TABLE III. THE PASSAGE OF WASTE MATERIALS FROM THE BODY Materials State How Formed Condition in How Removed in the Body the Blood from the Blood Carbon Gas By the Dissolved in Separated dioxide oxidation of the plasma from the the carbon and in loose blood at the of proteids, combination alveoli of carbohydrates, with salts the lungs and fats. in the and then blood. forced through the air passages into the atmosphere. Urea Solid By the Dissolved in Removed by oxidation in the plasma. the the liver of uriniferous nitrogenous tubules of compounds. the kidneys and to a small extent by the perspiratory glands. Water Liquid By the As water. Removed by oxidation of all the the hydrogen organs of of proteids, excretion, carbohydrates, but in the and fats. largest Amount formed quantities in the body is by the small. kidneys and the skin. Salts Solid Dissolved in By the the plasma. kidneys, liver, and skin.



HYGIENE

The separation of wastes from the body has such a close relation to the health that all conditions affecting it should receive the most careful attention. Their retention beyond the time when they should be discharged undoubtedly does harm and is the cause of many bodily disorders.

*Value of Water.*—As a rule the work of excretion is aided by drinking freely of pure water. As water is the natural dissolver and transporter of materials in the body, it is generally conceded by hygienists and physicians that the taking of plenty of water is a healthful practice. People do not as a rule drink a sufficient amount of water, about three pints per day being required by the average adult, in addition to that contained in the food. Most of the water should, of course, be taken between meals, although the sipping of a small amount during meals does not interfere with digestion. As stated elsewhere, the taking of a cup of water on retiring at night and again on rising in the morning is very generally recommended.

*Protection of Kidneys and Liver.*—The kidneys and liver are closely related in their work and in many instances are injured or benefited by the same causes. Both, as already stated (page 124), are liable to injury from an excess of proteid food, especially meats, and also by a condition of inactivity of the bowels (page 166). The free use of alcohol also has an injurious effect on both of these organs.(75) On the other hand, increasing the activity of the skin has a beneficial effect upon them, especially the kidneys. Exercise and bathing, which tend to make the skin more active, are valuable aids both in ridding the body of impurities and in lessening the work of the other excretory organs. One having a disease of the kidneys, however, needs to exercise great care in bathing on account of the bad results which follow getting chilled.

*Special Care after Certain Diseases.*—Certain diseases, as measles, diphtheria, scarlet fever, and typhoid fever, sometimes have the effect of weakening the kidneys (and other vital organs) and of starting disease in them. When this occurs it is usually the result of exposure or of over-exertion while the body is in a weakened condition. Severe chilling at such a time, by driving blood from the surface to the parts within, often causes inflammation of the kidneys. On recovering from any wasting disease one should exercise great caution both in resuming his regular work and in exposing his body to wet or cold.

*Misunderstood Symptoms.*—Pains in the small of the back, an increase in the secretions of the kidneys, and a sediment in the urine very naturally suggest some disorder of the kidneys. It is a fact, however, that these symptoms have little or no relation to the state of the kidneys and may occur when the kidneys are in a perfectly healthy condition. The kidneys are not located in the small of the back, but above this place, so that pains in this region are evidently not from the kidneys, while the increase in the flow of the urine may arise from a number of causes, one of which is an increase of certain waste products passed into the blood. The symptoms referred to are frequently the results of nervous exhaustion, resulting from overstudy, worry, eye strain, or some other condition that overtaxes the nervous system. When this is the case, relief is obtained through resting the nerves. Actual disease of the kidneys can only be determined through a chemical and microscopic examination of the urine. To resort to some patent medicine for kidney trouble without knowing that such trouble exists, as is sometimes done, is both foolish and unhygienic.

*Alcoholic Beverages and the Elimination of Waste.*—Causing as it does such serious diseases as cirrhosis of the liver and Bright's disease of the kidneys (footnote, page 210), alcohol will greatly interfere in this way with the elimination of waste. There is also evidence to the effect that it interferes with waste elimination before the stage is reached of causing disease of these organs. Researches have shown that alcohol increases the amount of uric acid in the body and decreases the amount of urea found in the urine. The conclusion to be drawn is that alcohol interferes in some way with the change of the harmful uric acid into the comparatively harmless urea—an interference which in some instances results in great harm. It has also been shown that malted liquors, such as beer and ale, contain substances which, like the caffein of tea and coffee (page 167), are readily converted into uric acid.(76) Wines contain acids which may also act injuriously. The harm which such substances do is, of course, additional to that caused by the alcohol.

*Summary.*—As a result of the oxidations and other changes at the cells, substances are produced that can no longer serve a purpose in the body. They are of the nature of waste, and their continuous removal from the body is as necessary to the maintenance of life as the introduction of food and oxygen. The organs whose work it is to remove the waste, excepting the lungs, are glands; and the material which they remove are of the nature of secretions. From the cells, the waste passes through the lymph in the blood. From the blood it is separated by the excretory organs and passed to the exterior of the body.

*Exercises.*—1. What general purposes are served by the glands in the body?

2. What are the parts common to all glands? What purpose is served by each of these parts?

3. How do tubular glands differ in structure from saccular glands? What is a racemose gland? Why so called?

4. Describe the nature of the secretory process.

5. What conditions render necessary the formation of waste materials in the body? Why must these be removed?

6. How do the waste materials get from the cells to the organs of excretion?

7. Show by a drawing the connections of the kidneys with the large blood vessels and the bladder. Name parts of drawing.

8. In what do the uriniferous tubes have their beginning? In what do they terminate? With what are they lined?

9. Why should the blood pass through two sets of capillaries in the kidneys?

10. Bright's disease of the kidneys affects the uriniferous tubes and interferes with their work. What impurity is then left in the blood?

11. Trace water and salts from the Malpighian capsules to the bladder, naming parts through which they pass.

12. Trace carbon dioxide from the cells to the outside atmosphere.

13. How does the quantity of material introduced into the body compare with that which is removed by the organs of excretion?

14. Name two ways of lessening the work of the kidneys.

15. Why is the drinking of plenty of pure water a healthful practice?



PRACTICAL WORK

*To suggest the Double Work of Glands.*—Prepare a simple filter by fitting a piece of porous paper into a glass funnel. Through this pass pure water and also water having salt dissolved in it and containing some sediment, as sand. The water and the dissolved salt pass through, while the sediment remains on the filter. Now substitute a fresh piece of paper in the funnel and drop on its surface a little solid coloring matter, such as cochineal. Again pass the liquid through the funnel. This time it comes through colored, the color being added by the filter. Compare the filter and materials filtered to the gland and the materials concerned in secretion (blood, the liquid secreted, substances added by the gland, etc.).

[Fig. 92]

Fig. 92—*The physiological scheme.* Diagram suggesting the essential relation of the bodily activities. See Summary of Part I, page 215, and Summary of Part II, page 413.



SUMMARY OF PART I

The body is an organization of different kinds of cells; it grows through the growth and reproduction of these cells; and its life as a whole is maintained by providing such conditions as will enable the cells to keep alive. Of chief importance in the work of the body is a nutrient fluid which supplies the cells with food and oxygen and relieves them of waste. A moving portion of this fluid, called the blood, serves as a transporting agent, while another portion, called the lymph, passes the materials between the blood and the cells. Through their effects upon the blood and the lymph, the organs of circulation, respiration, digestion, and excretion minister in different ways to the cells, and aid in the maintenance of life. By their combined action two distinct movements are kept up in the body, as follows:

1. An inward movement which carries materials from the outside of the body toward the cells.

2. An outward movement which carries materials from the cells to the outside of the body.

Passing inward are the oxygen and food materials in a condition to unite with each other and thereby change their potential into kinetic energy. Passing outward are the oxygen and the elements that formed the food materials after having united at the cells and liberated their energy.

As a final and all-important result, there is kept up a continuous series of chemical changes in the cells. These liberate the energy, provide special substances needed by the cells, and preserve the life of the body (Fig. 92).

In the chapters which follow, we are to consider the problem of adjusting the body to and of bringing it into proper relations with its surroundings.



PART II: MOTION, COORDINATION, AND SENSATION



CHAPTER XIV - THE SKELETON

One necessary means of establishing proper relations between the body and its surroundings is motion.(77) Not only can the body move itself from place to place, but it is able to move surrounding objects as well. In the production of motion three important systems are employed—the muscular system, the nervous system, and a system of mechanical devices which are found mainly in the skeleton. The muscular system supplies the energy for operating the mechanical devices, while the nervous system controls the movements.(78) Although the skeleton serves other purposes, such as giving shape to the body and protecting certain organs, its main use is that of an aid in the production of motion.

