The colon consists of four parts, described as the ascending colon, the transverse colon, the descending colon, and the sigmoid flexure, or sigmoid colon. The first three divisions are named from the direction of the movement of materials through them and the last from its shape, which is similar to that of the Greek letter sigma (Σ).

The rectum is the last division of the large intestine It is a nearly straight tube, from six to eight inches in length, and connects with the external surface of the body.

The general structure of the large intestine is similar to that of the small intestine, and, like the small intestine, it is held in place by the peritoneum. It differs from the small intestine, however, in its lining of mucous membrane and in the arrangement of the muscular coat. The mucous membrane presents a smooth appearance and has no villi, while the longitudinal layer of the muscular coat is limited to three narrow bands that extend along the greater length of the tube (Fig. 74). These bands are shorter than the coats, and draw the large intestine into a number of shallow pouches, by which it is readily distinguished from the small intestine (Fig. 71).

[Fig. 74]

Fig. 74—*Section of large intestine*, showing the coats. 1. Serous coat. 2. Circular layer of muscle. 3. Submucous coat. 4. Mucous membrane. 5. Muscular bands extending lengthwise over the intestine.

*Work of the Large Intestine.*—The large intestine serves as a receptacle for the materials from the small intestine. The digestive fluids from the small intestine continue their action here, and the dissolved materials also continue to be absorbed. In these respects the work of the large intestine is similar to that of the small intestine. It does, however, a work peculiar to itself in that it collects and retains undigested food particles, together with other wastes, and ejects them periodically from the canal.

*Work of the Alimentary Muscles.*—The mechanical part of digestion is performed by the muscles that encircle the food canal. Their uses, which have already been mentioned in connection with the different organs of digestion, may be here summarized: They supply the necessary force for masticating the food. They propel the food through the canal. They mix the food with the different juices. At certain places they partly or completely close the passage until a digestive process is completed. They may even cause a reverse movement of the food, as in vomiting. All of the alimentary muscles, except those around the mouth, are involuntary. Their work is of the greatest importance.

*Other Purposes of the Digestive Organs.*—The digestive organs serve other important purposes besides that of dissolving the foods. They provide favorable conditions for passing the dissolved material into the blood. They dispose of such portions of the foods as fail, in the digestive processes, to be reduced to a liquid state. A considerable amount of waste material is also separated from the blood by the glands of digestion (especially the liver), and this is passed from the body with the undigested portions of food. Then the food canal (stomach in particular) is a means of holding, or storing, food which is awaiting the processes of digestion. Considering the number of these purposes, the digestive organs are remarkably simple, both in structure and in method of operation.



HYGIENE OF DIGESTION

Many of the ills to which flesh is heir are due to improper methods of taking food and are cured by observing the simple rules of eating. Habit plays a large part in the process and children should, for this reason, be taught early to eat properly. Since the majority of the digestive processes are involuntary and the food, after being swallowed, is practically beyond control, careful attention must be given to the proper mastication of the food and to such other phases of digestion as are under control.

*Necessity for Thorough Mastication.*—Mastication prepares the food for the digestive processes which follow. Unless the food has been properly masticated, the digestive fluids in the stomach and intestines cannot act upon it to the best advantage. When the food is carefully chewed, a larger per cent of it is actually digested—a point of importance where economy in the use of food needs to be practiced.

A fact not to be overlooked is that one cannot eat hurriedly and practice thorough mastication. The food must not be swallowed in lumps, but reduced to a finely divided and pulpy mass. This requires time. The one who hurries through the meal is necessarily compelled to bolt his food. Thirty minutes is not too long to give to a meal, and a longer period is even better.

Perhaps the most important result of giving plenty of time to the taking of food is that of stimulating the digestive glands to a proper degree of activity. That both the salivary and gastric glands are excited by the sight, smell, and thought of food and, through taste, by the presence of food in the mouth, has been fully demonstrated. Food that is thoroughly masticated and relished will receive more saliva and gastric juice, and probably more of other juices, than if hastily chewed and swallowed. This has a most important bearing upon the efficiency of the digestive processes.

*Order of Taking Food.*—There has been evolved through experience a rather definite order of taking food, which our knowledge of the process of digestion seems to justify. The heavy foods (proteids for the most part) are eaten first; after which are taken starchy foods and fats; and the meal is finished off with sweetmeats and pastry.(64) The scientific arguments for this order are the following:

1. By receiving the first of the gastric flow the proteids can begin digesting without delay. Since these are the main substances acted on in the stomach, the time required for their digestion is shortened by eating them first.

2. Sugar, being of the nature of predigested starch, quickly gets into the blood and satisfies the relish for food. The result of taking sugar first may be to cause one to eat less than he needs and to diminish the activity of the glands.

3. Fat or grease, if taken first, tends to form a coating over the walls of the stomach and around the material to be digested. This prevents the juices from getting to and mixing with the foods upon which they are to act.

4. Starch following the proteids, for the most part, does not so quickly come in contact with the gastric juice. This enables the ptyalin of the saliva to continue its action for a longer time than if the starch were eaten first.

*Liquids during the Meal.*—Liquids as ordinarily taken during the meal are objectionable. They tend to diminish the secretion of the saliva and to cause rapid eating. Instead of eating slowly and swallowing the food only so fast as the glands can supply the necessary saliva, the liquid is used to wash the food down. Water or other drinks should be taken after the completion of the meal or when the mouth is completely free from food. Even then it should be taken in small sips. While the taking of a small amount of water in this way does no harm, a large volume has the effect of weakening the gastric juice. Most of the water needed by the body should be taken between meals.

*The State of Mind* has much to do with the proper digestion of the food. Worry, anger, fear, and other disturbed mental states are known to check the secretion of fluids and to interfere with the digestive processes. While the cultivation of cheerfulness is important for its general hygienic effects, it is of especial value in relation to digestion. Intense emotions, either during or following the meal, should if possible be avoided. The table is no place for settling difficulties or administering rebuke. The conversation, on the other hand, should be elevating and joy giving, thereby inducing a desirable reactionary influence upon the digestive processes.

*Care of the Teeth.*—The natural teeth are indispensable for the proper mastication of the food. Of especial value are the molars—the teeth that grind the food. The development of the profession of dentistry has made possible the preservation of the teeth, even when naturally poor, as long as one has need of them. To preserve the teeth they must be kept clean. They should be washed at least once a day with a soft-bristled brush, and small particles of food, lodged between them, should be removed with a wooden pick. The biting of hard substances, such as nuts, should be avoided, on account of the danger of breaking the enamel, although the chewing of tough substances is considered beneficial.

