The skin in this work is of course made to cooperate with other parts of the body. That it is not the only organ concerned in regulating the escape of heat is seen in the results that follow sensations either of chilliness or of heat at the surface.

*Effects of Heat and Cold Sensations.*—Sensations, or feelings, of heat and cold are made possible through the nerves which connect the brain with the temperature corpuscles, found in the skin (page 343). As the warm blood recedes from the skin, a sensation of cold is felt, but when the blood returns, there is again the feeling of warmth. The sensation of cold prompts one to seek a warmer place, or to put on more clothing; while the sensation of heat, if it be oppressive, leads to activities of an opposite kind. Prompted in this way by the sensations from the skin, one voluntarily supplies the external conditions, such as clothing and heat, that affect the body temperature.

*Alcohol and the Regulation of Temperature.*—Alcohol, through its effect upon the nervous system, interferes seriously with the regulation of the body temperature. By dilating the capillaries, it increases the circulation in the skin and leads to an undue loss of heat. At the same time the excess of blood in the skin causes a feeling of warmth which has led to the erroneous belief that alcohol is a heat producer. If taken on a cold day, it deceives one about his true condition and leads to a wasting of heat when it should be carefully economized. Not only is alcohol of no value in maintaining the body temperature, but if taken during severe exposure to cold, it becomes a menace to life itself. Arctic, explorers and others exposed to severe cold have found that they withstand cold far better when no alcohol at all is used.(92)



HYGIENE OF THE SKIN

Much of the hygiene of the skin is included in the problems of keeping it warm and clean. It is kept warm by clothing; bathing is the method of keeping it clean.

*Clothing* should be warm and loose-fitting. Woolen fabrics are to be preferred in winter to cotton because, being poorer conductors of heat, they afford better protection from the cold. But wool fails to absorb the perspiration rapidly from the skin and to pass it to the outside where it is evaporated. This, together with its tendency to irritate, makes woolen clothing somewhat objectionable for wearing next to the skin. This objection, however, is obviated by woolen underwear which is lined by a thin weaving of cotton.

*Bathing.*—The solid material from the perspiration, which is left on the skin, together with the oil from the oil glands and the dirt from the outside, tends to close up the pores and develop offensive odors. Keeping the skin clean is, for these reasons, necessary from both a health and a social standpoint. While one should always keep clean, the frequency of the bath will depend upon the season, the occupation of the individual, and the nature and amount of the perspiration. As to the kind of bath to be taken and the precautions to be observed, no specific rules can be laid down. These must be determined by the facilities at hand and by the health and natural vigor of the bather. Severe chilling of the body should be avoided, especially by those in delicate health. If a hot bath is taken, one should dash cold water over the body on finishing. One should then quickly dry and rub the body with a coarse towel. The dash of cold water closes the pores of the skin and lessens the liability of taking cold.

*The Tonic Bath.*—The cold bath has been found to have a beneficial effect upon the general health beyond its effect upon the skin. When taken with care as to the length of time and the degree of cold, decided tonic effects are observed on the circulation and on the nervous system. The rapid changes of temperature vigorously exercise the non-striated muscles of the blood vessels (page 57) and the nerves controlling them. The irritability of the nervous system in general is also lessened. For this reason the cold bath is one of the best means of keeping both mind and body in good condition during the warm months. Sponging off the body with cold or tepid water before retiring is also an excellent aid in securing sound sleep during the hot summer nights.

Danger from the cold bath arises through the shock to the nervous system and the loss of heat from the body. It is avoided by using water whose temperature is not too low and by limiting the time spent in the bath. A brisk rubbing with a coarse towel should always follow the cold bath. People past middle age are, as a rule, not benefited by the cold bath; and those in delicate health, especially if inclined toward rheumatism, are likely to be affected injuriously by it.

*Care of the Complexion.*—A good complexion is a natural accompaniment of good health and depends primarily upon two conditions—a clear skin and an active circulation of the blood through it. Clearness of the skin depends largely upon the elimination of waste material from the body, and where the solid wastes are not effectively removed through the natural channels (the liver, kidneys, and bowels), blotches, sallowness of the skin, and skin eruptions are likely to result. In seeking to clear the complexion, attention must be given to all those agencies that favor the elimination of waste, and especially should there be a free and thorough evacuation of the bowels each day. The general health should also be looked after, attention being given to exercise, fresh air, proper food,(93) sufficient sleep, etc.

Bathing is the chief means employed for increasing the circulation in the skin, although exercise which is sufficiently vigorous to cause one to perspire freely is a valuable aid. A daily bath of warm or hot water, finished off with a dash of cold, followed by a thorough rubbing of the entire surface, and this by a kneading of the skin with the thumbs and fingers, will in most cases bring about the desired results. A little olive oil, thoroughly worked into the skin during the kneading process, is beneficial where one lacks flesh or where the skin is dry and thin. The olive oil is also beneficial where the baths are exhausting or render one susceptible to cold. In rubbing and kneading, the skin should not be bruised or irritated.

The much advertised "complexion beautifiers" which are applied directly to the face frequently have the effect of clogging the pores and of causing eruptions of the skin. On the other hand, certain authorities state that the cold cream preparations may be of advantage in giving the skin a desired softness, and that when judiciously used (the face being cleansed after each application) they do no harm. Of the different kinds of face powder those prepared from rice are considered the least injurious.

*Treatment of Skin Wounds.*—Skin wounds which may not be serious in themselves frequently become so through getting infected with germs. Blood poisoning often results from such infections, one of the worst forms being tetanus, or lockjaw. A wound should be kept clean, and if it shows signs of infection, it should be washed with some antiseptic solution. Or, it may be cleansed with pure warm water and then covered with some antiseptic ointment,(94) of which there are a number on the market. A weak solution of carbolic acid (one part acid to twenty-five parts of water) makes an excellent antiseptic wash. It may be used not only for cleansing wounds, but also in counteracting the poisonous effects that follow the bites of insects.

A wound resulting from the bite of an animal (cat or dog), even though slight, should receive more serious attention, and as soon as possible after the occurrence. Such wounds should be cauterized, and for this purpose pure carbolic, acid (undiluted with water) may be used. A wooden toothpick is dipped into the acid and this is worked about in the wound. The acid is then washed out with warm water. A deep wound from a rusty nail or a thorn should be treated in the same manner and should be kept open, not being allowed to heal at the surface first. If one has reason to believe he has been bitten by a mad dog, the wound should be cauterized as above, and a physician should be summoned at once. Deep wounds from explosives, or other causes, should also receive the attention of the physician. Many cases of lockjaw result every year from wounds inflicted by the toy pistols, firecrackers, etc., used in our Fourth of July celebrations. These are due to the embedding in the skin or flesh of small solid particles on which are lockjaw germs. Wounds of this nature should, of course, receive the attention of the physician.

*Care of the Nails.*—Relief from a blood blister under the nail is secured by boring a small hole through the nail with the sharp point of a sterilized penknife (page 38). This simple bit of surgery not only relieves the pain, but is frequently the only means of saving the nail. Ingrown toe nails are relieved by scraping a broad strip in the middle of the nail until very thin. This relieves the pressure, preventing the sides of the nail from being forced into the toe. While the finger nails should be trimmed in a curve, corresponding to the end of the finger, it is recommended that the toe nails be cut straight across (Fig. 124), as this method diminishes the pressure from the shoe and keeps the nails from ingrowing. Shoes that pinch the toes should, of course, not be worn (page 238).

[Fig. 124]

Fig. 124—Proper method of trimming nails of toes.