*Skeleton Tissues.*—The tissues employed in the construction of the skeleton are the osseous, the cartilaginous, and the connective tissues. These are known as the supporting tissues of the body. They form the bones, supply the elastic pads at the ends of the bones, and furnish strong bands, called ligaments, for fastening the bones together. The skeleton forms about 16 per cent of the weight of the body. Its tissues, being of a more durable nature than the rest of the body, do not so readily decay. Especially is this true of the osseous tissue, which may be preserved indefinitely, after removal from the body, by simply keeping it dry.

*The Bones.*—The separate units, or parts, of which the skeleton is constructed are called bones. They are the hard structures that can be felt in all parts of the body, and they comprise nearly the entire amount of material found in the prepared skeleton. As usually estimated, the bones are 208 in number. They vary greatly in size and shape in different parts of the body.

*Composition and Properties of Bones.*—The most noticeable and important properties of the bones are those of hardness, stiffness, and toughness. Upon these properties the uses of the bones depend. These properties may, in turn, be shown to depend upon the presence in osseous tissue of two essentially different kinds of substance, known as the animal matter and the mineral matter. If a bone is soaked in an acid, the mineral matter is dissolved out, and as a result it loses its properties of hardness and stiffness. (See Practical Work.) This is because the mineral matter supplies these properties, being composed of substances which are hard and closely resemble certain kinds of rock. The chief materials forming the mineral matter are calcium phosphate and calcium carbonate.

On the other hand, burning a bone destroys the animal matter. When this is done the bone loses its toughness, and becomes quite brittle. The property of toughness is, therefore, supplied by the animal matter. This consists mainly of a substance called ossein, which may be dissolved out of the bones by boiling them. Separated from the bones it is known as gelatine. The blood vessels and nerves in the bones, and the protoplasm of the bone cells, are also counted in with the animal matter.

[Fig. 93]

Fig. 93—*Section of a long bone* (tibia), showing the gross structure.

If a dry bone from a full-grown, but not old, animal be weighed before and after being burned, it is found to lose about one third of its weight. From this we may conclude that about one third of the bone by weight is animal matter and two thirds is mineral matter. This proportion, however, varies with age, the mineral matter increasing with advance of years.

*Gross Structure of Bones.*—The gross structure of the bones is best learned by studying both dry and fresh specimens. (See Practical Work.) The ends of the bones are capped by a layer of smooth, elastic cartilage, while all the remaining surface is covered by a rather dense sheath of connective tissue, called the periosteum. Usually the central part of the long bones is hollow, being filled with a fatty substance known as the yellow marrow. Around the marrow cavity the bone is very dense and compact, but most of the material forming the ends is porous and spongy. These materials are usually referred to as the compact substance and the cancellous, or spongy, substance of the bones (Fig. 93).

The arrangement of the compact and spongy substance varies with the different bones. In the short bones (wrist and ankle bones, vertebrae, etc.) and also in the flat bones (skull bones, ribs, shoulder blades, etc.) there is no cavity for the yellow marrow, all of the interior space being filled with the spongy substance. The red marrow, relations of which to the red corpuscles of the blood have already been noted (page 27), occupies the minute spaces in the spongy substance.

[Fig. 94]

Fig. 94—*Cross section of bone showing minute structure.* Magnified. 1. Surface layer of bone. 2. Deeper portion. 3. Haversian canals from which pass the canaliculi. 4. A lacuna. Observe arrangement of lacunae at surface and in deeper portion.

*Minute Structure of Bone.*—A microscopic examination of a thin slice of bone taken from the compact substance shows this to be porous as well as the spongy substance. Two kinds of small channels are found running through it in different directions, known as the Haversian canals and the canaliculi (Fig. 94). These serve the general purpose of distributing nourishment through the bone. The Haversian canals are larger than the canaliculi and contain small nerves and blood vessels, chiefly capillaries (Fig. 95). They extend lengthwise through the bone. The canaliculi are channels for conveying lymph. They pass out from the Haversian canals at right angles, going to all portions of the compact substance except a thin layer at the surface. In the surface layer of the bone the canaliculi are in communication with the periosteum.

[Fig. 95]

Fig. 95—*Section showing Haversian canal and contents*, highly magnified (after Schaefer). 1. Arterial capillary. 2. Venous capillary. 3. Nerve fibers. 4. Lymph vessel.

*The Bone Cells.*—Surrounding the Haversian canals are thin layers of bone substance called the laminae, and within these are great numbers of irregular bodies, known as the lacunae. The walls of the lacunae are hard and dense, but within each is an open space. In this lies a flattened body, having a nucleus, which is recognized as the bone cell, or the bone corpuscle (Fig. 96). It appears to be the work of the bone cells to deposit mineral matter in the walls surrounding them and in this way to supply the properties of hardness and stiffness to the bones. The canaliculi connect with the lacunae in all parts of the bone, causing them to appear under the microscope like so many burs fastened together by their projecting spines (Fig. 94).

[Fig. 96]

Fig. 96—*Bone cell* removed from the lacuna and very highly magnified. (From Quain's Anatomy.)

*How the Bone Cells are Nourished.*—The bone cells, like all the other cells of the body, are nourished by the lymph that escapes from the blood. This passes through the canaliculi to the cells in the different parts of the bone, as follows:

1. The cells in the surface layer of the bone receive lymph from the capillaries in the periosteum.(79) It gets to them through the short canaliculi that run out to the surface.

2. The cells within the interior of the bone receive their nourishment from the small blood vessels in the Haversian canals. Lymph from these vessels is conveyed to the cells through the canaliculi that connect with the Haversian canals.

*Plan and Purpose of the Skeleton.*—The framework of the body is such as to adapt it to a movable structure. Obviously the different parts of the body cannot be secured to a foundation, as are those of a stationary building, but must be arranged after a plan that is conducive to motion. A moving structure, as a wagon or a bicycle, has within it some strong central part to which the remainder is joined. The same is true of the skeleton. That part to which the others are attached is a long, bony axis, known as the spinal column. Certain parts, as the ribs and the skull, are attached directly to the spinal column, while others are attached indirectly to it. The arrangement of all the parts is such that the spinal column is made the central, cohering portion of the skeleton and also of the whole body.

Besides the general arrangement of the parts of the skeleton, there is such a grouping of the bones in each of its main divisions as will enable them to serve definite purposes. In most places they form mechanical devices for supplying special movements, and in certain places they provide for the support or protection of important organs. In most cases there is a definite combination of different bones, forming what is called the bone group.

[Fig. 97]

Fig. 97—The human skeleton.

*Bone Groups.*—On account of the close relation between the bones of the same group, they cannot profitably be studied as individual bones, but each must be considered as a part of the group to which it belongs. By first making out the relation of a given bone to its group, its value to the whole body can be determined. The most important of the groups of bones are as follows:

1. The Spinal Column.—This group consists of twenty-four similarly shaped bones, placed one above the other, called the vertebrae, and two bones found below the vertebrae, known as the sacrum and the coccyx (Fig. 98). These twenty-six bones supply the central axis of the body, support the head and upper extremities, and inclose and protect the spinal cord.

[Fig. 98]

Fig. 98—The spinal column.

The upper seven vertebrae form the neck and are called the cervical vertebrae. They are smaller and have greater freedom of motion than the others. The first and second cervical vertebrae, known as the atlas and the axis, are specially modified to form a support for the head and provide for its movements. The head rests upon the atlas, forming with it a hinge joint (used in nodding to indicate "yes"); and the atlas turns upon an upward projection of the axis forming a pivot joint (used in shaking the head to indicate "no").

The next twelve vertebrae, in order below the cervical, are known as the thoracic vertebrae. They form the back part of the framework of the thorax and have little freedom of motion. The five vertebrae below the thoracic are known as the lumbar vertebrae. These bones are large and strong and admit of considerable motion. Below the last lumbar vertebra is a wedge-shaped bone which has the appearance of five vertebrae fused together. This bone, known as the sacrum, connects with the large bones which form the pelvic girdle. Attached to the lower end of the sacrum is a group of from two to four small vertebrae, more or less fused, called the coccyx.