Decayed places in the teeth should be promptly filled by the dentist. It is well, even when decayed places are not known to exist, to have the teeth examined occasionally in order to detect such places before they become large. On account of the expense, pain, and inconvenience there is a tendency to put off dental work which one knows ought to be done. Perhaps in no other instance is procrastination so surely punished. The decayed places become larger and new points of decay are started; and the pain, inconvenience, and expense are increased proportionately.

*The Natural Appetite* should be followed with reference to both the kind and the amount of food eaten. No system of knowledge will ever be devised which can replace the appetite as an aid in the taking of food. It is nature's means of indicating the needs of the body. The natural appetite may be spoiled, however, by overeating and by the use of highly seasoned foods, or by indulging in stimulants during the meal. It is spoiled in children by too free indulgence in sweetmeats. By cultivating the natural appetite and heeding its suggestions, one has at his command an almost infallible guide in the taking of food.

*Preparation of Meals.*—The cooking of food serves three important purposes. It renders the food more digestible, relieving the organs of unnecessary work; it destroys bacteria that may be present in the food, diminishing the likelihood of introducing disease germs into the body; and it makes the food more palatable, thereby supplying a necessary stimulus to the digestive glands. While the methods employed in the preparation of the different foods have much to do with the ease with which they are digested and with their nourishing qualities, the scope of our subject does not permit of a consideration of these methods.

*Quantity of Food.*—Overeating and undereating are both objectionable from a hygienic standpoint. Overeating, by introducing an unnecessary amount of food into the body, overworks the organs of digestion and also the organs of excretion. It may also lead to the accumulation of burdensome fat and of harmful wastes. On the other hand, the taking of too little food impoverishes the blood and weakens the entire body. As a rule, however, more people eat too much than too little, and to quit eating before the appetite is fully satisfied is with many persons a necessary precaution. The power of self-control, valuable in all phases of life, is indispensable in the avoidance of overeating.

*Frequency of Taking Food.*—Eating between meals is manifestly an unhealthful practice. The question has also been raised as to whether the common habit of eating three times a day is best suited to all classes of people. Many people of weak digestive organs have been benefited by the plan of two meals a day, while others adopt the plan of eating one heavy meal and two light ones. Either plan gives the organs of digestion more time to rest and diminishes the liability of overeating. On the other hand, those doing heavy muscular work can hardly derive the energy which they need from less than three good meals a day. Though no definite rule can be laid down, there is involved a hygienic principle which all should follow: Meals should not overlap. The stomach should be free from food taken at a previous meal before more is introduced into it. When this principle is not observed, material ferments in the stomach, causing indigestion and other disorders. It should be noted, however, that the overlapping may be due to overeating as well as to eating too frequently.

*Dangers from Impure Food.*—Food is frequently the carrier of disease germs and for this reason requires close inspection (page 128). Typhoid fever, a most dangerous disease, is usually contracted through either impure food or impure water (Chapter XXIII). One safeguard against disease germs, as stated above, is thorough cooking. Too much care cannot be exercised with reference to the water for drinking purposes. Water which is not perfectly clear, which smells of decaying material, or which forms a sediment on standing is usually not fit to drink. It can, however, be rendered comparatively harmless by boiling. The objections which many people have to drinking boiled water are removed when it is boiled the day before it is used, so as to give it time to cool, settle, and replace the air driven off by the boiling.

*Care of the Bowels.*—In considering the hygiene of the alimentary canal, the fact that it is used as a means of separating the impurities from the body must not be overlooked. Frequently, through lack of exercise, negligence in evacuating the bowels, or other causes, a weakened condition of the canal is induced which results in the retention of impurities beyond the time when they should be discharged. This is a great annoyance and at the same time a menace to the health.

In most cases this condition can be relieved, and prevented from recurring, by observing the following habits: 1. Have a regular time each day for evacuating the bowels. This is a most important factor in securing the necessary movements. 2. Drink a cup of cold water on rising in the morning and on retiring at night. 3. Eat generously of fruits and other coarse foods, such as corn bread, oatmeal, hominy, cabbage, etc. 4. Practice persistently such exercises as bring the abdominal muscles into play. These exercises strengthen indirectly the muscles of the canal. 5. Avoid overwork, especially of the nervous system.

*Alcohol and Digestion.*—Though exciting temporarily a greater flow of the digestive fluids, alcoholic drinks taken in any but very small quantities are considered detrimental to the work of digestion. Large doses retard the action of enzymes, inflame the mucous lining of the stomach,(65) and bring about a diseased condition of the liver. It may be noted, however, that the bad effects of alcoholic beverages upon the stomach, the liver, and the body in general are less pronounced when these are taken as a part of the regular meals.

*Effects of Tea and Coffee.*—In addition to the stimulating agent caffeine, tea and coffee contain a bitter, astringent substance, known as tannin. On account of the tannin these beverages tend to retard digestion and to irritate the lining of the stomach—effects that may be largely obviated by methods of preparing tea and coffee which dissolve little of the tannin. (They should be made without continued boiling or steeping.) The caffeine may do harm through its stimulating effect upon the nervous system (page 56) and through the introduction of a special waste into the body. In chemical composition caffeine closely resembles a waste, called uric acid, and in the body is converted into this substance. If one is in a weakened condition, the uric acid may fail to be oxidized to urea, as occurs normally, or to be thrown off as uric acid. In this case it accumulates in the body, causing rheumatism and related diseases. It thus happens that while some people may use tea and coffee without detriment, others are injured by them.

*Summary.*—The main structure in the digestive system is the alimentary canal. This provides cavities where important dissolving processes take place, and tubes for joining these cavities, while glands connecting with the canal supply the necessary liquids for changing and dissolving the foods. The general plan of digestion is that of passing the food through the canal, beginning with the mouth, and of acting on it at various places, with the final result of reducing most of it to the liquid state. The digestive fluids supply water which acts as a solvent and carries the active chemical agents, or enzymes, that convert the insoluble foods into substances that are soluble. The muscles in the walls of the canal perform the mechanical work of digestion, while the nervous system controls and regulates the activity of the various organs concerned in this work.

Exercises.—1. State the general purpose of digestion. How does digested food differ from that not digested?

2. Name all the divisions of the alimentary canal in the order in which the food passes through them.

3. What other work besides digestion is carried on by the alimentary canal?

4. What is gained by the mastication of the food? Why should mastication precede the other processes of digestion?

5. What is the work of the tongue in digestion?

6. State the purposes served by the gastric juice.

7. Give reasons for regarding the small intestine as the most important division of the food canal.

8. At what places, and by the action of what liquids, are fats, proteids, and starch digested?

9. What enzymes are found in the pancreatic juice? What is the digestive action of each?

10. Describe the work performed by the muscles of the stomach, the mouth, the esophagus, and the small intestine.

11. What advantages are derived from the use of cooked food?

12. State the advantages of drinking pure water.

13. If all the food that one needs to take at a single meal can be thoroughly masticated in fifteen minutes, why is it better to spend a longer time at the table?