*Care of the Hair.*—Occasional washing of the hair is beneficial, but too much wetting causes decay of the hair roots, which leads to its falling out. The worst enemy of the hair is dandruff. A method of removing dandruff which is highly recommended is that of rubbing olive oil into the scalp and later of removing this with a cleansing shampoo. The olive oil is placed on the scalp with a medicine dropper and thoroughly rubbed in with the fingers. After three or four hours the hair is washed with soap and water (any good toilet soap will do) and rinsed with pure water. The hair is then dried, the surplus water being removed with a coarse towel. Where the dandruff is very troublesome, this treatment may be given once or twice a week; but in mild cases once a month is sufficient. Massage of the scalp, by increasing the circulation at the hair roots, is beneficial, but irritation by a fine-tooth comb, a stiff hair brush, or by other means should be avoided. Frequent brushing and combing, however, are necessary both for the good appearance of the hair and for spreading the oil secreted by the glands at the hair roots.

*Summary.*—The skin forms the external covering of the body and also serves additional purposes. It is a most important agency in adapting the body to its physical surroundings, as shown by the part which it plays in the regulation of the body temperature. The skin should be kept clean and active, and skin wounds, even though small, should be guarded against infection.

*Exercises.*—1. Name an example of each of the protective coverings of the body.

2. Compare the dermis and the epidermis with reference to thickness, composition, and function.

3. To what is the color of the skin due? How is the color of the skin affected by the sunlight?

4. What modifications of the epidermis are found on our bodies? What are found on the body of a chicken?

5. What different kinds of protection are provided by the skin?

6. How does the perspiration cool the body?

7. What change occurs in the circulation in the skin when the body is becoming too cold? When becoming too warm? What is the purpose of these changes?

8. How does alcohol cause one to feel warm when he may be losing too much of his heat?

9. What precaution should be observed by one in poor health, in taking a bath?

10. How may the cold bath be a means of improving the general health?



PRACTICAL WORK

*Observations on the Skin and its Appendages.*—Examine the palm of the hand with a lens. Note the small ridges which correspond to the rows of papillae beneath the cuticle. In these find small pits, which are the openings of the sweat glands.

2. Examine the epidermis on the back of the hand and palm. At which place is it thickest and most resisting? Is it of uniform thickness over the palm? Try picking it with a pin at the thickest place, noting if pain is felt. Inference?

3. Examine a finger nail. Is the free edge or the root the thickest? Trim closely the thumb nail and the nail of the middle finger of one hand and try to pick up a pin, or other minute object, from a smooth, hard surface. The result indicates what use of the nails? Suggest other uses.

4. Examine with a microscope under a low power hairs from a variety of animals, as the horse, dog, cat, etc., noting peculiarities of form and surface.

*To illustrate Cooling Effects of Evaporation.*—1. Wet the back of the hand and move it through the air to hasten evaporation. Observe that, as the hand dries, a sensation of cold is felt. Repeat the experiment, using ether, alcohol, or gasolene instead of the water, noting the differences in results. These liquids evaporate faster than water.

2. Wet the bulb of a thermometer with alcohol or water. Move it through the air to hasten evaporation. Note and account for the fall of the mercury.



CHAPTER XVII - STRUCTURE OF THE NERVOUS SYSTEM

*Cooerdination and Adjustment.*—If we consider for a moment the movements of the body, we cannot fail to note the cooeperation of organs, one with another. In the simple act of whittling a stick one hand holds the stick and the other the knife, while the movements of each hand are such as to aid in the whittling process. Examples of cooeperation are also found in the taking of food, in walking, and in the performance of different kinds of work. Not only is cooeperation found among the external organs, but our study of the vital processes has shown that the principle of cooeperation is carried out by the internal organs as well. The fact that all the activities of the body are directed toward a common purpose makes the cooeperation of its parts a necessity. The term "cooerdination" is employed to express this cooeperation, or working together, of the different parts of the body.

A further study of the movements of the body shows that many of them have particular reference to things outside of it. In going about one naturally avoids obstructions, and if anything is in the way he walks around or steps over it. Somewhat as a delicate instrument (the microscope for example) is altered or adjusted, in order to adapt it to its work, the parts of the body, and the body as a whole, have to be adjusted to their surroundings. This is seen in the attitude assumed in sitting and in standing, in the position of the hands for different kinds of work, in the variations of the circulation of the blood in the skin, and in the movements for protecting the body.(95)

*Work of the Nervous System.*—How are the different activities of the body controlled and cooerdinated? How is the body adjusted to its surroundings? The answer is found in the study of the nervous system. Briefly speaking, the nervous system controls, cooerdinates, and adjusts the different parts of the body by fulfilling two conditions:

1. It provides a complete system of connections throughout the body, thereby bringing all parts into communication.

2. It supplies a means of controlling action (the so-called impulse) which it passes along the nervous connections from one part of the body to another.

The present chapter deals with the first of these conditions; the chapter following, with the second.

*The Nerve Skeleton.*—If all the other tissues are removed, leaving only the nervous tissue, a complete skeleton outline of the body still remains. This nerve skeleton, as it has been called, has the general form of the framework of bones, but differs from it greatly in the fineness of its structures and the extent to which it represents every portion of the body. An examination of a nerve skeleton, or a diagram of one (Fig. 125), shows the main structures of the nervous system and their connection with the different parts of the body.

Corresponding to the skull and the spinal column is a central nervous axis, made up of two parts, the brain and the spinal cord. From this central axis white, cord-like bodies emerge and pass to different parts of the body. These are called nerve trunks, and the smaller branches into which they divide are called nerves. The nerves also undergo division until they terminate as fine thread-like structures in all parts of the body. The distribution of nerve terminations, however, is not uniform, as might be supposed, but the skin and important organs like the heart, stomach, and muscles are the more abundantly supplied. On many of the nerves are small rounded masses, called ganglia, and from many of these small nerves also emerge. At certain places the nerves and ganglia are so numerous as to form a kind of network, known as a plexus.

[Fig. 125]

Fig. 125—*Diagram of nerve skeleton.* The illustration shows the extent and general arrangement of the nervous tissue. A. Brain. B. Spinal cord. N. Nerve trunks and nerves. G. Ganglia.

It is through these structures—brain and spinal cord, nerve trunks and nerves, ganglia and nerve terminations—that connections are established between all parts of the body, but more especially between the surface of the body and the organs within.

*The Neurons, or Nerve Cells.*—While a hasty examination of the nerve skeleton is sufficient to show the connection of the nervous system with all parts of the body, no amount of study of its gross structures reveals the nature of its connections or suggests its method of operation. Insight into the real nature of the nervous system is obtained only through a study of its minute structural elements. These, instead of being called cells, as in the case of the other tissues, are called neurons. The use of this term, instead of the simpler one of nerve cell, is the result of recent advances in our knowledge of the nervous system.(96)

[Fig. 126]

Fig. 126—*Diagram of a mon-axonic neuron* (greatly enlarged except as to length). The central thread in the axon is the axis cylinder.

The neurons are in all respects cells. They differ widely, however, from all the other cells of the body and are, in some respects, the most remarkable of all cells. They are characterized by minute extensions, or prolongations, which in some instances extend to great distances. Though the neurons in certain parts of the body differ greatly in form and size from those in other parts of the body, most of them may be included in one or the other of two classes, known as mon-axonic neurons and di-axonic neurons.

*Mon-axonic Neurons.*—Neurons of this class consist of three distinct parts, known as the cell-body, the dendrites, and the axon (Fig. 126).

The cell-body has in itself the form of a complete cell and was at one time so described. It consists of a rounded mass of protoplasm, containing a well-defined nucleus. The protoplasm is similar to that of other cells, but is characterized by the presence of many small granules and has a slightly grayish color.

The dendrites are short extensions from the cell-body. They branch somewhat as the roots of a tree and form in many instances a complex network of tiny rootlets. Their protoplasm, like that of the cell-body, is more or less granular. The dendrites increase greatly the surface of the cell-body, to which they are related in function.