[Fig. 99]

Fig. 99—*Two views of a lumbar vertebra.* A. From above. B. From the side. 1. Body. 2, 3, 4, 5. Projections from the neural arch.

*The Joining of the Vertebrae.*—A typical vertebra consists of a heavy, disk-shaped portion in front, called the body, which is connected with a ring-like portion behind, called the neural arch. The body and the neural arch together encircle a round opening which is a part of the canal that contains the spinal cord (Fig. 99). From the neural arch are seven bony projections, or processes, three of which serve for the attachment of muscles and ligaments, while the other four, two above and two below, are for the interlocking of the vertebrae with each other. The separate vertebrae are joined together in the spinal column, as follows:

a. Between the bodies of adjacent vertebrae are disks of elastic cartilage. Each disk is about one fourth of an inch thick and is grown tight onto the face of the vertebra above and also onto the face of the vertebra below. By means of these disks a very close connection is secured between the vertebrae on the front side of the column.

b. On the back of the column, the downward projections from the neural arch of each vertebra above fit into depressions found in the neural arch of the vertebra below. This interlocking of the vertebrae, which is most marked in the lumbar region, strengthens greatly the back portion of the column.

c. To further secure one bone upon the other, numerous ligaments pass from vertebra to vertebra on all sides of the column.

2. The Skull.—The skull is formed by the close union of twenty-two irregular bones. These fall naturally into two subgroups—the cranium and the face (Fig. 100). The cranium consists of eight thin, curved bones which inclose the space, called the cranial cavity, that holds the brain. The face group, consisting of fourteen bones, provides cavities and supports for the different organs of the face, and supplies a movable part (the inferior maxillary) which, with the bones above (superior maxillary), forms the machine for masticating the food.

[Fig. 100]

Fig. 100—*The skull (Huxley).* The illustration shows most of the bones of the skull.

3. The Thorax.—This group contains twenty-four bones of similar form, called ribs, and a straight flat bone, called the sternum, or breastbone (Fig. 101). The ribs connect with the spinal column behind, and all but the two lowest ones connect with the sternum in front, and, by so doing, inclose the thoracic cavity. As already stated (page 85), the bones of the thorax form a mechanical device, or machine, for breathing. The ribs are so arranged that the volume of the thorax is increased by elevating them and diminished by depressing them, enabling the air to be forced into and out of the lungs.

[Fig. 101]

Fig. 101—*Bone groups of trunk.*

4. The Shoulder and Pelvic Girdles.—These groups form two bony supports—one at the upper and the other at the lower portion of the trunk—which serve for the attachment of the arms and legs (Fig. 101). The shoulder girdle is formed by four bones—two clavicles, or collar bones, and two scapulae, or shoulder blades. The clavicle on either side connects with the upper end of the sternum and serves as a brace for the shoulder, while the scapula forms a socket for the humerus (the large bone of the arm) and supplies many places for the attachment of muscles.

The pelvic girdle consists of two large bones of irregular shape, called the innominate bones. They connect behind with the sacrum and in front they connect, through a small pad of cartilage, with each other. On the inside of the girdle is a smooth, basin-shaped support for the contents of the abdomen, but on the outside the bones are rough and irregular and provide many places for the attachment of muscles and ligaments. Each innominate bone has a deep, round socket into which the end of the femur (the long bone of the leg) accurately fits.

5. The Arm and Hand Groups.—A long bone, the humerus, connects the arm with the shoulder and gives form to the upper arm. In the forearm are two bones, the radius and the ulna, which connect at one end with the humerus and at the other with the bones of the wrist (Fig. 102).

[Fig. 102]

Fig. 102—*Bone groups of arm and leg.*

A group of eight small, round bones is found in the wrist, known as the carpal bones. These are arranged in two rows and are movable upon one another. Five straight bones, the metacarpals, connect with the wrist bones and form the framework for the palm of the hand. Attached to the metacarpals are the bones of the fingers and thumb. These form an interesting group of fourteen bones, called the phalanges of the fingers (Fig. 102).

The bones of the hand provide a mechanical device, or machine, for grasping, and the arm serves as a device for moving this grasping machine from place to place. The work of the arm, in this respect, is not unlike that of a revolving crane upon the end of which is a grab-hook. The hand without the arm to move it about would be of little use.

6. The Leg and Foot Groups.—These correspond in form and arrangement to the bones of the arm and hand. Since, however, the leg and foot are used for purposes different from those of the arm and hand, certain differences in structure are to be found. The patella, or kneepan, has no corresponding bone in the arm; and the carpus, or ankle, which corresponds to the wrist, contains seven instead of eight bones. The bones of the foot and toes are the same in number as those of the hand and fingers, but they differ greatly in size and form and have less freedom of motion. The femur, which gives form to the thigh, is the longest bone of the body. The tibia, or shin bone, and the fibula, the slender bone by its side, give form to the lower part of the leg (Fig. 102).

The legs are mechanical devices (walking machines) for moving the body from place to place. The feet serve both as supports for the body and as levers for pushing the body forward. By their attachment to the legs they may be placed in all necessary positions for supporting and moving the body.

The different bone groups are shown in Fig. 97 and named in Table IV.

*Adaptation to Special Needs.*—When any single bone is studied in its relation to the other members of the group to which it belongs or with particular reference to its purpose in the body, its adaptation to some special place or use is at once apparent. Each bone serves some special purpose, and to this purpose it is adapted by its form and structure. Long bones, like the humerus and femur, are suited to giving strength, form, and stiffness to certain parts, while irregular bones, like the vertebrae and the pelvic bones, are fitted for supporting and protecting organs. Others, like the wrist and ear bones, make possible a peculiar kind of motion, and still others, like the ribs, are adapted to more than one purpose. The vast differences in shape, size, structure, and surface among the various bones are but the conditions that adapt them to particular forms of service in the body.

TABLE IV - THE PRINCIPAL BONES AND THEIR GROUPING IN THE BODY

I. AXIAL SKELETON

A. Skull, 28.

1. Cranium, 8.

a. Frontal, forehead 1 b. Parietal 2 c. Temporal, temple 2 d. Occipital 1 e. Sphenoid 1 f. Ethmoid 1

2. Face, 14.

a. Inferior maxillary 1 b. Superior maxillary 2 c. Palatine, palate 2 d. Nasal bones 2 e. Vomer 1 f. Inferior turbinated 2 g. Lachrymal 2 h. Malar, cheek bones 2

3. Bones of the Ears, 6.

a. Malleus 2 b. Incus 2 c. Stapes 2

B. Spinal Column, 26.

1. Cervical, or neck, vertebrae 7 2. Dorsal, or thoracic, vertebrae 12 3. Lumbar vertebrae 5 4. Sacrum 1 5. Coccyx 1

C. Thorax, 25.

1. Ribs 24 2. Sternum 1

D. Hyoid, 1 (at base of tongue).

II. APPENDICULAR SKELETON

A. Shoulder girdle 4.

1. Clavicle, collarbone. 2 2. Scapula, shoulder blade 2

B. Upper extremities, 60.

1. Humerus 2 2. Radius 2 3. Ulna 2 4. Carpal, wrist bones 16 5. Metacarpal 10 6. Phalanges of fingers 28

C. Pelvic girdle, 2.

1. Osinnominatum 2

D. Lower extremities, 60.

1. Femur, thigh bone 2 2. Tibia, shin bone 2 3. Fibula 2 4. Patella, kneepan 2 5. Tarsal, ankle bones 14 6. Metatarsal, instep bones 10 7. Phalanges of toes 28



ARTICULATIONS

Any place in the body where two or more bones meet is called an articulation, or joint. At the place of meeting the bones are firmly attached to each other, thereby securing the necessary coherence of the skeleton. The large number of bones, and consequently of articulations, are necessary for the different movements of the body and also on account of the manner in which the skeleton develops, or grows. Articulations are classed with reference to their freedom of motion, as movable, slightly movable, and immovable articulations.

Most of the immovable articulations are found in the skull. Here irregular, tooth-like projections from the different bones enable them to interlock with one another, while they are held firmly together by a thin layer of connective tissue. The wavy lines formed by articulations of this kind are called sutures (Fig. 100).