14. What is meant by the overlapping of meals? What bad results follow? How avoided?



PRACTICAL WORK

Examine a dissectible model of the human abdomen (Fig. 75), noting the form, location, and connection of the different organs. Find the connection of the esophagus with the stomach, of the stomach with the small intestine, and of the small intestine with the large intestine. Sketch a general outline of the cavity, and locate in this outline its chief organs.

Where it is desirable to learn something of the actual structure of the digestive organs, the dissection of the abdomen of some small animal is necessary. On account of unpleasant features likely to be associated with such a dissection, however, this work is not recommended for immature pupils.

[Fig. 75]

Fig. 75—Model for demonstrating the abdomen and its contents.

*Dissection of the Abdomen.* (Optional)—For individual study, or for a small class, a half-grown cat is perhaps the best available material. It should be killed with chloroform, and then stretched, back downward, on a board, the feet being secured to hold it in place.

The teacher should make a preliminary examination of the abdomen to see that it is in a fit condition for class study. If the bladder is unnaturally distended, its contents may be forced out by slight pressure. The following materials will be needed during the dissection, and should be kept near at hand: a sharp knife with a good point, a pair of heavy scissors, a vessel of water, some cotton or a damp sponge, and some fine cord. During the dissection the specimen should be kept as clean as possible, and any escaping blood should be mopped up with the cotton or the sponge. The dissection is best carried out by observing the following order:

1. Cut through the abdominal wall in the center of the triangular space where the ribs converge. From here cut a slit downward to the lower portion of the abdomen, and sideward as far as convenient. Tack the loosened abdominal walls to the board, and proceed to study the exposed parts. Observe the muscles in the abdominal walls, and the fold of the peritoneum which forms an apron-like covering over the intestines.

2. Observe the position of the stomach, liver, spleen, and intestines, and then, by pushing the intestines to one side, find the kidneys and the bladder.

3. Study the liver with reference to its location, size, shape, and color. On the under side, find the gall bladder, from which a small tube leads to the small intestine. Observe the portal vein as it passes into the liver. As the liver is filled with blood, neither it nor its connecting blood vessels should be cut at this time.

4. Trace out the continuity of the canal. Find the esophagus where it penetrates the diaphragm and joins the stomach. Find next the union of the stomach with the small intestine. Then, by carefully following the coils of the small intestine, discover its union with the large intestine.

5. Within the first coil of the small intestine, as it leaves the stomach, find the pancreas. Note its color, size, and branches. Find its connection with the small intestine.

6. Beginning at the cut portion of the abdominal wall, lift the thin lining of the peritoneum and carefully follow it toward the back and central portion of the abdomen. Observe whether it extends back of or in front of the kidneys, the aorta, and the inferior vena cava. Find where it leaves the wall as a double membrane, the mesentery, which surrounds and holds in place the large and small intestines. Sketch a coil of the intestine, showing the mesentery.

7. Find in the center of the coils of small intestine a long, slender body having the appearance of a gland. This is the beginning of the thoracic duct and is called the receptacle of the chyle. From this the thoracic duct rapidly narrows until it forms a tiny tube difficult to trace in a small animal.

8. Cut away about two inches of the small intestine from the remainder, having first tied the tube on the two sides of the section removed. Split it open for a part of its length, and wash out its contents. Observe its coats. Place it in a shallow vessel containing water, and examine the mucous membrane with a lens to find the villi. Make a drawing of this section, showing the coats.

9. Study the connection of the small intestine with the large. Split them open at the place of union, wash out the contents, and examine the ileo-caecal valve.

10. Observe the size, shape, and position of the kidneys. Do they lie in front of or back of the peritoneum? Do they lie exactly opposite each other? Note the connection of each kidney with the aorta and the inferior vena cava by the renal artery and the renal vein. Find a slender tube, the ureter, running from each kidney to the bladder. Do the ureters connect with the top or with the base of the bladder? Show by a sketch the connection of the kidneys with the large blood vessels and the bladder.

*To demonstrate the Teeth.*—Procure from the dentist a collection of different kinds of teeth, both sound and decayed.

(a) Examine external surfaces of different kinds of teeth, noting general shape, cutting or grinding surfaces, etc. Make a drawing of an incisor and also of a molar.

(b) After soaking some of the teeth for a couple of days in warm water saw one of them in two lengthwise, and another in two crosswise, and smooth the cut surfaces with fine emery or sand paper. Examine both kinds of sections, noting arrangement and extent of dentine, enamel, and pulp. Make drawings.

(c) Examine a decayed tooth. Which substance of the tooth appears to decay most readily? Why is it necessary to cut away a part of the tooth before filling?

(d) Test the effect of acids upon the teeth by leaving a tooth over night in a mixture of one part hydrochloric acid to four parts water, and by leaving a second tooth for a couple of days in strong vinegar. Examine the teeth exposed to the action of acids, noting results.

*To show the Importance of Mastication.*—Fill two tumblers each half full of water. Into one put a lump of rock salt. Into the other place an equal amount of salt that has been finely pulverized. Which dissolves first and why?

*To illustrate Acid and Alkaline Reactions.*—To a tumbler half full of water add a teaspoonful of hydrochloric or other acid, as vinegar. To a second tumbler half full of water add an equal amount of cooking soda. Taste each liquid, noting the sour taste of the acid, and the alkaline taste of the soda. Hold a piece of red litmus paper in the soda solution, noting that it is turned blue. Then hold a piece of blue litmus paper in the acid solution, noting that it is turned red. Add acid to the soda solution, and soda to the acid solution, until the conditions are reversed, testing with the red and blue litmus papers.

Hold, for a minute or longer, a narrow strip of red litmus paper in the mouth, noting any change in the color of the paper. Repeat, using blue litmus paper. What effect, if any, has the saliva upon the color of the papers? Has the mouth an acid or an alkaline reaction?

*To show the Action of Saliva on Starch.*—1 (Optional). Prepare starch paste by mixing half a teaspoonful of starch in half a pint of water and heating the mixture to boiling. Place some of this in a test tube and thin it by adding more water. Then add a small drop of iodine solution (page 136) to the solution of starch. It should turn a deep blue color. This is the test for starch.