The axon, or nerve fiber, is a long, slender extension from the cell-body, which connects with some organ or tissue. It was at one time described as a distinct nervous element, but later study has shown it to be an outgrowth from the cell-body. The mon-axonic neurons are so called from their having but a single axon.

*Di-axonic Neurons.*—Neurons belonging to this class have each a well-defined cell-body and two axons, but no parts just like the dendrites of mon-axonic neurons. The cell-body is smooth and rounded, and its axons extend from it in opposite directions (Fig. 127).

[Fig. 127]

Fig. 127—*Diagram of a di-axonic neuron.* The diagram shows only the conducting portion of the axon, or axis cylinder.

*Structure of the Axon.*—The axon, or nerve fiber, has practically the same structure in both classes of neurons, being composed in most cases of three distinct parts. In the center, and running the entire length of the axon, is a thread-like body, called the axis cylinder (Fig. 126). The axis cylinder is present in all axons and is the part essential to their work. It may be considered as an extension of the protoplasm from the cell-body. Surrounding the axis cylinder is a thick, whitish-looking layer, known as the medullary sheath, and around this is a thin covering, called the primitive sheath, or neurilemma. The medullary sheath and the primitive sheath are not, strictly speaking, parts of the nerve cell, but appear to be growths that have formed around it. Certain of the axons have no primitive sheath and others are without a medullary sheath.(97)

*Form and Length of Axons.*—Where the axons terminate they usually separate into a number of small divisions, thereby increasing the number of their connections. Certain axons are also observed to give off branches before the place of termination is reached (Fig. 131). These collateral branches, by distributing themselves in a manner similar to the main fiber, greatly extend the influence of a single neuron.

In the matter of length, great variation is found among the axons in different parts of the body. In certain parts of the brain, for example, are fibers not more than one one-hundredth of an inch in length, while the axons that pass all the way from the spinal cord to the toes have a length of more than three feet. Between these extremes practically all variations in length are found.

*Arrangements of the Neurons.*—Nowhere in the body do the neurons exist singly, but they are everywhere connected with each other to form the different structures observed in the nerve skeleton. Two general plans of connection are to be observed, known as the anatomical and the physiological, or, more simply speaking, as the "side-by-side" and "end-to-end" plans. The side-by-side plan is seen in that disposition of the neurons which enables them to form the nerves and the ganglia, as well as the brain and spinal cord. The end-to-end connections are necessary to the work which the neurons do.

*Side-by-side Connections.*—On separating the ganglia and nerves into their finest divisions, it is found that the nerves consist of axons, while the ganglia are made up mainly of cell-bodies and dendrites. The axons lie side by side in the nerve, being surrounded by the same protective coverings, while the cell-bodies form a rounded mass or cluster, which is the ganglion (Fig. 128). But the axons, in order to connect with the cell-bodies, must terminate within the ganglion, so that they too form a part of it. To some extent, also, axons pass through ganglia with which they make no connection. The neurons in the brain and spinal cord also lie side by side, but their arrangement is more complex than that in the nerves and ganglia.

[Fig. 128]

Fig. 128—*Diagrams illustrating arrangement of neurons.* A, B. Ganglia and short segments of nerves. 1. Ganglion. 2. Nerve. In the ganglion of A are end-to-end connections of different neurons; in the ganglion of B are the cell-bodies of di-axonic neurons. C. Section of a nerve trunk. 1. Epineurium consisting chiefly of connective tissue. 2. Bundles of nerve fibers. 3. Covering of fiber bundle, or perineurium. 4. Small artery and vein.

The side-by-side arrangement of the neurons shows clearly the structure of the ganglia and nerves. The nerve is seen to be a bundle of axons, or nerve fibers, held together by connective tissue, while the ganglion is little more than a cluster of cell-bodies. Their connection is necessarily very close, for the same group of neurons will form, with their axons, the nerve, and, with their cell-bodies, the ganglion (Fig. 128).

*End-to-end Connections.*—These consist of loose end-to-end unions of the fiber branches of certain neurons with the dendrites of other neurons. The purpose of such connections is to provide the means of communication between different parts of the body. There appears to be no actual uniting of the fiber branches with the dendrites, but they come into relations sufficiently close to establish conduction pathways, and these extend throughout the body (Fig. 129). They connect all parts of the body with the brain and spinal cord, while connections within the brain and cord bring the parts into communication with each other.

[Fig. 129]

Fig. 129—*Diagram of a nerve path* starting at the skin, extending through the spinal cord, and passing out to muscles. A division of this path also reaches the brain.

*Nature of the Nervous System.*—The nervous system represents the sum total of the neurons in the body. In some respects it may be compared to the modern telephone system. The neurons, like the electric wires, connect different places with a central station (the brain and spinal cord), and through the central station connections are established between the different places in the system. As the separate wires are massed together to form cables, the neurons are massed to form the gross structures of the nervous system. The nervous system, however, is so radically different from anything found outside of the animal body that no comparison can give an adequate idea of it. We now pass to a study of the gross structures observed in the nerve skeleton.

*Divisions of the Nervous System.*—While all of the nervous structures are very closely blended, forming one complete system for the entire body, this system presents different divisions which may, for convenience, be studied separately. As physiologists have become better acquainted with the human nervous system, different schemes of classification have been proposed. The following outline, based upon the location of the different parts, presents perhaps the simplest view of the entire group of nervous structures:

[Table]

*The Central Division.*—This division of the nervous system lies within the cranial and spinal cavities, and consists of the brain and the spinal cord. The brain occupying the cranial cavity and the spinal cord in the spinal cavity connect with each other through the large opening at the base of the skull to form one continuous structure. The brain and cord are the most complicated portions of the nervous system, and the ones most difficult to understand.

[Fig. 130]

Fig. 130—*Diagram of divisions of brain.*

*The Brain.*—The brain, which is the largest mass of nervous tissue in the body, weighs in the average sized man about 50 ounces, and in the average sized woman about 44 ounces.(98) It may be roughly divided into three parts, which are named from their positions (in lower animals) the forebrain, the midbrain, and the hindbrain (Fig. 130). The forebrain consists almost entirely of a single part, known as

*The Cerebrum.*—The cerebrum comprises about seven eighths of the entire brain, and occupies all the front, middle, back, and upper portions of the cranial cavity, spreading over and concealing, to a large extent, the parts beneath. The surface layer of the cerebrum is called the cortex. This is made up largely of cell-bodies, and has a grayish appearance.(99) The cortex is greatly increased in area by the presence everywhere of ridge-like convolutions, between which are deep but narrow depressions, called fissures. The interior of the cerebrum consists mainly of nerve fibers, or axons, which give it a whitish appearance. These fibers connect with the cell-bodies in the cortex (Fig. 131).

The cerebrum is a double organ, consisting of two similar divisions, called the cerebral hemispheres. These are separated by a deep groove, extending from the front to the back of the brain, known as the median fissure. The hemispheres, however, are closely connected by a great band of underlying nerve fibers, called the corpus callosum.

[Fig. 131]

Fig. 131—*Microscope drawing* of a neuron from cerebral cortex. a. Short segment of the axis cylinder with collateral branches.

At the base of the cerebrum three large masses of cell-bodies are to be found. One of these, a double mass, occupies a central position between the hemispheres, and is called the optic thalami. The other two occupy front central positions at the base of either hemisphere, and are known as the corpora striata, or the striate bodies.

*The Midbrain* is a short, rounded, and compact body that lies immediately beneath the cerebrum, and connects it with the hindbrain. On account of the great size of the cerebrum, the midbrain is entirely concealed from view when the other parts occupy their normal positions. However, if the cerebrum is pulled away from the hindbrain, it is brought into view somewhat as in Fig. 130.