The best examples of joints that are slightly, but not freely, movable are found in the front of the spinal column. The cartilaginous pads between the vertebrae permit, by their elasticity, of a slight bending of the column in different directions. These movements are caused, not by one bone gliding over another, but by compressions and extensions of the cartilage. Between the vertebrae in the back of the spinal column, however, there is a slight movement of the bone surfaces upon one another.

*Structure of the Movable Joints.*—By far the most numerous and important of the joints are those that are freely movable. Such joints are strongly constructed and endure great strain without dislocation, and yet their parts move over each other easily and without friction. The ends of the bones are usually enlarged and have specially formed projections or depressions which fit into corresponding depressions or elevations on the bones with which they articulate. In addition to this the articular surfaces are quite smooth and dense, having no Haversian canals, and they are covered with a layer of cartilage. Strong ligaments pass from one bone to the other to hold each in its place (A, Fig. 103). Some of these consist simply of bands, connecting the joint on its different sides, while others form continuous sheaths around the joint.

[Fig. 103]

Fig. 103—*Outside and inside view of knee joint.* 1. Tendons. 2. Ligaments. 3. Cartilage. 4. Space containing synovial fluid. This space is lined, except upon the articular surfaces, by the synovial membrane.

The interior of the joint, except where the bone surfaces rub upon each other, is covered with a serous lining, called the synovial membrane (B, Fig. 103). This secretes a thick, viscid liquid, the synovial fluid, which prevents friction. The synovial membrane does not cover the ends of the bones, but passes around the joint and connects with the bones at their edges so as to form a closed sac in which the fluid is retained.

*Kinds of Movable Joints.—*The different kinds of movable joints are the ball and socket joint, the hinge joint, the pivot joint, the condyloid joint, and the gliding joint. These are constructed and admit of motion, as follows:

1. In the ball and socket joint the ball-shaped end of one bone fits into a cup-shaped cavity in another bone, called the socket. The best examples of such joints are found at the hips and shoulders. The ball and socket joint admits of motion in all directions.

2. In the hinge joint the bones are grooved and fit together after the manner of a hinge. Hinge joints are found at the elbows and knees and also in the fingers. The hinge joint gives motion in but two directions—forward and backward.

3. A pivot joint is formed by the fitting of a pivot-like projection of one bone into a ring-like receptacle of a second bone, so that one, or the other, is free to turn. A good example of the pivot joint is found at the elbow, where the radius turns upon the humerus. Another example is the articulation of the atlas with the axis vertebra as already noted. The pivot joint admits of motion around an axis.

4. The condyloid joint is formed by the fitting of the ovoid (egg-shaped) end of one bone into an elliptical cavity of a second bone. Examples of condyloid joints are found at the knuckles and where the wrist bones articulate with the radius and ulna. They move easily in two directions, like hinge joints, and slightly in other directions.

5. Gliding joints are formed by the articulation of plain (almost flat) surfaces. Examples of gliding joints are found in the articulations between the bones of the wrist and those of the ankle. They are the simplest of the movable joints and are formed by one bone gliding, or slipping, upon the surface of another.

*The Machinery of the Body.*—A machine is a contrivance for directing energy in doing work. A sewing machine, for example, so directs the energy of the foot that it is made to sew. Through its construction the machine is able to produce just that form of motion needed for its work, and no other forms, so that energy is not wasted in the production of useless motion. The places in machines where parts rub or turn upon each other are called bearings, and extra precautions are taken in the construction and care of the bearings to prevent friction.

The body cannot properly be compared to any single machine, but must be looked upon as a complex organization which employs a number of different kinds of machines in carrying on its work. The majority of these machines are found in the skeleton. The bones are the parts that are moved, and the joints serve as bearings. Connected with the bones are the muscles that supply energy, and attached to the muscles are the nerves that control the motion. Other parts also are required for rendering the machines of the body effective in doing work. These are supplied by the tissues connected with the bones and the muscles.



HYGIENE OF THE SKELETON

Of chief concern in the hygiene of the skeleton is the proper adjustment of its parts. The efficiency of any of the body machines is impaired by lack of proper adjustment. Not only this, but because of the fact that the skeleton forms the groundwork of the whole body—muscles, blood vessels, nerves, everything in fact, being arranged with reference to it—any lack of proper adjustment of the bones interferes generally with the arrangement and work of tissues and organs. The displaced bones may even compress blood vessels and nerves and interfere, in this way, with the nourishment and control of organs remote from the places where the displacements occur. For these reasons the proper adjustment of the different parts of the skeleton supplies one of the essential conditions for preserving the health.

*Hygienic Importance of the Spinal Column.*—What has been said about the adjustment of the skeleton in general applies with particular force to the spinal column. The spinal column serves both as the central axis of the body and as the container of the spinal cord. Thirty-one pairs of nerves pass between the vertebrae to connect the spinal cord with different parts of the body, and two important arteries (the vertebral) pass through a series of small openings in the bones of the neck to reach the brain. Unnatural curves of the spine throw different parts of the body out of their natural positions, diminish the thoracic and abdominal cavities, and, according to the belief of certain physicians, compress the nerves that pass from the cord to other parts of the body. Slightly misplaced vertebrae in the neck, by compressing the vertebral arteries, may also interfere with the supply of blood

[Fig. 104]

Fig. 104—A tendency toward spinal curvature (after Mosher)

[Fig. 105]

Fig. 105—Effect on spinal column of improper position in writing. (From Pyle's Personal Hygiene.)

*How the Skeleton becomes Deformed*—We are accustomed to look upon the skeleton as a rigid framework which can get out of its natural form only through severe strain or by violence. This view is far from being correct. On account of their necessary freedom of motion, the bones, especially those of the spinal column, are easily slipped from their normal positions; and where improper attitudes are frequently assumed, or continued through long periods of time, the skeleton gradually becomes deformed (Fig. 104). For example, the habit of always sleeping on the same side with a high pillow may develop a bad crook in the neck; and the ugly curves, assumed so frequently in writing (80) (Fig. 105), and also in standing, when the weight is shifted too much on one foot, may become permanent. Then the habit of reclining in a chair with the hips resting on the front of the seat often deforms the back and causes a drooping of the shoulders. In fact, slight displacements of the vertebrae come about so easily through incorrect positions, that they may almost be said to "occur of themselves" where active measures are not taken to preserve the natural form of the body. The very few people who have perfectly formed bodies show to what an extent has been overlooked an essential law of hygiene.

*Prevention of Skeletal Deformities.*—Those deformities of the skeleton that are acquired through improper positions are prevented by giving sufficient attention to the positions assumed in sitting, standing, and sleeping, and also to the posture in various kinds of work. In sitting the trunk should be erect and the hips should touch the back of the chair. One should not lounge in the ordinary chair. In standing the body should be erect, the shoulders back and down, the chest pushed slightly up and forward, and the chin slightly depressed, while the weight should, as a rule, rest about equally on the two feet. The habit of leaning against some object when standing (the pupil in reciting often leans on his desk) should be avoided. In sleeping the pillow should be of the right thickness to support the head on a level with the spinal column and should not be too soft. If one sleeps on his back, no pillow is required. It is best not to acquire the habit of sleeping always on the same side.

Where one is compelled by his work to assume harmful positions, these should be corrected by proper exercises, and by cultivating opposing positions during the leisure hours. Much is to be accomplished through those forms of physical exercise which develop the muscles whose work it is to keep the body in an upright position.

*School Furniture.*—It has long been observed that school children are more subject to curvature of the spine and other deformities of the skeleton than the children who do not attend school. While this is due largely to faulty positions assumed by the pupils at their work, it has been suggested that the school furniture may be in part to blame for these positions. Investigations of this problem have shown that most of the school desks and seats in use in our public schools are unhygienically constructed, in that they force pupils into unnatural positions. School seats should support the pupil in a natural position, both in the use of his books and in writing, and there are many arguments in favor of the so-called "adjustable" school furniture. Fig. 106 shows the seat and desk designed by the Boston, Mass., Schoolhouse Commission after much study and experimenting and used in the Boston schools. This furniture, which provides a seat adjustable for height, having a back rest also adjustable for height, and a desk which is likewise provided with a vertical adjustment, supplies all essential hygienic requirements. It is to be hoped that school furniture of this character may in the near future come into general use.

[Fig. 106]

Fig. 106—Adjustable seat and desk used in schools of Boston, Mass.