Now collect from the mouth, in a clean test tube, two or three teaspoonfuls of saliva. Add portions of this to small amounts of fresh starch solution in two test tubes. Let the tubes stand for five or ten minutes surrounded by water having about the temperature of the body. Test for changes that have occurred as follows:

(a) To one tube add a little of the iodine solution. If it does not turn blue, it shows that the starch has been converted into some other substance by the saliva, (b) To the other tube add a few drops of a very dilute solution of copper sulphate. Then add sodium (or potassium) hydroxide, a few drops at a time, until the precipitate which first forms dissolves and turns a deep blue. Then gradually heat the upper portion of the liquid to boiling. If it turns an orange or yellowish red color, the presence of a form of sugar (maltose or dextrose) is proved. See page 136.

2. Hold some powdered starch in the mouth until it completely dissolves and observe that it gradually acquires a sweetish taste. This shows the change of starch into sugar.

*To illustrate the Action of the Gastric Juice.*—Add to a tumbler two thirds full of water as much scale pepsin (obtained from a drug store) as will stay on the end of the large blade of a penknife. Then add enough hydrochloric acid to give a slightly sour taste. Place in the artificial gastric juice thus prepared some boiled white of egg which has been finely divided by pressing it through a piece of wire gauze. Also drop in a single large lump. Keep in a warm place (about the temperature of the body) for several hours or a day, examining from time to time. What is the general effect of the artificial gastric juice upon the egg?

*To illustrate Effect of Alcohol upon Gastric Digestion.*—Prepare a tumbler half full of artificial gastric juice as in the above experiment, and add 10 cubic centimeters of this to each of six clean test tubes bearing labels. To five of the tubes add alcohol from a burette as follows: (1) .5 c.c., (2) 1 c.c., (3) 1.5 c.c., (4) 2 c.c., and (5) 3 c.c., leaving one tube without alcohol. Now add to each tube about 1/4 gram of finely divided white of egg from the experiment above, and place all of the tubes in a beaker half full of water. Keep the water a little above the temperature of the body for several hours, examining the tubes at intervals to note the progress of digestion. Inferences.



CHAPTER XI - ABSORPTION, STORAGE, AND ASSIMILATION

The dissolved nutrients, to reach the cells, must be transferred from the alimentary canal to the blood stream. This process is known as absorption. In general, absorption means the penetration of a liquid into the pores of a solid, and takes place according to the simple laws of molecular movements. The absorption of food is, however, not a simple process, and the passage takes place through an active (living) membrane. Another difference is that certain foods undergo chemical change while being absorbed.

*Small Intestine as an Organ of Absorption.*—While absorption may occur to a greater or less extent along the entire length of the alimentary canal, most of it takes place at the small intestine. Its great length, its small diameter, and its numerous blood vessels all adapt the small intestine to the work of absorption. The transverse folds in the mucous membrane, by retarding the food in its passage and by increasing the absorbing surface, also aid in the process. But of greatest importance are the minute elevations that cover the surface of the mucous membrane, known as

*The Villi.*—Each single elevation, or villus, has a length of about one fiftieth of an inch and a diameter about half as great (A, Fig. 76), and contains the following essential parts:

1. An outer layer of epithelial cells, resting upon a connective tissue support.

2. A small lymph tube, called a lacteal, which occupies the center of the villus and connects at the base with other lymph tubes, also called lacteals (B, Fig. 76).

3. A network of capillaries.

The villi are structures especially adapted to the work of absorption, and they are found only in the small intestine. The mucous membrane in all parts of the canal, however, is capable of taking up some of the digested materials.

[Fig. 76]

Fig. 76—*The villi.* A. Diagram of a small section of mucous membrane of small intestine. 1. Villi. 2. Small glands, called crypts.

B. Diagram showing structure of villi. 1. Small artery. 2. Lacteal. 3. Villus showing termination of the lacteal. 4. Villus showing capillaries. 5. Villus showing both the lacteal and the capillaries. 6. Small vein. 7. Layer of epithelial cells.

*Work of Capillaries and Lacteals.*—The capillaries and lacteals act as receivers of material as it passes through the layer of epithelial cells covering the mucous membrane. The lacteals take up the digested fats,(66) and the capillaries receive all the other kinds of nutrients. These vessels do not, of course, retain the absorbed materials, but pass them on. Their final destination is the general circulation, which they reach by two well-defined channels, or routes.

*Routes to the Circulation.*—The two routes from the place of absorption to the general circulation are as follows:

1. Route taken by the Fat.—The fat is conveyed by the lacteals from the villi to the receptacle of the chyle. At this place it mingles with the lymph from the lower parts of the body, and with it passes through the thoracic duct to the left subclavian vein. Here it enters the general circulation. Thus, to reach the general circulation, the fat has to pass through the villi, the lacteals, the receptacle of the chyle, and the thoracic duct (Fig. 77). Its passage through these places, like the movements in all lymph vessels, is slow, and it is only gradually admitted to the blood stream.

[Fig. 77]

Fig. 77—*Diagram of routes* from food canal to general circulation. See text.

2. Route of All the Nutrients except Fat.—Water and salts and the digested proteids and carbohydrates, in passing into the capillaries, mix there with the blood. But this blood, instead of flowing directly to the heart, is passed through the portal vein to the liver, where it enters a second set of capillaries and is brought very near the liver cells. From the liver it is passed through the hepatic veins into the inferior vena cava, and by these it is emptied into the right auricle. This route then includes the capillaries in the mucous membrane of the stomach and intestines, the branches of the portal vein, the portal vein proper, the liver, and the hepatic veins (Fig. 77). In passing through the liver, a large portion of the food material is temporarily retained for a purpose and in a manner to be described later (page 177).

*Absorption Changes.*—During digestion the insoluble foods are converted into certain soluble materials, such as peptones, maltose, and glycerine,—the conversion being necessary to their solution. A natural supposition is that these materials enter and become a part of the blood, but examination shows them to be absent from this liquid. (See Composition of the Blood, page 30.) There are present in the blood, however, substances closely related to the peptones, maltose, glycerine, etc.; substances which have in fact been formed from them. During their transfer from the food canal, the dissolved nutrients undergo changes, giving rise to the materials in the blood. Thus are the serum albumin and serum globulin of the blood derived from the peptones and proteoses; the dextrose, from the maltose and other forms of sugar; and the fat droplets, from the glycerine, fatty acid, and soluble soap.

While considerable doubt exists as to the cause of these changes and as to the places also where some of them occur, their purpose is quite apparent. The materials forming the dissolved foods, although adapted to absorption, are not suited to the needs of the body, and if introduced in this form are likely to interfere with its work.(67) They are changed, therefore, into the forms which the body can use.