The midbrain carries upon its back and upper surface four small rounded masses of cell-bodies, called the corpora quadrigemina. The upper two of these bodies are connected with the eyes; the lower two appear to have some connection with the organs of hearing. On the front and under surface, the midbrain separates slightly as if to form two pillars, which are called the crura cerebri, or cerebral peduncles. These contain the great bundles of nerve fibers that connect the cerebrum with the parts of the nervous system below.

*The Hindbrain* lies beneath the back portion of the cerebrum, and occupies the enlargement at the base of the skull. It forms about one eighth of the entire brain, and is composed of three parts—the cerebellum, the pons, and the bulb.

*The Cerebellum* is a flat and somewhat triangular structure with its upper surface fitting into the triangular under surface of the back of the cerebrum. It is divided into three lobes—a central lobe and two lateral lobes—and weighs about two and one half ounces. In its general form and appearance, as well as in the arrangement of its cell-bodies and axons, the cerebellum resembles the cerebrum. It differs from the cerebrum, however, in being more compact, and in having its surface covered with narrow, transverse ridges instead of the irregular and broader convolutions (Fig. 132).

*The Pons*, or pons Varolii, named from its supposed resemblance to a bridge, is situated in front of the cerebellum, and is readily recognized as a circular expansion which extends forward from that body. It consists largely of bands of nerve fibers, between which are several small masses of cell-bodies. The fibers connect with different parts of the cerebellum and with parts above.

[Fig. 132]

Fig. 132—*Human brain* viewed from below. C. Cerebrum. Cb. Cerebellum. M. Midbrain. P. Pons. B. Bulb. I-XII. Cranial nerves.

*The Bulb*, or medulla oblongata, is, properly speaking, an enlargement of the spinal cord within the cranial cavity. It is somewhat triangular in shape, and lies immediately below the cerebellum. It contains important clusters of cell-bodies, as well as the nerve fibers that pass from the spinal cord to the brain.

*The Spinal Cord.*—This division of the central nervous system is about seventeen inches in length and two thirds of an inch in diameter. It does not extend the entire length of the spinal cavity, as might be supposed, but terminates at the lower margin of the first lumbar vertebra.(100) It connects at the upper end with the bulb, and terminates at the lower extremity in a number of large nerve roots, which are continuous with the nerves of the hips and legs (Fig. 133). Two deep fissures, one in front and the other at the back, extend the entire length of the cord, and separate it into two similar divisions. These are connected, however, along their entire length by a central band consisting of both gray and white matter.

[Fig. 133]

Fig. 133—*Spinal cord*, showing on one side the nerves and ganglia with which it is closely related in function. A. Bulb. B. Cervical enlargement. C. Lumbar enlargement. D. Termination of cord. E. Nerve roots that occupy the spinal cavity below the cord. P. Pons. D.G. Dorsal root ganglia. S.G. Sympathetic ganglia. N. Nerve trunks to upper and lower extremities.

The arrangement of the neurons of the spinal cord is just the reverse of that in the cerebrum—the center being occupied by a double column of cell-bodies, which give it a grayish appearance, while the fibers occupy the outer portion of the cord, giving it a whitish appearance.

The spinal cord is not uniform in thickness, but tapers slightly, though not uniformly, from the upper toward the lower end. At the places where the nerves from the arms and legs enter the cord two enlargements are to be found, the upper being called the cervical and the lower the lumbar enlargement. These, on account of the difference in length between the cord and the spinal cavity, are above—the lower one considerably above—the places where the limbs which they supply join the trunk (Fig. 133).

*Arrangement of the Neurons of the Brain and Cord.*—The cell-bodies in the brain and spinal cord are collected into groups, and their fibers extend from these groups to places that may be near or remote. Guided by the white and gray colors of the nervous tissue, and also by the structures revealed by the microscope, physiologists have made out three general schemes in the grouping of cell-bodies, as follows:

1. That of surface distribution, the cell-bodies forming a thin but continuous layer over a given surface. This is the plan in the cerebrum and cerebellum, and here are found devices for increasing the surface: the cerebrum having convolutions, the cerebellum transverse ridges.

2. That of collections of cell-bodies into rounded masses. Such masses are found in the bulb, the pons, the midbrain, and the base of the cerebrum.

3. That of arrangement in a continuous column. This is the plan in the spinal cord. It matters not at what place the spinal cord be cut, a central area of gray matter, resembling in form the capital letter H, is always found.

The fibers connecting with the cell-bodies in the brain and spinal cord are gathered into bundles or tracts, and these pass through different parts somewhat as follows:

1. In the cerebrum they extend in three general directions, forming three classes of fibers. The first connect different localities in the same hemisphere, and are known as association fibers (A, Fig. 134). The second make connection between the two hemispheres, and form the corpus callosum. These are known as commissural fibers (C, Fig. 134). The third connect the cerebrum with the parts of the nervous system below, and are called projection fibers (P, Fig. 134).

2. In the cerebellum both association and commissural fibers are found. Bands of fibers, passing upward toward the cerebrum and downward toward the cord, connect this part of the brain with other parts of the nervous system.

[Fig. 134]

Fig. 134—*Semi-diagrammatic representation of a section through the right cerebral hemisphere*, showing fiber tracts. A. Association fibers. C. Commissural fibers. P. Projection fibers. The cell-bodies with which the fiber bundles connect are in the surface layer or cortex.

3. In the midbrain, bulb, and spinal cord fibers are found: first, that connect these parts with the cerebrum(101) and cerebellum above; second, that pass into and become a part of the nerves of the body; and third, that connect the opposite sides of these parts together.

*The Peripheral Division.*—The peripheral division of the nervous system includes all the nervous structures found outside of the brain and spinal cord. These consist of the cranial, spinal, and sympathetic nerves, and of various small ganglia, all of which are closely connected with the central system.

*Spinal Nerves and Dorsal-root Ganglia.*—The spinal nerves comprise a group of thirty-one pairs, which connect the spinal cord with different parts of the trunk, with the upper, and with the lower extremities. Each nerve joins the cord by two roots, these being named from their positions the ventral, or anterior, root and the dorsal, or posterior, root. The two roots blend together within the spinal cavity to form a single nerve trunk, which passes out between the vertebrae. On the dorsal root of each spinal nerve is a small ganglion which is named, from its position, the dorsal-root ganglion. (Consult Figs. 133 and 135, and also Fig. 125.)

*Double Nature of Spinal Nerves.*—Charles Bell, in 1811, made the remarkable discovery that each spinal nerve is double in function. He found the portion connecting with the cord by the dorsal root to be concerned in the production of feeling and the portion connecting by the ventral root to be concerned in the production of motion. In keeping with these functions, the two divisions of the nerve are made up of different kinds of fibers, as follows:

1. The dorsal-root divisions, of the fibers of di-axonic neurons, the cell-bodies of which form the dorsal-root ganglia (Fig. 135).

2. The ventral-root divisions, of the fibers of mon-axonic neurons, the cell-bodies of which are in the gray matter of the cord.

The first convey impulses to the cord and are called afferent neurons;(102) the second convey impulses from the cord and are known as efferent neurons. Thus, by forming a part of the nerve pathways between the skin and the brain, the dorsal divisions of these nerves aid in the production of feeling; and by completing pathways to the muscles, the ventral divisions aid in the production of motion (Figs. 129, 135, and 141).

[Fig. 135]

Fig. 135—*Connection of spinal nerves with the cord.* On the right is shown a nerve pathway from the skin to the muscle. A division of this pathway reaches the brain.

*The Cranial Nerves.*—From the under front surface of the brain, twelve pairs of nerves emerge and pass to the head, neck, and upper portions of the trunk. These, the cranial nerves, have names suggestive of their function or distribution and, in addition, are given numbers which indicate the order in which they leave the brain (Fig. 136). Unlike the spinal nerves, the cranial nerves present great variety among themselves, scarcely any two of them being alike in function or in their connection with different parts of the body. Several of them have to do with the special senses, and are for this reason very important. They connect the brain with the different parts of the head, neck, and trunk, as follows:

1. The first pair (olfactory nerves; nerves of smell; afferent) connect with the mucous membrane of the nostrils (Fig. 136).