*Correction of Skeletal Deformities.*—It is, of course, easier to prevent deformities of the skeleton by giving attention to proper positions, than to correct them after they have occurred. It should also be noted that severe deformities cannot be corrected by the individual for himself, but these must come under the treatment of specialists in this line of medical work. In mild cases of spinal curvature, drooping of the head, and round shoulders, the individual can benefit his condition. By working to "substitute a correct attitude for the faulty one,"(81) he can by persistence bring about marked improvements. It is better, however, to have the advice and aid of a physical director, where this is possible. It should also be borne in mind that the correction of skeletal deformities requires effort through a long period of time, especially where the deformities are pronounced; and one lacking the will power to persist will not secure all the results which he seeks.

*"Setting Up" Exercises.*—The splendid carriage of students from military schools shows what may be accomplished in securing erectness of form where proper attention is given to this matter. The military student gets his fine form partly through his exercises in handling arms, but mainly through his so-called "setting up" drill. As a suggestion to one desiring to improve the form of his body, a modification of the usual "setting up" drill is here given:

1. Standing erect, with the heels together, the feet at an angle of 45 deg., and hands at the sides, bring the arms to a horizontal position in front, little fingers touching and nails down. From this position raise the hands straight over the head, bringing the palms gradually together. Then with a backward sweeping movement, return the hands again to the sides. Repeat several times.

2. With the feet as in the above exercise, bring the hands and the arms to a level with the shoulders, palms down, elbows bent, middle fingers of the two hands touching, and the extended thumbs touching the chest. Keeping the palms down and the arms on a level with the shoulders, extend the hands as far sideward and backward as possible, returning each time to the first position. As the hands move out, inhale deeply (through the nose), and as they are brought back, exhale quickly (through the mouth). Repeat several times.

3. With the arms at the sides and the feet side by side and touching, bring the hands in a circular movement to a vertical position over the head, and lock the thumbs. Keeping the knees straight and the thumbs locked, bend forward, letting the hands touch the ground if possible, and then bring the body and hands again to the vertical position. Then by a backward sweeping movement, return the hands again to the sides. Repeat.

While these exercises may be practiced whenever convenient, it is best to set apart some special time each day for them, as on retiring at night or on rising in the morning.

*Hygienic Footwear.*—A necessary aid to erectness of position in standing and walking is a properly fitting shoe. Heels that are too high tilt the body unnaturally forward, and shoes that cause any kind of discomfort in walking lead to unnatural positions in order to protect the feet. Shoes should fit snugly, being neither too large nor too small. Many shoes, however, are unhygienically constructed, and no attempt should be made to wear them. Certainly is this true of styles that approach the "French heel" or the "toothpick toe" (Fig. 107). However, many styles of shoes are manufactured that are both hygienic and neat fitting. Rubber heels, on account of their elasticity, are to be preferred to those made of leather.

[Fig. 107]

Fig. 107—Heels and toes of unhygienic and of hygienic footwear.

*The Skeleton in Childhood and Old Age.*—Certain peculiarities are found to exist in the bones of children and of old people which call for special care of the skeleton during the first and last periods of life. The bones of children are soft, lacking mineral matter, and are liable to become bent For this reason, children who are encouraged to walk at too early an age may bend the thigh bones, causing the too familiar "bow-legs." These bones may also be bent by having children sit on benches and chairs which are too high for the feet to reach the floor, and which do not provide supports for the feet. Wholesome food, fresh air, sunlight, and exercise are also necessary to the proper development of the bones of children. Where these natural conditions are lacking, as in the crowded districts of cities, children often suffer from a disease known as "rickets," on account of which their bones are unnaturally soft and easily bent.

On account of the accumulation of mineral matter, the bones of elderly people become brittle and are easily broken, and from lack of vigor of the bone cells they heal slowly after such injuries occur. This makes the breaking of a bone by an aged person a serious matter. Old people should, as far as possible, avoid liabilities to falls, such as going rapidly up and down stairs, or walking on icy sidewalks, and should use the utmost care in getting about. In old people also the cartilage between the bones softens, increasing the liability of getting misshaped. Special attention, therefore, should be given to erectness of form, and to such exercises as tend to preserve the natural shape of the body.

*Treatment of Fractures.*—A fractured bone always requires the aid of a surgeon, and no time should be lost in securing his services. In the meantime the patient should be put in a comfortable position, and the broken limb supported above the rest of the body. Though the breaking of a bone is not, as a rule, a serious mishap, it is necessary that the very best skill be employed in setting it. Any failure to bring the ends of the broken bone into their normal relations permanently deforms the limb and interferes with its use.

*Dislocations and Sprains.*—Dislocations, if they be of the larger joints, also require the aid of the surgeon in their reduction and sometimes in their subsequent treatment. Simple dislocations of the finger joints, however, may be reduced by pulling the parts until the bones can be slipped into position.

A sprain, which is an overstrained condition of the ligaments surrounding a joint, frequently requires very careful treatment. When the sprain is at all serious, a physician should be called. Because of the limited supply of blood to the ligaments, they are slow to heal, and the temptation to use the joint before it is fully recovered is always great. Massage(82) judiciously applied to a sprained joint, by bringing about a more rapid change in the blood and the lymph, is beneficial both in relieving the pain, and in hastening recovery.

*Summary.*—The skeleton, or framework of the body, is a structure which is movable as a whole and in most of its parts. It preserves the form of the body, protects important organs, and supplies the mechanical devices, or machines, upon which the muscles act in the production of motion. The skeleton is adapted to its purposes through the number and properties of the bones, and through the cartilage and connective tissue associated with the bones. The places where the different bones connect one with another are known as joints, and most of these admit of motion. The preservation of the natural form of the skeleton is necessary, both for its proper action and for the health of the body.

*Exercises.*—1. State the main purpose of the skeleton. What is the necessity for so many bones in its construction?

2. How may the per cent of animal and of mineral matter in a bone be determined?

3. What properties are given the bones by the animal matter? What by the mineral matter?

4. Locate the bone cells. What is their special function?

5. State the plan by which nourishment is supplied to the bone cells in different parts of the bone.

6. Give the uses of the periosteum.

7. State the purpose of the Haversian canals. Of the canaliculi.

8. Give functions of the spinal column.

9. Name the different materials used in the construction of a joint and the purpose served by each.

10. Name four mechanical devices, or machines, found in the skeleton and state the purpose served by each.

11. Name one or more of the body machines not located in the skeleton.

12. Of what advantage is the peculiar shape of the lower jaw? Of the ribs? Of the bones of the pelvic girdle?

13. State the importance of preserving the natural form of the skeleton. How are unnatural curves produced in the spinal column?

14. How may slight deformities of the skeleton be corrected?

15. What different systems are employed in the body in the production of motion? What is the special function of each?



PRACTICAL WORK

To obtain clear ideas of the form and functions of the bones, a careful examination of a prepared and mounted skeleton is necessary. Many of the bones, however, may be located and their general form made out from the living body. Bones of the lower animals may also be studied to advantage.

*Experiments to show the Composition of Bone.*—1. Examine a slender bone, like that in a chicken's leg. Note that it resists bending and is difficult to break. Note also that it is elastic—that, when slightly bent, it will spring back.

2. Soak such a bone over night in a mixture of one part hydrochloric acid and four parts water. Then ascertain by bending, stretching, and twisting what properties the bone has lost. The acid has dissolved out the mineral matter.

3. Burn a small piece of bone in a clear gas flame, or on a bed of coals, until it ceases to blaze and turns a white color. Can the bone now be bent or twisted? What properties has it lost and what retained? What substance has been removed from the bone by burning?

*Observation on the Gross Structure of Bone.*—1. Procure a long, dry bone. (One that has lain out in the field until it has bleached will answer the purpose excellently.) Test its hardness, strength, and stiffness. Saw it in two a third of the distance from one end, and saw the shorter piece in two lengthwise. Compare the structure at different places. Find rough elevations on the outside for the attachment of muscles, and small openings into the bone for the entrance of blood vessels and nerves. Make drawings to represent the sections.

2. Procure a fresh bone from the butcher shop. Note the difference between it and the dry bone. Examine the materials surrounding the sides and covering the ends of the bone. Saw through the enlarged portion at the end and examine the red marrow. Saw through the middle of the bone and observe the yellow marrow.