*A Second Purpose of Digestion.*—Comparing the digestive changes with those of absorption, it is found that they are of a directly opposite nature; that while digestion is a process of tearing down, or separating,—one which reduces the food to a more finely divided condition—there is in absorption a process of building up. From the comparatively simple compounds formed by digestion, there are formed during absorption the more complex compounds of the blood. The one exception is dextrose, which is a simple sugar; but even this is combined in the liver and the muscles to form the more complex compound known as glycogen. (See Methods of Storage, below.) These facts have suggested a second purpose of digestion—that of reducing foods to forms sufficiently simple to enable the body to construct out of them the more complex materials that it needs. Evidence that digestion serves such a purpose is found in the fact that both proteids and carbohydrates are reduced to a simpler form than is necessary for dissolving them.(68)

*The Storage of Nutriment.*—For some time after the taking of a meal, food materials are being absorbed more rapidly than they can be used by the cells. Following this is an interval when the body is taking no food, but during which the cells must be supplied with nourishment. It also happens that the total amount of food absorbed during a long interval may be in excess of the needs of the cells during that time; and it is always possible, as in disease, that the quantity absorbed is not equal to that consumed. To provide against emergencies, and to keep up a uniform supply of food to the cells, it is necessary that the body store up nutrients in excess of its needs.

*Methods of Storage.*—The general plan of storage varies with the different nutrients as follows:

1. The carbohydrates are stored in the form of glycogen. This, as already stated (page 120), is a substance closely resembling starch. It is stored in the cells of both the liver and the muscles, but mainly in the liver (Fig. 78). It is a chief function of the liver to collect the excess of dextrose from the blood passing through it, and to convert it into glycogen, which it then stores within its cells. It does not, however, separate all of the dextrose from the blood, a small amount being left for supplying the immediate needs of the tissues. As this is used, the glycogen in the liver is changed back to dextrose and, dissolving, again finds its way into the blood. In this way, the amount of dextrose in the blood is kept practically constant. The carbohydrates are stored also by converting them into fat.

[Fig. 78]

Fig. 78—*Liver cells* where is stored the glycogen. C. Capillaries.

[Fig. 79]

Fig. 79—*Stored-up fat.* The figure shows four connective tissue cells containing small particles of fat. 1. Nucleus. 2. Protoplasm. 3. Fat. 4. Connective tissue fibers.

2. The fat is stored for the most part in the connective tissue. Certain of the connective tissue cells have the property of taking fat from the blood and of depositing it within their inclosing membranes (Fig. 79). When this is done to excess, and the cells become filled with fat, they form the so-called adipose tissue. Most of this tissue is found under the skin, between the muscles, and among the organs occupying the abdominal cavity. If one readily takes on fat, it may also collect in the connective tissue around the heart. The stored-up fat is redissolved as needed, and enters the blood, where it again becomes available to the active cells.

3. The proteids form a part of all the tissues, and for this reason are stored in larger quantities than any of the other food substances. The large amount of proteid found in the blood may also be looked upon as storage material. The proteids in the various tissues are spoken of as tissue proteids, and those in the blood as circulating proteids. The proteids of the tissues serve the double purpose of forming a working part of the cell protoplasm, and of supplying reserve food material. That they are available for supplying energy, and are properly regarded as storage material, is shown by the rapid loss of proteid in starving animals. When the proteids are eaten in excess of the body's need for rebuilding the tissues, they are supposed to be broken up in such a manner as to form glycogen and fat, which may then be stored in ways already described.

*General Facts Relating to Storage.*—The form into which the food is converted for storage in the body is that of solids—the form that takes up the least amount of space. These solids are of such a nature that they can be changed back into their former condition and, by dissolving, reenter the blood.

Only energy-yielding foods are stored. Water and salts, though they may be absorbed in excess of the needs of the body, are not converted into other substances and stored away. Oxygen, as already stated (page 108), is not stored. The interval of storage may be long or short, depending upon the needs of the body. In the consumption of stored material the glycogen is used first, then as a rule the fat, and last of all the proteids.

*Storage in the Food Canal.*—Not until three or four hours have elapsed are all the nutrients, eaten at a single meal, digested and passed into the body proper. The undigested food is held in reserve, awaiting digestion, and is only gradually absorbed as this process takes place. It may properly, on this account, be regarded as stored material. That such storage is of advantage is shown by the observed fact that substances which digest quickly (sugar, dextrin, "predigested foods," etc.) do not supply the needs of the body so well as do substances which, like starch and proteids, digest slowly. Even substances digesting quite slowly (greasy foods and pastry), since they can be stored longer in the food canal, may be of real advantage where, from hard work or exposure, the body requires a large supply of energy for some time. These "stay by" the laborer, giving him strength after the more easily digested foods have been used up. Storage by the food canal is limited chiefly to the stomach.

*Regulation of the Food Supply to the Cells.*—The storage of food materials is made to serve a second purpose in the plan of the body which is even more important than that of supplying nourishment to the cells during the intervals when no food is being taken. It is largely the means whereby the rate of supply of materials to the cells is regulated. The cells obtain their materials from the lymph, and the lymph is supplied from the blood. Should food substances, such as sugar, increase in the blood beyond a low per cent, they are converted into a form, like glycogen, in which they are held in reserve, or, for the time being, placed beyond the reach of the cells. When, however, the supply is reduced, the stored-up materials reenter the blood and again become available to the cells. By this means their rate of supply to the cells is practically constant.

We are now in a position to understand why carbohydrates, fats, and proteids are so well adapted to the needs of the body, while other substances, like alcohol, which may also liberate energy, prove injurious. It is because foods are of such a chemical nature that they are adapted in all respects to the body plan of taking up and using materials, while the other substances are lacking in some particular.

[Fig. 80]

Fig. 80—*Diagrams illustrating the relation of nutrients* and the non-relation of these to alcohol. A. Inter-relation and convertibility of proteids, fats, and carbohydrates (after Hall).

B. Diagram showing disposition of alcohol if this substance is taken in quantity corresponding to that of the nutrients (F.M.W.). The alcohol thrown off as waste is unoxidized and yields no energy.

*Why Alcohol is not a Food.*—If the passage of alcohol through the body is followed, it is seen, in the first place, that it is a simple liquid and undergoes no digestive change; and in the second place, that it is rapidly absorbed from the stomach in both weak and concentrated solutions. This introduces it quickly into the blood, and once there, it diffuses rapidly into the lymph and then into the cells. Since the body cannot store alcohol or convert it into some nutrient that can be stored (Fig. 80), there is no way of regulating the amount that shall be present in the blood, or of supplying it to the cells as their needs require. They must take it in excess of their needs, regardless of the effect, at least until the organs of excretion can throw off the surplus as waste. Compared with proteid, carbohydrates, or fats, alcohol is an unmanageable substance in the body. Attempting to use it as a food is as foolish as trying to burn gasolene or kerosene in an ordinary wood stove. It may be done to a limited extent, but is an exceedingly hazardous experiment. Not being adapted to the body method of using materials, alcohol cannot be classed as a food.