2. The second pair (optic nerves; nerves of sight; afferent) connect with the retina of the eyes.

3. The third, fourth, and sixth pairs (motores oculi; control muscles of the eyes; efferent) connect with the internal and external muscles of the eyeballs (Fig. 136).

[Fig. 136]

Fig. 136—*Diagram suggesting the distribution and functions of the cranial nerves* (Colton). See also Fig. 132.

4. The fifth pair (trigeminal nerves; nerves of feeling to the face, of taste to the front of the tongue, and of control of muscles of mastication; afferent and efferent) connect with the skin of the face, the mucous membrane of the mouth, the teeth, and the muscles of mastication.

5. The seventh pair (facial nerves; control muscles that give the facial expressions; efferent) connect with the muscles just beneath the skin of the face.

6. The eighth pair (auditory nerves; nerves of hearing; afferent) connect with the internal ear.

7. The ninth pair (glossopharyngeal nerves; nerves of taste to back of tongue and of muscular control of pharynx; afferent and efferent) connect with the back surface of the tongue and with the muscles of the pharynx.

8. The tenth pair (vagus, or pneumogastric, nerves; nerves of feeling and of muscular control; afferent and efferent) connect with the heart, larynx, lungs, and stomach. They have the widest distribution of any of the cranial nerves.

9. The eleventh pair (spinal accessory nerves; control muscles of neck; efferent) connect with the muscles of the neck.

10. The twelfth pair (hypoglossal nerves; control muscles of the tongue; efferent) connect with the muscles of the tongue.

*Sympathetic Ganglia and Nerves.*—The sympathetic ganglia are found in different parts of the body, and vary in size from those which are half an inch in diameter to those that are smaller than the heads of pins. The largest and most important ones are found in two chains which lie in front, and a little to either side, of the spinal column, and extend from the neck to the region of the pelvis (Figs. 125 and 133). The number of ganglia in each of these chains is about twenty-four. They are connected on either side by the right and left sympathetic nerves which extend vertically from ganglion to ganglion. In addition to the ganglia forming these chains, important ones are found in the head (outside of the cranial cavity) and in the plexuses of the thorax and the abdomen.

The sympathetic ganglia receive nerves from the central division of the nervous system, but connect with glands, blood vessels, and the intestinal walls through fibers from their own cell-bodies. Some of these latter fibers join the spinal nerves, and some blend with each other to form small sympathetic nerves.

*Protection of Brain and Spinal Cord.*—On account of their delicate structure, the brain and spinal cord require the most complete protection. In the first place, they are surrounded by the bones of the head and spinal column; these not only shield them from the direct effects of physical force, but by their peculiar construction prevent, to a large degree, the passage of jars and shocks to the parts within. In the second place, they are surrounded by three separate membranes, as follows:

1. The dura, or dura mater, a thick, dense, and tough membrane which lines the bony cavities and forms supporting partitions.

2. The pia, or pia mater, a thin, delicate membrane, containing numerous blood vessels, that covers the surface of the brain and cord.

3. The arachnoid, a membrane of loose texture, that lies between the dura and the pin.

Finally, within the spaces of the arachnoid is a lymph-like liquid which completely envelops the brain and the cord, and which, by serving as a watery cushion, protects them from jars and shocks. Thus the brain and cord are directly shielded by bones, by membranes, and by the liquid which surrounds them. They are also protected from jars resulting from the movements of the body by the general elasticity of the skeleton.

*Summary.*—The nervous system establishes connections between all parts of the body, and provides a stimulus by means of which they are controlled. It is made up of a special form of cells, called neurons. The neurons form the different divisions of the nervous system, and also serve as the active agents in carrying on its work. Through a side-by-side method of joining they form the nerves, ganglia, spinal cord, and brain; and by a method of end-to-end joining they connect places remote from each other, and provide for nervous movements through the body. The nervous system, may in some respects be compared to a complicated system of telephony, in which the chains of neurons correspond to the wires, and the brain and spinal cord to the central station.

Exercises.—1. Give the meaning of the term "cooerdination." Supply illustrations.

2. What two general conditions are supplied in the body by the nervous system?

3. Compare the skeleton outline of the nervous system with the bony skeleton.

4. Sketch outlines of mon-axonic and di-axonic neurons.

5. Give two differences between the neurons and the other cells of the body.

6. Describe the two general methods of connecting neurons in the body. What purpose is accomplished by each method?

7. Name and locate the principal divisions of the nervous system.

8. Draw an outline of the brain (side view), locating each of its principal divisions.

9. If a pencil were placed over the ear, what portions of the brain would be above it and what below?

10. Describe briefly the cerebrum, the cerebellum, the midbrain, the pons, and the bulb.

11. Locate and describe the cortex. State purpose of the convolutions.

12. State the general differences between the cranial and the spinal nerves.

13. Locate and give the number of the dorsal-root ganglia. Locate and give the approximate number of the sympathetic ganglia.

14. Show how the two portions of the spinal nerves are formed—the one from the mon-axonic and the other from the di-axonic neurons.

15. Enumerate the different agencies through which the brain and spinal cord are protected.

16. What cranial nerves contain afferent fibers? What ones contain efferent fibers? What ones contain both afferent and efferent fibers?

17. In what respects is the nervous system similar to a system of telephony? In what respects is it different?



PRACTICAL WORK

Examine a model of the brain, identifying the different divisions and noting the position and relative size of the different parts (Fig. 137). Observe the convolutions of the cerebrum and compare these with the parallel ridges of the cerebellum. If the model is dissectible, study the arrangement of the cell-bodies (gray matter) and the distribution of the fiber bundles (white matter). Note the connection of the cranial nerves with the under side.

[Fig. 137]

Fig. 137—Model for demonstrating the brain (dissectible).

A prepared nervous system of a frog (such as may be obtained from supply houses) should also be examined. Observe the appearance and general distribution of the nerves and their connection with the brain and spinal cord. If such a preparation is not at hand, some small animal may be dissected to show the main divisions of the nervous system, as follows:

*Dissection of the Nervous System* (by the teacher).—For this purpose a half-grown cat is generally the best available material. This should be killed with chloroform and secured to a board as in the dissection of the abdomen (page 169). Open the abdominal cavity and remove the contents, tying the alimentary canal where it is cut, and washing out any blood which may escape. Dissect for the nervous system in the following order:

1. Cut away the front of the chest, exposing the heart and lungs. Find on each side of the heart a nerve which passes by the side of the pericardium to the diaphragm. These nerves assist in controlling respiration and are called the phrenic nerves. Find other nerves going to different parts of the thorax.

2. Remove the heart and lungs. Find in the back part of the thoracic cavity, on each side of the spinal column, a number of small "knots" of nervous matter joined together by a single nerve. These are sympathetic ganglia. Where the neck joins the thorax, find two sympathetic ganglia much larger than the others.

3. Cut away the skin from the shoulder and upper side of the fore leg. By separating the muscles and connective tissue where the leg joins the thorax, find several nerves of considerable size. These connect with each other, forming a network called the brachial plexus. From here nerves pass to the thorax and to the fore leg.

4. From the brachial plexus trace out the nerves which pass to different parts of the fore leg. In doing this separate the muscles with the fingers and use the knife only where it is necessary to expose the nerves. Note that some of the branches pass into the muscles, while others connect with the skin.

5. Remove the skin from the upper portion of one of the hind legs and separate the muscles carefully until a large nerve is found. This is one of the divisions of the sciatic nerve. Carefully trace it to the spinal cord, cutting away the bone where necessary, and find the connections of its branches with the cord. Then trace it toward the foot, discovering its branches to different muscles and to the skin.