*To show the Minute Structure of the Bone.*—Prepare a section of bone for microscopic study as follows: With a jeweler's saw cut as thin a slice as possible. Place this upon a good-sized whetstone, not having too much grit, and keeping it wet rub it under the finger, or a piece of leather, until it is thin enough to let the light shine through. The section may then be washed and examined with the microscope. If the specimen is to be preserved for future study, it may be mounted in the usual way, but with hard balsam. Prepare and study both transverse and longitudinal sections, making drawings. The sections should be prepared from bones that are thoroughly dry but which have not begun to decay.

*To show the Structure of a Joint.*—Procure from a butcher the joint of some small animal (hog or sheep). Cut it open and locate the cartilage, synovial membrane, and ligaments. Observe the shape and surface of the rubbing parts and the strength of the ligaments.



CHAPTER XV - THE MUSCULAR SYSTEM

As already stated, the skeleton, the nervous system, and the muscular system are concerned in the production of motion. The skeleton and the nervous system, however, serve other purposes in the body, while the muscular system is devoted exclusively to the production of motion. For this reason it is looked upon as the special motor system. The muscular tissue is the most abundant of all the tissues, forming about 41 per cent of the weight of the body.

*Properties of Muscles.*—The ability of muscular tissue to produce motion depends primarily upon two properties—the property of irritability and the property of contractility. Irritability is that property of a substance which enables it to respond to a stimulus, or to act when acted upon. Contractility is the property which enables the muscle when stimulated to draw up, thereby becoming shorter and thicker (a condition called contraction), and when the stimulation ceases, to return to its former condition (of relaxation). The property of contractility enables the muscles to produce motion. Irritability is a condition necessary to their control in the body.

*Kinds of Muscular Tissue.*—Three kinds of muscular tissue are found in the body. These are known as the striated, or striped, muscular tissue; the non-striated, or plain, muscular tissue; and the muscular tissue of the heart. These are made up of different kinds of muscle cells and act in different ways to cause motion. The striated muscular tissue far exceeds the others in amount and forms all those muscles that can be felt from the surface of the body. The non-striated muscle is found in the walls of the food canal, blood vessels, air passages, and other tubes of the body; while the muscular tissue of the heart is confined entirely to that organ.

*Striated Muscle Cells.*—The cells of the striated muscles are slender, thread-like structures, having an average length of 1-1/2 inches (35 millimeters) and a diameter of about 1/400 of an inch (60 μ). Because of their great length they are called fibers, or fiber cells. They are marked by a number of dark, transverse bands, or stripes, called striations,(83) which seem to divide them into a number of sections, or disks (Fig. 108). A thin sac-like covering, called the sarcolemma, surrounds the entire cell and just beneath this are a number of nuclei.(84)

[Fig. 108]

Fig. 108—*A striated muscle cell* highly magnified, showing striations and nuclei. Attached to the cell is the termination of a nerve fiber.

Within the sarcolemma are minute fibrils and a semiliquid substance, called the sarcoplasm. At each end the cell tapers to a point from which the sarcolemma appears to continue as a fine thread, and this, by attaching itself to the inclosing sheath, holds the cell in place. Most of the muscle cells receive, at some portion of their length, the termination of a nerve fiber. This penetrates the sarcolemma and spreads out upon a kind of disk, having several nuclei, known as the end plate.

*The "Muscle-organ."*—We must distinguish between the term "muscle" as applied to the muscular tissue and the term as applied to a working group of muscular tissue, which is an organ. In the muscle, or muscle-organ, is found a definite grouping of muscle fibers such as will enable a large number of them to act together in the production of the same movement. An examination of one of the striated muscles shows the individual fibers to lie parallel in small bundles, each bundle being surrounded by a thin layer of connective tissue. (See Practical Work.) These small bundles are bound into larger ones by thicker sheaths and these in turn may be bound into bundles of still larger size (Fig. 109). The sheaths surrounding the fiber bundles are connected with one another and also with the outer covering of the muscle, known as

[Fig. 109]

Fig. 109—*Diagram* of a section of a muscle, showing the perimysium and the bundles of fiber cells.

[Fig. 110]

Fig. 110—*A muscle-organ in position.* The tendons connect at one end with the bones and at the other end with the fiber cells and perimysium. (See text.)

*The Perimysium.*—The plan of the muscle-organ is revealed through a study of the perimysium. This is not limited to the surface of the muscle, as the name suggests, but properly includes the sheaths that surround the bundles of fibers. Furthermore, the surface perimysium and that within the muscle are both continuous with the strong, white cords, called tendons, that connect the muscles with the bones. By uniting with the bone at one end and blending with the perimysium and fiber bundles at the other, the tendon forms a very secure attachment for the muscle. The perimysium and the tendon are thus the means through which the fiber cells in any muscle-organ are made to pull together upon the same part of the body (Fig. 110).

*Purpose of Striated Muscles.*—The striated muscles, by their attachments to the bones, supply motion to all the mechanical devices, or machines, located in the skeleton. Through them the body is moved from place to place and all the external organs are supplied with such motion as they require. Because of the attachment of the striated muscles to the skeleton, and their action upon it, they are called skeletal muscles. As most of them are under the control of the will, they are also called voluntary muscles. They are of special value in adapting the body to its surroundings.

*Structure of the Non-striated Muscles.*—The cells of the non-striated muscles differ from those of the striated muscles in being decidedly spindle-shaped and in having but a single well-defined nucleus (Fig. 111). Furthermore, they have no striations, and their connection with the nerve fibers is less marked. They are also much smaller than the striated cells, being less than one one-hundredth of an inch in length and one three-thousandth of an inch in diameter.

In the formation of the non-striated muscles, the cells are attached to one another by a kind of muscle cement to form thin sheets or slender bundles. These differ from the striated muscles in several particulars. They are of a pale, whitish color, and they have no tendons. Instead of being attached to the bones, they usually form a distinct layer in the walls of small cavities or of tubes (Fig. 111). Since they are controlled by the part of the nervous system which acts independently of the will, they are said to be involuntary. They contract and relax slowly.

[Fig. 111]

Fig. 111—*Non-striated muscle cells.* A. Cross section of small artery magnified, showing (1) the layer of non-striated cells. B. Three non-striated cells highly magnified.

*Work of the Non-striated Muscles.*—The work of the non-striated muscles, both in purpose and in method, is radically different from that of the striated. They do not change the position of parts of the body, as do the striated muscles, but they alter the size and shape of the parts which they surround. Their purpose, as a rule, is to move, or control the movement of, materials within cavities and tubes, and they do this by means of the pressure which they exert. Examples of their action have already been studied in the propulsion of the food through the alimentary canal and in the regulation of the flow of blood through the arteries (pages 159 and 49). While they do not contract so quickly, nor with such great force as the striated muscles, their work is more closely related to the vital processes.

*Structure of the Heart Muscle.*—The cells of the heart combine the structure and properties of the striated and the non-striated muscle cells, and form an intermediate type between the two. They are cross-striped like the striated cells, and are nearly as wide, but are rather short (Fig. 112). Each cell has a well-defined nucleus, but the sarcolemma is absent. They are placed end to end to form fibers, and many of the cells have branches by which they are united to the cells in neighboring fibers. In this way they interlace more or less with each other, but are also cemented together. They contract quickly and with great force, but are not under control of the will. Muscular tissue of this variety seems excellently adapted to the work of the heart.

[Fig. 112]

Fig. 112—*Muscle cells from the heart*, highly magnified (after Schaefer).

*The Muscular Stimulus.*—The inactive, or resting, condition of a muscle is that of relaxation. It does work through contracting. It becomes active, or contracts, only when it is being acted upon by some force outside of itself, and it relaxes again when this force is withdrawn. Any kind of force which, by acting on muscles, causes them to contract, is called a muscular stimulus. Electricity, chemicals of different kinds, and mechanical force may be so applied to the muscles as to cause them to contract. These are artificial stimuli. So far as known, muscles are stimulated naturally in but one way. This is through the nervous system. The nervous system supplies a stimulus called the nervous impulse, which reaches the muscles by the nerves, causing them to contract. By means of nervous impulses, all of the muscles (both voluntary and involuntary) are made to contract as the needs of the body for motion require.