*Assimilation.*—Digestion, absorption, circulation, and storage of foods are the processes that finally make them available to the cells in the different parts of the body. There still remains another process for these materials to undergo before they serve their final purposes. This last process, known as assimilation, is the appropriation of the food material by the cell protoplasm. In a sense the storage of fat by connective tissue cells and of glycogen by the liver cells is assimilation. The term is limited, however, to the disposition of material with reference to its final use. Whether all the materials used by the cells actually become a part of the protoplasm is not known. It is known, however, that the cells are the places where most of the oxidations of the body occur and that materials taking part in these oxidations must, at least, come in close contact with the protoplasm. Assimilation, then, is the last event in a series of processes by which oxygen, food materials, and cell protoplasm are brought into close and active relations. The steps leading up to assimilation are shown in Table II.

TABLE II. THE PASSAGE OF MATERIALS TO THE CELLS MATERIALS DIGESTION ABSORPTION ROUTE TO STORAGE CONDITION THE GENERAL IN THE CIRCULATION BLOOD Proteids Changed In passing Through the Become a As proteids into into the portal vein part of the in proteoses capillaries, to the protoplasm colloidal and the liver and of all the solution. peptones by proteoses from there cells. the action and through the of the peptones hepatic gastric and change into veins into pancreatic the the juices. proteids of inferior the blood. vena cava. Fat Changed In passing Through the As fat in Chiefly as into fatty into the lacteals to the cells minute oil acid, lacteals, the of droplets. glycerine, the thoracic collective and glycerine duct, by tissue. soluable unites with which it is soap by the the soluable emptied bile and soap and into the pancreatic fatty acid left juice. to form the subclavian oil droplets vein. of the blood. Starch Reduced to Enters the Through the As glycogen As dextrose some of the capillaries portal chiefly by in different as dextrose. vein, the liver, solution. forms of liver, but to some sugar, as hepatic extent by maltose, veins, into muscle dextrose, inferior cells. etc. vena cava. Water Undergoes Taken up by Both Is not As the no change. both the routes, but stored in water which lacteals and mostly by the sense serves as a capillaries, way of the that energy carrier of but to the liver. foods are. all the greater other extent by constituents the of the capilaries. blood. Common salt Undergoes Taken up by By way of Not stored. In solution. no change. the portal capillaries vein, without liver, and undergoing hepatic apparent veins into change. inferior vena cava. Oxygen Taken up by Already in Is not United with the the general stored. the capillaries circulation. hemoglobin at the and to a lungs. small extent in solution in the plasma.

*Tissue Enzymes.*—The important part played by enzymes in the digestion of the food has suggested other uses for them in the body. It has been recently shown that many of the chemical changes in the tissues are in all probability due to the presence of enzymes. An illustration of what a tissue enzyme may do is seen in the changes which fat undergoes. In order for the body to use up its reserve fat, it must be transferred from the connective tissue cells, where it is stored, to the cells of the active tissues where it is to be used. This requires that it be reduced to the form of a solution and that it reenter the blood. In other words, it must be redigested. For bringing about these changes a substance identical in function with the steapsin of the pancreatic juice has been shown to exist in several of the tissues.

Although this subject is still under investigation, it may be stated with certainty that there are present in the tissues, enzymes that change dextrose to glycogen and vice versa, that break down and build up the proteids, and that aid in the oxidations at the cells. The necessity for such enzymes is quite apparent.

*Summary.*—The digested nutrients are taken up by the capillaries and the lymph vessels and transferred by two routes to the circulation. In passing from the alimentary canal into the circulation the more important of the foods undergo changes which adapt them to the needs of the body. Since materials are absorbed more rapidly than they are used, means are provided for storing them and for supplying them to the cells as their needs require. Capability of storage is an essential quality of energy-yielding foods; and substances, such as alcohol, which lack this quality are not adapted to the needs of the body. For causing the chemical changes that occur in the storage of foods, as well as the oxidations at the cells, the presence of active agents, or enzymes, is necessary.

*Exercises.*—1. In what respects does the absorption of food materials from the alimentary canal differ from the absorption of a simple liquid by a solid?

2. In what different ways is the small intestine especially adapted to the work of absorption?

3. What are the parts of a villus? What are the lacteals? Account for the name.

4. What part is played by the capillaries and the lacteals in the work of absorption? How does their work differ?

5. What changes, if any, take place in water, common salt, fat, proteids, and carbohydrates during absorption?

6. What double purpose is served by the processes of digestion?

7. Trace the passage of proteids, fats, and carbohydrates from the small intestine into the general circulation.

8. What is the necessity for storing nutrients in the body? Why is it not also necessary to store up oxygen?

9. In what form and at what places is each of the principal nutrients stored?

10. How is the rate of supply of food to the cells regulated? Why is the body unable to regulate the supply of alcohol to the cells when this substance is taken?

11. Explain Fig. 80, page 181. What becomes of the alcohol if this is taken in any but very small quantities?

12. State the general purpose of enzymes in the body. Name the enzymes found in each of the digestive fluids. What ones are found in the tissues?



PRACTICAL WORK

Illustrate the ordinary meaning of the term "absorption" by bringing the end of a piece of crayon in contact with water, or a piece of blotting paper in contact with ink, noting the passage of the liquid into the crayon or the paper. Show how absorption from the food canal differs from this kind of absorption.

Show by a diagram similar to Fig. 77 the two routes by which the foods pass from the alimentary canal into the blood stream.



CHAPTER XII - ENERGY SUPPLY OF THE BODY

If one stops taking food, it becomes difficult after a time for him to move about and to keep warm. These results show that food has some relation to the energy of the body, for motion and heat are forms of energy. The relation of oxygen to the supply of energy has already been discussed (Chapter VIII). We are now to inquire more fully into the energy supply of the body, and to consider those conditions which make necessary the introduction of both food and oxygen for this purpose.

*Kinds of Bodily Energy.*—The healthy body has at any time a considerable amount of potential, or reserve, energy,—energy which it is not using at the time, but which it is able to use as its needs require. When put to use, this energy is converted into such forms of kinetic energy(69) as are indicated by the different kinds of bodily power. These are as follows:

1. Power of Motion.—The body can move itself from place to place and it can give motion to things about it.

2. Heat Power.—The body keeps itself warm and is able to communicate warmth to its surroundings.

3. Nervous Power.—Through the nervous system the body exercises the power of control over its different parts.

As motion, heat, and nervous power the body uses most of its energy.

*The Source of Bodily Energy.*—As already indicated, the energy of the body is supplied through the food and the oxygen. These contain energy in the potential form, which becomes kinetic (active) through their uniting with each other in the body. Somewhat as the power of the steam engine is derived from the combustion of fuel in the furnaces, the energy of the body is supplied through the oxidations at the cells. How the food and oxygen come to possess energy is seen by a study of the general methods by which energy is stored up and used.