6. Unjoint the neck and remove the head. Examine the spinal cord where exposed. Cut away the bone sufficiently to show the connection between the cord and one of the spinal nerves. On the dorsal root of one of the nerves find a small ganglion. What is it called?

7. Fasten the head to a small board and remove the scalp. Saw through the skull bones in several directions. Pry off the small pieces of bones, exposing the upper surface of the brain. Study its membranes, convolutions, and divisions.

8. With a pair of bone forceps, or nippers, break away the skull until the entire brain can be removed from the cavity. Examine the different divisions, noting the relative position and size of the parts.

9. With a sharp knife cut sections through the different parts, showing the positions of the "gray matter" and of the "white matter."

NOTE.—If the entire class is to examine one specimen, it is generally better to have the dissecting done beforehand and the parts separated and tacked to small boards. This will permit of individual examination. Sketches of the sciatic nerve, brachial plexus, and of sections through the brain and spinal cord should be made.

*Location of Nerves in the Body.*—Several of the nerves of the body lie sufficiently near the surface to be located by pressure and are easily recognized as sensitive cords. Slight pressure from the fingers reveals the presence of nerves in the grooves of the elbow (the crazy bone), between the muscles on the inner side of the arm near the shoulder, and in the hollow part of the leg back of the knee. These are all large nerves. Small nerves may be located in the same manner in the face and neck.



CHAPTER XVIII - PHYSIOLOGY OF THE NERVOUS SYSTEM

In the preceding chapter was pointed out the method by which the different parts of the body are brought into communication by the neurons or nerve cells. We are now to study the means whereby the neurons are made to control and cooerdinate the different parts of the body and bring about the necessary adjustment of the body to its surroundings. This work of the neurons naturally has some relation to their properties.

*Properties of Neurons.*—The work of the neurons seems to depend mainly upon two properties—the property of irritability and the property of conductivity. Irritability was explained, in the study of the muscles (page 243), as the ability to respond to a stimulus. It has the same meaning here. The neurons, however, respond more readily to stimuli than do the muscles and are therefore more irritable. Moreover, they are stimulated by all the forces that induce muscular contraction and by many others besides. They are by far the most irritable portions of the body.

Conductivity is the property which enables the effect of a stimulus to be transferred from one part of a neuron to another. On account of this property, an excitation, or disturbance, in any part of a neuron is conducted or carried to all the other parts. Thus a disturbance at the distant ends of the dendrites causes a movement toward the cell-body and, reaching the cell-body, the disturbance is passed through it into the axon. This movement through the neuron is called the nervous impulse.

*Purpose of the Impulse. *—Though the nature of the nervous impulse is not understood, (103) its purpose is quite apparent. It is the means employed by the nervous system for controlling and cooerdinating the different parts of the body. The arrangement of the neurons enables impulses to be started in certain parts of the nervous system, and the property of conductivity causes them to be passed as stimuli to other parts. This enables excitation at one place to bring about action at another place.

Acting as stimuli, the impulses seem able to produce two distinct effects: first, to throw resting organs into action and to increase the activity of organs already at work; and second, to diminish the rate, or check entirely, the activity of organs. Impulses producing the first effect are called excitant impulses; those producing the second effect, inhibitory impulses.

*Functions of the Parts of Neurons.*—The cell-body serves as a nutritive center from which the other parts derive nourishment. Proof of this is found in the fact that when any part of the neuron is separated from the cell-body, it dies, while the cell-body and the parts attached to the cell-body may continue to live. In addition to this the cell-body probably reenforces the nervous impulse.

The dendrites serve two purposes: first, they extend the surface of the cell-body, thereby enabling it to absorb a greater amount of nourishment from the surrounding lymph; second, they act as receivers of stimuli from other neurons. The same impulse does not pass from one neuron to another. An impulse in one neuron, however, is able to excite the neuron with which it makes an end-to-end connection, so that a series of impulses is produced along a given nerve path (Fig. 129).

The special function of the axon is to transmit the impulse. By its length, structure, and property of conductivity it is especially adapted to this purpose. The axis cylinder, however, is the only part of the axon concerned in the transmission. The primitive sheath and the medullary layer protect the axis cylinder, and, according to some authorities, serve to insulate it. The medullary sheath may also aid in the nourishment of the axis cylinder.

*Nerve Stimuli.*—While the properties of irritability and conductivity supply a necessary cause for the production and transmission of nervous impulses, these alone are not sufficient to account for their origin. An additional cause is necessary—a force not found in the nerve protoplasm, but one which, by its action on the protoplasm, makes it produce the impulse. In this respect, the neuron does not differ essentially from the cell of a muscle. Just as the muscle cell requires a stimulus to make it contract, so does the neuron require a stimulus to start the impulse. Hence, in accounting for the activities of the body, it is not sufficient to say they are caused by nervous impulses. We must also investigate the nerve stimuli—the means through which the nervous impulses are started. Most of these are found outside of the body and are known as external stimuli.

*Action of External Stimuli.*—In the arrangement of the nervous system the most favorable conditions are provided for the reception of external stimuli. Not only do vast numbers of neurons terminate at the surface of the body,(104) but they connect there with delicate structures, called sense organs. The purpose of the sense organs is to sensitize (make sensitive) the terminations of the neurons. This they do by supplying special structures through which the stimuli can act to the best advantage upon the nerve endings. Moreover, there are different kinds of sense organs, and these cause the neurons to be sensitive to different kinds of stimuli. Acting through the sense organs adapted for receiving them, light, sound, heat, cold, and odors all act as stimuli for starting impulses. Indeed, the arrangement is so complete that the nervous system is subjected to the action of external stimuli in some form practically all the time. The work of the sense organs is further considered in Chapters XX, XXI, and XXII.

*How External Stimuli act on Internal Organs.*—For stimulating the neurons not connected with the body surface we are dependent, so far as known, upon the nervous impulses. An impulse started by the external stimulus goes only so far as its neuron extends. But it serves as a stimulus for the neuron with which the first connects and starts an impulse in this connecting neuron, the point of stimulation being where the fiber terminations of the first neuron make connection with the dendrites of the second. This impulse in turn stimulates the next neuron, and so on, producing a series of impulses along a given nerve path. In this way the effect of an external stimulus may reach and bring about action in any part of the body. This is in brief the general plan of inducing action in the various organs of the body. This plan, however, is varied according to circumstances, and at least three well-defined forms of action are easily made out. These are known as reflex action, voluntary action, and secondary reflex action.

*Reflex Action.*—When some sudden or strong stimulus acts upon the nerve terminations at the surface of the body, an immediate response is frequently observed in some quick movement. The jerking away of the hand on accidentally touching a hot stove, the winking of the eyes on sudden exposure to danger, and the quick movements from slight electrical shocks are familiar examples. The explanation of reflex action is that external stimuli start impulses in neurons terminating at the surface of the body and these, in turn, excite impulses in neurons which pass from the spinal cord or brain to the muscles (Fig. 138). Since there is an apparent turning back of the impulses by the cord or brain, the resulting movements are termed reflex.(105)

[Fig. 138]

Fig. 138—*Diagram illustrating reflex action of an external organ.*

*Reflex Action and the Mind.*—If one carefully studies the reflex actions of his own body, he will find that they occur at the time, or even a little before the time, that he realizes what has happened. If a feather is brought in contact with the more sensitive parts of the face of a sleeping person, there is a twitching of the skin and sometimes a movement of the hand to remove the offending substance. Surgeons operating upon patients completely under the influence of chloroform, and therefore completely unconscious, have observed strong reflex actions. These and other similar cases indicate clearly that reflex action occurs independently of the mind—that the mind neither causes nor controls it. If a further proof of this fact were needed, it is supplied by experiments upon certain of the lower animals,(106) which live for a while after the removal of the brain. These experiments show that the nervous impulses that produce reflex action need only pass through the spinal cord and do not reach the cerebrum, the organ of the mind.