*Energy Transformation in the Muscle.*—The muscle serves as a kind of engine, doing work by the transformation of potential into kinetic energy. Evidences of this are found in the changes that accompany contraction. Careful study shows that during any period of contraction oxygen and food materials are consumed, waste products, such as carbon dioxide, are produced, and heat is liberated. Furthermore, the blood supply to the muscle is such that the materials for providing energy may be carried rapidly to it and the products of oxidation as rapidly removed. Blood vessels penetrate the muscles in all directions and the capillaries lie very near the individual cells (Fig. 113). Provision is made also, through the nervous system, for increasing the blood supply when the muscle is at work. From these facts, as well as from the great force with which the muscle contracts, one must conclude that the muscle is a transformer of energy—that within its protoplasm, chemical changes take place whereby the potential energy of oxygen and food is converted into the kinetic energy of motion.

[Fig. 113]

Fig. 113—*Capillaries* of muscles.

*Plan of Using Muscular Force.*—Two difficulties have to be overcome in the using of muscular force in the body. The first of these is due to the fact that the muscles exert their force only when they contract. They can pull but not push. Hence, in order to bring about the opposing movements(85) of the body, each muscle must work against some force that produces a result directly opposite to that which the muscle produces. Some of the muscles (those of breathing) work against the elasticity of certain parts of the body; others (those that hold the body in an upright position), to some extent against gravity; and others (the non-striated muscle in arteries), against pressure. But in most cases, muscles work against muscles.

[Fig. 114]

Fig. 114—*The muscle pair* that operates the forearm. For names of these muscles, see Fig. 119.

The striated, or skeletal, muscles are nearly all arranged after the last-named plan. As a rule a pair of muscles is so placed, with reference to a joint, that one moves the part in one direction, and the other moves it in the opposite direction. From the kinds of motion which the various muscle pairs produce, they are classified as follows:

1. Flexors and Extensors.—The flexor muscles bend and the extensors straighten joints (Fig. 114).

2. Adductors and Abductors.—The adductors draw the limbs into positions parallel with the axis of the body and the abductors draw them away.

3. Rotators (two kinds).—The rotators are attached about pivot joints and bring about twisting movements.

4. Radiating and Sphincter Muscles. —The radiating muscles open and the sphincter muscles close the natural openings of the body, such as the mouth.

The pupil should locate examples of the different kinds of muscle pairs in his own body.

*Exchange of Muscular Force for Motion.*—The second difficulty to be overcome in the use of muscular force in the body is due to the fact that the muscles contract through short distances, while it is necessary for most of them to move portions of the body through long distances. It may be easily shown that the longest muscles of the body do not shorten more than three or four inches during contraction. To bring about the required movements of the body, which in some instances amount to four or five feet, requires that a large proportion of the muscular force be exchanged for motion. The machines of the skeleton, while providing for motion in definite directions, also provide the means whereby strong forces, acting through short distances, are made to produce movements of less force, through long distances. The mechanical device employed for this purpose is known as

*The Lever.*—The lever may be described as a stiff bar which turns about a fixed point of support, called the fulcrum. The force applied to the bar to make it turn is called the power, and that which is lifted or moved is termed the weight. The weight, the power, and the fulcrum may occupy different positions along the bar and this gives rise to the three kinds of levers, known as levers of the first class, the second class, and the third class (Fig. 115). In levers of the first class the fulcrum occupies a position somewhere between the power and the weight. In the second class the weight is between the fulcrum and the power. In the third class the power is between the fulcrum and the weight.

[Fig. 115]

Fig. 115—*Classes of levers. I.* Two levers of first class showing fulcrums in different positions. II. Lever of second class. III. Lever of third class. F. Fulcrum. P. Power. W. Weight. a. Power-arm. b. Weight-arm.

*Application to the Body.*—In the body the bones serve as levers; the turning points, or fulcrums, are found at the joints; the muscles supply the power; and parts of the body, or things to be lifted, serve as weights. For these levers to increase the motion of the muscles, it is necessary that the muscles be attached to the bones near the joints, and that the parts to be moved be located at some distance from the joints. In other words the (muscle) power-arm must be shorter than the (body) weight-arm.(86)

Examining Fig. 116, it is seen that the distances moved by the power and weight vary as their respective distances from the fulcrum. That is to say, if the weight is twice as far from the fulcrum as the power, it will move through twice the distance, and if three times as far, through three times the distance. Thus the muscles, by acting through short distances (on the short arms of levers), are able to move portions of the body (located on the long arms) through long distances. Can all three classes of levers be used in this way in the body?

[Fig. 116]

Fig. 116—*Motion producing levers.* Diagrams show relative distances moved by the power and weight in levers having the power nearer the fulcrum than is the weight. F. Fulcrum. P, P'. Power. W, W'. Weight.

*Classes of Levers found in the Body.*—Practically all of the levers of the body belong either to the first class or the third class. In both of these the muscle power can be applied to the short arm of the lever, thereby moving the body weight through a longer distance than the muscle contracts (Fig. 116). In the levers of the second class, however, the weight occupies this position, being situated between the power and fulcrum (Fig. 117). The weight, therefore, cannot move farther than the power in this lever. It must always move a shorter distance. While such a lever is of great advantage in lifting heavy weights outside of the body, it cannot be used for increasing the motion of the muscles. For this reason no well-defined levers of the second class are present in the body.(87)

[Fig. 117]

Fig. 117—*Weight lifting levers.* Diagrams show relative distances moved by the power and weight in levers having the weight nearer the fulcrum than is the power. F. Fulcrum. P, P'. Power. W, W'. Weight.

[Fig. 118]

Fig. 118—*Diagram of the foot lever.* F. Fulcrum at ankle joint. W. Body weight expressed as pressure against the earth. While the muscle power acts through the distance ab, the fulcrum support (body) is forced through the distance FE.

*Loss of Muscular Force.*—Using a small spring balance for measuring the power, a light stick for a lever, and a small piece of metal for a weight, and arranging these to represent some lever of the body (as the forearm), it is easily shown that the gain in motion causes a corresponding loss in muscular power. (See Practical Work.) If, for example, the balance is attached two inches from the fulcrum and the weight twelve inches, the pull on the balance is found to be six times greater than the weight that is being lifted. If other positions are tried, it is found that the power exerted in each case is as many times greater than the weight as the weight-arm is times longer than the power-arm.

Applying this principle to the levers of the body, it is seen that the gain in motion is at the expense of muscular force, or, as we say, muscular force is exchanged for motion. This exchange is greatly to the advantage of the body; for while the ability to lift heavy weights is important, the ability to move portions of the body rapidly and through long distances is much more to be desired.

*Important Muscles.*—There are about five hundred separate muscles in the body. These vary in size, shape, and plan of attachment, to suit their special work. Some of those that are prominent enough to be felt at the surface are as follows:

Of the head: The temporal, in the temple, and the masseter, in the cheek. These muscles are attached to the lower jaw and are the chief muscles of mastication.

Of the neck: The sterno-mastoids, which pass between the mastoid processes, back of the ears, and the upper end of the sternum. They assist in turning the head and may be felt at the sides of the neck (Fig. 119).

Of the upper arm: The biceps on the front side, the triceps behind, and the deltoid at the upper part of the arm beyond the projection of the shoulder.

[Fig. 119]

Fig. 119—Back and front views of important muscles.

Of the forearm: The flexors of the fingers, on the front side, and the extensors of the fingers, on the back of the forearm (Fig. 119).

Of the hand: The adductor pollicis between the thumb and the palm.

Of the trunk: The pectoralis major, between the upper front part of the thorax and the shoulder; the trapezius, between the back of the shoulders and the spine; the rectus abdominis, passing over the abdomen from above downward; and the erector spinae, found in the small of the back.

Of the hips: The glutens maximus, fastened between the lower back part of the hips and the upper part of the femur.

Of the upper part of the leg: The rectus femoris, the large muscle on the front of the leg which connects at the lower end with the kneepan.

Of the lower leg: The tibialis anticus on the front side, exterior to the tibia, and the gastrocnemius, the large muscle in the calf of the leg. This is the largest muscle of the body, and is connected with the heel bone by the tendon of Achilles (Fig. 119).

The use of these muscles is, in most instances, easily determined by observing the results of their contraction.



HYGIENE OF THE MUSCLES

The hygiene of the muscles is almost expressed by the one word exercise. It is a matter of everyday knowledge that the muscles are developed and strengthened by use, and that they become weak, soft, and flabby by disuse. The effects of exercise are, however, not limited to the large muscles attached to the skeleton, but are apparent also upon the involuntary muscles, whose work is so closely related to the vital processes. While it is true that exercise cannot be applied directly to the involuntary muscles, it is also true that exercise of the voluntary muscles causes a greater activity on the part of those that are involuntary and is indirectly a means of exercising them.