[Fig. 81]

Fig. 81—*Simple device* for storing energy through gravity.

*Simple Methods of Storing Energy.*—Energy is stored by converting the kinetic into the potential form. Two of the simplest ways of doing this are the following:

1. Storing of Energy through Gravity.—On account of the attraction between the earth and all bodies upon the earth, the mere lifting of a weight puts it in a position where gravity can cause it to move (Fig. 81). As a consequence the raising of bodies above the earth's surface is a means of storing energy—the energy remaining stored until the bodies fall. As they fall, the stored-up (potential) energy becomes kinetic and can be made to do work.

2. Storing of Energy through Elasticity.—Energy is stored also by doing work in opposition to elasticity, as in bending a bow or in winding a clock spring. The bending, twisting, stretching, or compressing of elastic substances puts them in a condition of strain which causes them to exert a pressure (called elastic force) that tends to restore them to their former condition. Energy stored by this means becomes active as the distorted or compressed substance returns to its former shape or volume.

These simple methods of storing energy will serve to illustrate the general principles upon which such storage depends:

1. To store energy, energy must be expended, or work done.

2. The work must be against some force, such as gravity or elasticity, which can undo the work, i.e., bring about an effect opposite to that of the work.

3. The stored energy becomes active (kinetic) as the force through which the energy was stored undoes the work, or puts the substance upon which the work was done into its former condition (gravity causing bodies to fall, etc.).

These principles are further illustrated by the

*Storing of Energy through Chemical Means.*—A good example of storing energy by chemical means is that of decomposing water with electricity. If a current of electricity is passed through acidulated water in a suitable apparatus (Fig. 82), the water separates into its component gases, oxygen and hydrogen. These gases now have power (energy) which they did not possess before they were separated. The hydrogen will burn in the oxygen, giving heat; and if the two gases are mixed in the right proportions and then ignited, they explode with violence. This energy was derived from the electricity. It was stored by decomposing the water.

[Fig. 82]

Fig. 82—*Storing energy by chemical means.* Apparatus for decomposing water with electricity.

Energy is stored by chemical means by causing it to do work in opposition to the force of chemism, or chemical affinity. Instead of changing the form of bodies or moving them against gravity, it overcomes the force that causes atoms to unite and to hold together after they have united. Since in most cases the atoms on separating from any given combination unite at once to form other combinations, we may say that energy is stored when strong chemical combinations are broken up and weak ones formed. Energy stored by this means becomes active when the atoms of weak combinations unite to form combinations that are strong.(70)

*How Plants store the Sun's Energy.*—The earth's supply of energy comes from the sun. While much of this, after warming and lighting the earth's surface, is lost by radiation, a portion of it is stored up and retained. The sun's energy is stored both through the force of gravity(71) and by chemical means, the latter being the more important of the two methods. Plants supply the means for storing it chemically (Fig. 83). Attention has already been called to the fact (page 112) that growing plants are continually taking carbon dioxide into their leaves from the air. This they decompose, adding the carbon to compounds in their tissues and returning the oxygen to the air. It is found, however, that this process does not occur unless the plants are exposed to sunlight. The sunlight supplies the energy for overcoming the attraction between the atoms of oxygen and the atoms of carbon, while the plant itself serves as the instrument through which the sunlight acts. The energy for decomposing the carbon dioxide then comes from the sun, and through the decomposition of the carbon dioxide the sun's energy is stored—becomes potential. It remains stored until the carbon of the plant again unites with the oxygen of the air, as in combustion.

[Fig. 83]

Fig. 83—*Nature's device* for storing energy from the sun. See text.

*The Sun's Energy in Food and Oxygen.*—Food is derived directly or indirectly from plants and sustains the same relation to the oxygen of the air as do the plants themselves. (The elements in the food have an attraction for the oxygen, but are separated chemically from it.) On account of this relation they have potential energy—the energy derived through the plant from the sun. When a person eats the food and breathes the oxygen, this energy becomes the possession of the body. It is then converted into kinetic energy as the needs of the body require.

[Fig. 84]

Fig. 84—*Simple apparatus* for illustrating transformation of energy. Potential energy is converted into heat and heat into motion.

*From the Sun to the Cells.*—It thus appears that the body comes into possession of energy, and is able to use it, through a series of transferences and transformations that can be traced back to the sun.(72) Coming to the earth as kinetic energy, it is transformed into potential energy and stored in the compounds of plants and in the oxygen of the air. Through the food and the oxygen the potential energy is transferred to the cells of the body. Then by the uniting of the food and the oxygen at the cells (oxidation), the potential becomes kinetic energy and is used by the body in doing its work. The phrase "Child of the Sun" has sometimes been applied to man to express his dependence upon the sun for his supply of energy.

*Why Oxygen and Food are Both Necessary.*—The necessity for introducing both oxygen and food into the body for the purpose of supplying energy is now apparent. The energy which is used in the body is not the energy of food alone. Nor is it the energy of oxygen alone. It belongs to both. It is due to their attraction for each other and their condition of separation. It cannot, therefore, become kinetic except through their union. To introduce one of these substances into the body without the other, would neither introduce the energy nor set it free. They must both be introduced into the body and there caused to unite.

*Bodily Control of Energy.*—A fact of importance in the supply of energy to the body is that the rate of transformation (changing of potential to kinetic) is just sufficient for its needs. It is easily seen that too rapid or too slow a rate would prove injurious. The oxidations at the cells are, therefore, under such control that the quantity of kinetic energy supplied to the body as a whole, and to the different organs, is proportional to the work that is done. This is attained, in part at least, through the ability of the body to store up the food materials and hold them in reserve until they are to be oxidized (page 180).

*Animal Heat and Motion.*—Most of the body's energy is expended as heat in keeping warm. It is estimated that as much as five sixths of the whole amount is used in this way. The proportion, however, varies with different persons and is not constant in the same individual during different seasons of the year. This heat is used in keeping the body at that temperature which is best suited to carrying on the vital processes. All parts of the body, through oxidation, furnish heat. Active organs, however, such as the muscles, the brain, and the glands (especially the liver), furnish the larger share. The blood in its circulation serves as a heat distributer for the body and keeps the temperature about the same in all its parts (page 33).

Next to the production of heat, in the consumption of the body's energy, is the production of motion. This topic will be considered in the study of the muscular system (Chapter XV).