*The Reflex Action Pathway.*—By study of the impulses that produce any reflex action, a rather definite pathway may be made out, having the following divisions:

1. From the surface of the body to the central nervous system (usually the spinal cord). This, the afferent division, is made up of di-axonic neurons, and these have (in the case of the spinal nerves) their cell-bodies in the dorsal root ganglia (page 295). They are acted upon by external stimuli, while their impulses in turn act on the neurons in the spinal cord.

2. Through the central system (spinal cord or base of brain). This, the intermediate division, may be composed of mon-axonic neurons, or it may consist of branches from the afferent neurons. In the case of separate neurons, these are acted upon by impulses from the afferent neurons, while their impulses serve in turn as stimuli to other neurons within the cord (Fig. 129).

3. From the central nervous system to the muscles. This, the efferent division, is made up of mon-axonic neurons. Most of these have their cell-bodies in the gray matter of the cord, while their fibers pass into the spinal nerves by the ventral roots.(107) They may be stimulated by impulses either from the intermediate neurons, or from branches of the afferent neurons. Their impulses reach and stimulate the muscles.

*Reflex Action in Digestion.*—The flowing of the saliva, when food is present in the mouth, is an example of reflex action. In this case, however, the organ excited to activity is a gland instead of a muscle. The food starts the impulses, and these, acting through the bulb, reach and stimulate the salivary glands. In a similar manner food excites the glands that empty their fluids into the stomach and intestines, and stimulates the muscular coats of these organs to do their part in the digestive process. To a considerable extent, neurons having their cell-bodies in the sympathetic ganglia are concerned in these actions (Fig. 139).

[Fig. 139]

Fig. 139—Diagram illustrating reflex action in its relation to the food canal. The nerve path in this case includes sympathetic neurons.

*Reflex Action in the Circulation of the Blood.*—On sudden exposure to cold, the small arteries going to the skin quickly diminish in size, check the flow of blood to the surface, and prevent too great a loss of heat. In this case, impulses starting at the surface of the body are transmitted to the bulb and then through the efferent neurons to the muscles in the walls of the arteries. In a somewhat similar manner, heat leads to a relaxation of the arterial walls and an increase in the blood supply to the skin. Other changes in the blood supply to different parts of the body are also of the nature of reflex actions. As in the work of digestion, neurons having their cell-bodies in the sympathetic ganglia aid in the control of the circulation.

*Purposes of Reflex Action.*—The examples of reflex action so far considered illustrate its two main purposes—(1) protection, and (2) a means of controlling important processes.

The pupil has but to study carefully the reflex actions of his own body for a period, say of two or three weeks, in order to be convinced of their protective value. He will observe that portions of his body have, on exposure to danger, been moved to places of safety, while in some instances, like falling, his entire body has been adjusted to new conditions. He will also find that reflex action is quicker, and for that reason offers in some cases better protection, than movements directed by the mind. In digestion and circulation are found the best examples of the control of important processes through reflex action.

*Voluntary Action.*—It is observed that reflex action, in the sense that it has so far been considered, is not the usual mode of action of the external organs, but is, instead, a kind of emergency action, due to unusual conditions and excitation by strong stimuli. Voluntary actions, on the other hand, represent the ordinary, or normal, action of these organs. They comprise the movements of the body of which we are conscious and which are controlled by the mind. But while they are of a higher order than reflex actions and are under intelligent direction, they are brought about in much the same manner.

*Voluntary Action Pathways* differ in but one essential respect from those of reflex action. They pass through the cerebrum, the organ of the mind (Fig. 140). This is necessary in order that the mind may control the action. From all portions of the body surface, afferent pathways may be traced to the cerebrum; and from the cerebrum efferent pathways extend to all the voluntary organs. A complex system of intermediate neurons, found mostly in the brain, join the afferent with the efferent pathways. The voluntary pathways are not distinct from, but include, reflex pathways, a fact which explains why the same external stimulus may excite both reflex and voluntary action (Fig. 141).

[Fig. 140]

Fig. 140—*Diagram of a voluntary action pathway.*

*Choice in Voluntary Action.*—In reflex action a given stimulus, acting in a certain way; produces each time the same result. This is not the case with voluntary action, the difference being due to the mind. In these actions the external stimulus first excites the mind, and the resulting mental processes—perhaps as memory of previous experiences—supply a variety of facts, any of which may act as stimuli to action. Before the action takes place, however, some one fact must be singled out from among the mental processes excited. This fact becomes the exciting stimulus and leads to action. It follows, therefore, that the action which finally occurs is not necessarily the result of an immediate external stimulus, but of a selected stimulus—one which is the result of choice.

[Fig. 141]

Fig. 141—*Diagram of voluntary action pathways* including reflex pathways.

Not only does the element of choice enter into the selection of the proper stimulus, but it also enters into the time, nature, and intensity of the action. For these reasons it is frequently impossible to trace voluntary actions back to their actual stimuli. The pupil will recognize the element of choice in such simple acts as picking up some object from the street, complying with a request, and purchasing some article from a store.

*Reflex and Voluntary Action Compared.*—Certain likenesses and differences, already suggested in these two forms of action, may now be more fully pointed out. Reflex and voluntary action are alike in that the primary cause of each is some outside force or condition which has impressed itself upon the nervous system. They are also alike in the general direction taken by the impulses in producing the action. The impulses are, first, from the surface of the body to the central nervous system; second, through the central system; and third, from the central nervous system to the active tissues of the body.

Their chief differences are to be found, first, in the pathways followed by the impulses, which are through the cerebrum (the organ of the mind) in voluntary action, but in reflex action are only through the spinal cord or the lower parts of the brain; and second, in the fact that voluntary action is under the direction of the mind, while reflex action is not. It would seem, therefore, that the statement sometimes made that "voluntary action is reflex action plus the mind" is not far from correct. Mind, however, is the important factor in this kind of action.

*Secondary Reflex Action.*—Everyday experience teaches that any voluntary action becomes easier by repetition. A given act performed a number of times under conscious direction establishes a condition in the nervous system that enables it to occur without that direction and very much as reflex actions occur. Actions of this kind are known as secondary reflex actions, or as acquired reflexes. Walking, writing, and numerous other movements pertaining to the occupation which one follows are examples of such reflexes. These activities are at first entirely voluntary, but by repetition they gradually become reflex, requiring only the stimulus to start them.

The advantages to the body of its acquired reflexes are quite apparent. The mind does not have to attend to the selection and direction of stimuli and, to that extent, is left free for other work. A good example of this is found in writing, where the mind apparently gives no heed to the movements of the hand and is only concerned in what is being written. The student will easily supply other illustrations of the advantages of secondary reflex action.

The development of secondary reflexes probably consists in the establishment of fixed pathways for impulses through the nervous system. Through the branching of the nerve fibers many pathways are open to the impulses. But in repeating the same kind of action the impulses are guided into particular paths, or channels. In time these paths become so well established that the impulses flow along them without conscious direction and it is then simply necessary that some stimulus starts the impulses. By following the established pathways, these reach the right destination and produce the desired result. According to this view, secondary reflex action is but a higher phase of ordinary reflex action—a kind of reflex action, the conditions of which have been established by the mind through repetition. (See functions of the cerebellum, page 317.)

*Habits.*—People are observed to act differently when exposed to the same conditions, or when acted upon by the same stimuli. This is explained by saying they have different habits. By habits are meant certain general modes of action that have been acquired by repetition. Certain acts repeated again and again have established conditions in the nervous system which enable definite forms of action to be excited, somewhat after the manner of reflex action. On account of habits, therefore, the actions of the individual are more or less predisposed. What he will do under certain conditions may be foretold from his habits. Habits simply represent, a higher order of secondary reflexes—those more closely associated with the mental life and character than are the lower forms.