*Exercise and Health.*—In addition to its effects upon the muscles themselves, exercise is recognized as one of the most fundamental factors in the preservation of the health. Practically every process of the body is stimulated and the body as a whole invigorated by exercise properly taken. On the other hand, a lack of exercise has an effect upon the entire body somewhat similar to that observed upon a single muscle. It becomes weak, lacks energy, and in many instances actually loses weight when exercise is omitted. This shows exercise to supply an actual need and to be in harmony with the nature and plan of the body.

*How Exercise benefits the Body.*—In accounting for the healthful effects of exercise, it must be borne in mind that the body is essentially a motion-producing structure. Furthermore, its plan is such that the movements of its different parts aid indirectly the vital processes. The student will recall instances of such aid, as, for example, the assistance rendered by muscular contractions in the circulation of the blood and lymph, due to the valves in veins and lymph vessels, and the assistance rendered by abdominal movements in the propulsion of materials through the food canal. A fact not as yet brought out, however, is that exercise stimulates nutritive changes in the cells, thereby imparting to them new vigor and vitality. While this effect of exercise cannot be fully accounted for, two conditions that undoubtedly influence it are the following:

1. Exercise causes the blood to circulate more rapidly.

2. Exercise increases the movement of the lymph through the lymph vessels.

The increase in the flow of the blood and the lymph causes changes to take place more rapidly in the liquids around the cells, thereby increasing the supply of food and oxygen, and hastening the removal of waste.

*One should plan for Exercise.*—Since exercise is demanded by the nature and plan of the body, to neglect it is a serious matter. People do not purposely omit exercise, but from lack of time or from its interference with the daily routine of duties, the needed amount is frequently not taken. Especially is this true of students and others who follow sedentary occupations. People of this class should plan for exercise as they plan for the other great needs of the body—food, sleep, clothing, etc. It is only by making a sufficient amount of muscular work or play a regular part of the daily program that the needs of the body for exercise are adequately supplied.

*Amount and Kind of Exercise.*—The amount of exercise required varies greatly with different individuals, and definite recommendations cannot be made. For each individual also the amount should vary with the physical condition and the other demands made upon the energy. One in health should exercise sufficiently to keep the muscles firm to the touch and the body in a vigorous condition.

Of the many forms of exercise from which one may choose, the question is again one of individual adaptability and convenience. While the different forms of exercise vary in their effects and may be made to serve different purposes, the consideration of these is beyond the scope of an elementary text. As a rule one will not go far wrong by following his inclinations, observing of course the conditions under which exercise is taken to the best advantage.

*General Rules for Healthful Exercise.*—That exercise may secure the best results from the standpoint of health, a number of conditions should be observed: 1. It should not be excessive or carried to the point of exhaustion. Severe physical exercise is destructive to both muscular and nervous tissues. 2. It should, if possible, be of an interesting nature and taken in the open air. 3. It should be counter-active, that is, calling into play those parts of the body that have not been used during the regular work.(88) 4. It should be directed toward the weak rather than toward the strong parts of the body. 5. When one is already tired from study, or other work, it should be taken with moderation or omitted for the time being. (For exercise of the heart muscle and the muscular coat of the blood vessels see pages 55 and 57.)

*Massage.*—In lieu of exercise taken in the usual way, similar effects are sometimes obtained by a systematic rubbing, pressing, stroking, or kneading of the skin and the muscles by one trained in the art. This process, known as massage, may be gentle or vigorous and is subject to a variety of modifications. Massage is applied when one is unable to take exercise, on account of disease or accident, and also in the treatment of certain bodily disorders. A weak ankle, wrist, or other part of the body, or even a bruise, may be greatly benefited by massage. The flow of blood and lymph is stimulated, causing new materials to be passed to the affected parts and waste materials to be removed. Massage, however, should never be applied to a boil, or other infected sore. The effect in this case would be to spread the infection and increase the trouble.

*Summary.*—Motion is provided for in the body mainly through the muscle cells. These are grouped into working parts, called muscles, which in turn are attached to the movable parts of the body. The striated muscles, as a rule, are attached to the mechanical devices found in the skeleton, and bring about the voluntary, movements. The non-striated muscles surround the parts on which they act, and produce involuntary movements. Both, however, are under the control of the nervous system. To bring about the opposing movements of the body, the striated muscles are arranged in pairs; and to increase their motion, the bones are used as levers. Physical exercise is necessary both for the development of the muscles and for the health and vigor of the entire body.

*Exercises.*—1. Compare the striated and non-striated muscles with reference to structure, location, and method of work.

2. In what respects is the muscular tissue of the heart like the striated, and in what respects like the non-striated, muscular tissue?

3. If muscles could push as well as pull, would so many be needed in the body? Why?

4. Locate muscles that work to some extent against elasticity and gravity.

5. Locate five muscles that act as flexors; five that act as extensors; two that act as adductors; and two as abductors. Locate sphincter and radiating muscles.

6. By what means does the nervous system control the muscles?

7. Give proofs of the change of potential into kinetic energy during muscular contraction.

8. Define the essential properties of muscular tissue and state the purpose served by each.

9. Describe a lever. For what general purpose are levers used in the body? What other purpose do they serve outside of the body?

10. Why are levers of the second class not adapted to the work of the body?

11. Name the class of lever used in bending the elbow; in straightening the elbow; in raising the knee; in elevating the toes; and in biting. Why is one able to bite harder with the back teeth than with the front ones when the same muscles are used in both cases?

12. Measure the distance from the middle of the palm of the hand to the center of the elbow joint. Find the attachment of the tendon of the biceps muscle to the radius and measure its distance to the center of the elbow joint. From these distances calculate the force with which the biceps contracts in order to support a weight of ten pounds on the palm of the hand.

13. How does exercise benefit the health? How does a short walk "clear the brain" and enable one to study to better advantage?

14. When exercisers taken for its effects upon the health, what conditions should be observed?



PRACTICAL WORK

The reddish muscle found in a piece of beef is a good example of striated muscle. The clear ring surrounding the intestine of a cat (shown by cross section) and the outer portion of the preparation from the cow's stomach, sold at the butcher shop under the name of tripe, are good examples of non-striated muscular tissue. The heart of any animal, of course, shows the heart muscle.

*To show the Structure of Striated Muscle.*—Boil a tough piece of beef, as a cut from the neck, until the connective tissue has thoroughly softened. Then with some pointed instrument, separate the main piece into its fiber bundles and these in turn into their smallest divisions. The smallest divisions obtainable are the muscle cells or fibers.

*To show Striated Fibers.*—Place a small muscle from the leg of a frog in a fifty-per-cent solution of alcohol and leave it there for half a day or longer. Then cover with water on a glass slide, and with a couple of fine needles tease out the small muscle threads. Protect with a cover glass and examine with a microscope, first with a low and then with a high power. The striations, sarcolemma, and sometimes the nuclei and nerve plates, may be distinguished in such a preparation.

*To show Non-striated Cells.*—Place a clean section of the small intestine of a cat in a mixture of one part of nitric acid and four parts of water and leave for four or five hours. Thoroughly wash out the acid with water and separate the muscular layer from the mucous membrane. Cover a small portion of the muscle with water on a glass slide and tease out, with needles, until it is as finely divided as possible. Examine with a microscope, first with a low and then with a high power. The cells appear as very fine, spindle-shaped bodies.

*To illustrate Muscular Stimulus and Contraction.*—Separate the muscles at the back of the thigh of a frog which has just been killed and draw the large sciatic nerve to the surface. Cut this as high up as possible and, with a sharp knife and a small pair of scissors, dissect it out to the knee. Now cut out entirely the large muscle of the calf of the leg (the gastrocnemius), but leave attached to it the nerve, the lower tendon, and the bones of the knee. Mount on an upright support, as shown in Fig. 120, and fasten the tendon to a lever below by a thread or small wire hook:

[Fig. 120]

Fig. 120—*Apparatus* for demonstrating properties of muscles.

1. Lay the nerve over the ends of the wires from a small battery which are attached to the support at A, and arrange a second break in the circuit at B. At this place the battery circuit is made and broken either by a telegraph key or by simply touching and separating the wires. Note that the muscle gives a single contraction, or twitch, both when the current is made and when it is broken.