Habits, in common with other forms of secondary reflex action, serve the important purpose of economizing the nervous energy. However, if pernicious habits are formed instead of those that are useful, they are detrimental from both a moral and physical standpoint. Youth is recognized as the period in which fundamental habits are formed and character is largely determined. Therefore parents and teachers do wisely when they insist upon the formation of right habits by the young.

*Functions of Divisions of the Nervous System.*—The relationship between the different parts of the nervous system is very close and one part does not work independently of other parts. At the same time the general work of the nervous system requires that its different divisions serve different purposes:

1. The peripheral divisions of the nervous system are concerned in the transmission of impulses between the surface of the body and the central system and between the central system and the active tissues. The nerves are the carriers of the impulses. The ganglia contain the cell-bodies which serve as nutritive centers; and, in the case of the sympathetic ganglia, these cell-bodies are the places where the fiber terminations of one neuron connect with, and stimulate, other neurons.

2. The gray matter in the spinal cord, bulb, pons, and midbrain (through the cell-bodies, fiber terminations, and short neurons which they contain) completes the reflex action pathways between the surface of the body and the voluntary muscles, and also between the surface of the body and the organs of circulation and digestion.

3. The white matter of the spinal cord, bulb, pons, and midbrain (by means of the fibers of which they are largely composed) forms connections with, and passes impulses between, the various parts of the central nervous system.

4. The bulb, because of certain special reflex-action pathways completed through it, is the portion of the central nervous system concerned in the control of respiration, circulation, and the secretion of liquids.

*Work of the Sympathetic Ganglia and Nerves.*—The neurons which form these ganglia aid in controlling the vital processes, especially digestion and circulation. These neurons are controlled for the most part by fibers from the bulb and spinal cord, and cannot for this reason be looked upon as forming an independent system. Their chief purpose seems to be that of spreading the influence of neurons from the central system over a wider area than they would otherwise reach. For example, a single neuron passing out from the spinal cord may, by terminating in a sympathetic ganglion, stimulate a large number of neurons, each of which will in turn stimulate the cells of muscles or of glands. Because of this function, the sympathetic neurons are sometimes called distributing neurons.

*Functions of the Cerebellum.*—Efforts to discover some special function of the cerebellum have been in the main unsuccessful. Its removal from animals, instead of producing definite results, usually interferes in a mild way with a number of activities. The most noticeable results are a general weakness of the muscles and an inability on the part of the animal to balance itself. This and other facts, including the manner of its connection with other parts of the nervous system, have led to the belief that the cerebellum is the chief organ for the reflex cooerdination of muscular movements, especially those having to do with the balancing of the body. In this connection it is subordinate to and under the control of the cerebrum. Of the relations which the cerebellum sustains to the cerebrum and to the different parts of the body, the following view is quite generally held:

In the development of secondary reflexes, as already described, conditions are established in the cerebellum, such that given stimuli may act reflexively through it and produce definite results in the way of muscular contraction. After the establishment of these conditions, afferent impulses from the eyes, ears, skin, and other places, under the general direction of the cerebrum, may cause such actions as the balancing of the body, walking, etc., as well as the delicate and varied movements of the hand. This view of its functions makes of the cerebellum the great center of secondary reflex action.

*Functions of the Cerebrum.*—While the work of the cerebrum is closely related to that of the general nervous system, it, more than any other part, exercises functions peculiar to itself. The cerebrum is the part of the nervous system upon which our varied experiences leave their impressions and through which these impressions are made to influence the movements of the body. But the power to alter, postpone, or entirely inhibit, nervous movements is but a part of the general work ascribed to the cerebrum as the organ of the mind. Numerous experiments performed upon the lower animals, together with observations on man, show the cerebrum to be the seat of the mental activities, and to make possible, in some way, the processes of consciousness, memory, volition, imagination, emotion, thought, and sensation.

*Localization of Cerebral Functions.*—Many experiments have been performed with a view to determining whether the entire cerebrum is concerned in each of its several activities or whether special functions belong to its different parts. These experiments have been made upon the lower animals and the results thus obtained compared with observations made upon injured and imperfectly developed brains in man. The results have led to the conclusion that certain forms of the work of the cerebrum are localized and that some of its parts are concerned in processes different from those of others.

[Fig. 142]

Fig. 142—*Location of cerebral functions.* Diagram of cerebrum, showing most of the areas whose functions are known.

The work of locating the functions of different parts of the cerebrum forms one of the most interesting chapters in the history of brain physiology. The portions having to do with sight, voluntary motion, speech, and hearing have been rather accurately determined, while considerable evidence as to the location of other functions has been secured. Much of the cerebral surface, however, is still undetermined (Fig. 142).



NERVOUS CONTROL OF IMPORTANT PROCESSES

*Circulation of the Blood.*—1. Control of the Heart.—The ability to contract at regular intervals has been shown to reside in the heart muscle. Among other proofs is that furnished by cold-blooded animals, like the frog, whose heart remains active for quite a while after its removal from the body. These automatic contractions, however, are not sufficient to meet all the demands made upon the circulation. The needs of the tissues for the constituents of the blood vary with their activity, and it is therefore necessary to vary frequently the force and rapidity of the heart's contractions. Such changes the heart itself is unable to bring about.

For the purpose of controlling the rate and force of its contractions, the heart is connected with the central nervous system by two kinds of fibers:

a. Fibers that convey excitant impulses to the heart to quicken its movements.

b. Fibers that convey inhibitory impulses to the heart to retard its movements.

The cell-bodies of the excitant fibers are found in the sympathetic ganglia, but fibers from the bulb connect with and control them. The cell-bodies of the inhibitory fibers are located in the bulb, from where their fibers pass to the heart as a part of the vagus nerve.

In addition to the fibers above mentioned, are those that convey impulses from the heart to the bulb. These connect with neurons that in turn connect with blood vessels and with them act reflexively, when the heart is likely to be overstrained, to cause a dilation of the blood vessels. This lessens the pressure which the heart must exert to empty itself of blood. These fibers serve, in this way, as a kind of safety valve for the heart.

2. Control of Arteries.—Changes in the rate and force of the heart's contractions can be made to correspond only to the general needs of the body. When the blood supply to a particular organ is to be increased or diminished, this is accomplished through the muscular coat in the arteries. The connection of the arterial muscle with the sympathetic ganglia and the method by which they vary the flow of blood to different organs has already been explained (pages 311 and 49), so that only the location of the controlling neurons need be noted here. These, like the controlling neurons of the heart, have their cell-bodies in the bulb. It thus appears that the entire control of the circulation is effected in a reflex manner through the nerve centers in the bulb. These centers are stimulated by conditions that relate to the movement of the blood through the body.

*Respiration.*—Efferent fibers connect the different muscles of respiration with a cluster of cell-bodies in the bulb, called the respiratory center. This center together with the nerves and muscles in question form an automatic, or self-acting, mechanism similar in some respects to that of the heart. Through the impulses passing from the respiratory center to the muscles, a rhythmic action is maintained sufficient to satisfy the usual needs of the body for oxygen. The demand of the body for oxygen, however, varies with its activities, and to such variations the respiratory center alone is unable to respond. The regulating factor in the respiratory movements has been found to be the condition of the blood with reference to the presence of oxygen and carbon dioxide. If the blood contains much carbon dioxide and little oxygen, it acts as a strong stimulus to the respiratory center, causing it, in turn, to stimulate the respiratory muscles with greater intensity and frequency. On the other hand, if the blood contains much oxygen and little carbon dioxide, it acts only as a mild stimulus. This explains how physical exercise increases the breathing, since the muscles at work consume more oxygen than when resting and give more carbon dioxide and other wastes to the blood.