*Some Questions of Hygiene.*—The heat-producing capacity of the body sustains a very important relation to the general health. A sudden chill may result in a number of derangements and is supposed to be a predisposing cause of colds. One's capacity for producing heat may be so low that he is unable to respond to a sudden demand for heat, as in going from a warm room into a cold one. As a consequence, the body is unable to protect itself against unavoidable exposures.

Impairment of the heat-producing capacity is brought about in many ways. Several diseases do this directly, or indirectly, to quite an extent. In health too great care in protecting the body from cold is the most potent cause of its impairment. Staying in rooms heated above a temperature of 70 deg. F., wearing clothing unnecessarily heavy, and sleeping under an excess of bed clothes, all diminish the power of the body to produce heat. They accustom it to producing only a small amount, so that it does not receive sufficient of what might be called heat-producing exercise. Lack of physical exercise in the open air, as well as too much time spent in poorly lighted and ventilated rooms, tends also to reduce one's ability to produce heat. Moreover, since most of the heat of the body comes from the union of oxygen and food materials at the cells, a lack of either of these will interfere with the production of heat.

*Results of Exhaustion.*—Through overwork, or excesses in pleasurable pursuits, one may make greater demands upon the energy of his body than it can properly supply. The resulting condition, known as exhaustion, is not only a matter of temporary inconvenience, but may through repetition lead to a serious impairment of the health. It should be noted, in this connection, that the energy of the body is spent in two general ways: first, in carrying on the vital processes; and second, in the performance of voluntary activities. Since, in all cases, there is a limit to one's energy, it is easily possible to expend so much in the voluntary activities that the amount left is not sufficient for the vital processes. This leads to various disturbances and, among other things, renders the body less able to supply itself with energy.

*The Problem of Increasing One's Energy.*—Since the energy supply is kept up through the food and the oxygen, it might be inferred that the introduction of these substances into the body in larger amounts would increase the energy at one's disposal. This does not necessarily follow. Oxidation at the cells is preceded by digestion, absorption, circulation, and assimilation. It is followed and influenced by the removal of wastes from the body. A careful study of the problem leads to the conclusion that while the energy supply to the body does depend upon the introduction of the proper amounts of food and oxygen, it also depends upon the efficiency of the vital processes. The maximum amount of energy may, therefore, be expected when the body is in a condition of perfect health. Hence, one desiring to increase the amount of his energy must give attention to all those conditions that improve the health.

*Effect of Stimulants on the Energy Supply.*—In the effort to get out of the body as much as possible of work or of pleasure, various stimulants, such as alcohol, tobacco, and strong tea and coffee, have been used. Though these have the effect of giving a temporary feeling of strength and of enabling the individual in some instances to accomplish results which he could not otherwise have brought about, the general effect of their use is to lessen, rather than to increase, the sum total of bodily power. The student, for example, who drinks strong coffee in order to study late at night is able to command less energy on the day following. While enabling him to draw upon his reserve of nervous power for the time being, the coffee deprives him of sleep and needed rest.

The danger of stimulants, so far as energy is concerned, is this: they tend to exhaust the bodily reserve so that there is not sufficient left for properly running the vital processes. Evidences of their weakening effect are found in the feeling of discomfort and lassitude which result when stimulants to which the body has become accustomed are withdrawn. Not until one gets back his bodily reserve is he able to work normally and effectively. Increase in bodily energy comes through health and not through the use of stimulants.

*Summary.*—The body requires a continuous supply of energy. To obtain this supply, materials possessing potential, or stored-up, energy are introduced into it. The free oxygen of the air and the substances known as foods, on account of the chemical relations which they sustain to each other, contain potential energy and are utilized for supplying the body. So long as the foods are not oxidized, the energy remains in the potential form, but in the process of oxidation the potential energy is changed to kinetic energy and made to do the work of the body.

*Exercises.*—1. In what different ways does the body use energy?

2. Show that a stone lying against the earth has no energy, while the same stone above the earth has energy.

3. How does potential energy differ from kinetic energy?

4. What kind of energy is possessed by a bent bow? By a revolving wheel? By a coiled spring? By the wind? By gunpowder?

5. How does decomposing water with electricity store energy?

6. Account for the energy possessed by the oxygen of the air and food substances.

7. Trace the energy supply of the body back to the sun.

8. Why must both oxygen and food be introduced into the body in order to supply it with energy?

9. How may overwork and overexercise diminish the energy supply of the body?

10. How may one increase the amount of his energy?



PRACTICAL WORK

*Suggested Experiments.*—1. The change of kinetic into potential energy may be shown by stretching a piece of rubber, by lifting a weight, and by separating the armature from a magnet.

2. The change of potential into kinetic energy may be shown by letting weights fall to the ground, by releasing the end of a piece of stretched rubber, and by burning substances.

3. The change of one form of kinetic energy to another may be illustrated by rubbing together two pieces of wood until they are heated, by ringing a bell, and by causing motion in air or in water by heating them. If suitable apparatus is at hand, the transformation of electrical energy into heat, light, sound, and mechanical motion can easily be shown.

4. A weight connected by a cord with some small machine and made to run it, will help the pupil to grasp the general principles in the storage of energy through gravity. A vessel of water on a high support from which the water is siphoned on to a small water wheel will serve the same purpose.

5. The storing of energy by chemical means may be illustrated by decomposing potassium chlorate with heat or by decomposing water by means of a current of electricity.

6. Study the transfer of energy from the body to surrounding objects, as in moving substances and lifting weights.

Fill a half gallon jar two thirds full of water and carefully take the temperature with a chemical thermometer. Hold the hand in the water for four or five minutes and take the temperature again. Inference.



CHAPTER XIII - GLANDS AND THE WORK OF EXCRETION

In our study so far we have been concerned mainly with the introduction of materials into the body. We are now to consider the removal of materials from the body. The structures most directly concerned in this work are known as

*Glands.*—As generally understood, glands are organs that prepare special liquids in the body and pour them out upon free surfaces. These liquids, known as secretions, are used for protecting exposed parts, lubricating surfaces that rub against each other, digesting food, and for other purposes. They differ widely in properties as well as in function, but are all alike in being composed chiefly of water. The water, in addition to being necessary to the work of particular fluids, serves in all cases as a carrier of solid substances which are dissolved in it.

*General Structure of Glands.*—While the various glands differ greatly in size, form, and purpose, they present striking similarities in structure. All glands contain the following parts:

1. Gland, or secreting, cells. These are specialized cells for the work of secretion and are the active agents in the work of the gland. They are usually cubical in shape.

2. A basement membrane. This is a thin, connective tissue support upon which the secreting cells rest.

3. A network of capillary and lymph vessels. These penetrate the tissues immediately beneath the secreting cells.

4. A system of nerve fibers which terminate in the secreting cells and in the walls of the blood vessels passing to the glands.

These structures—secreting cells, basement membrane, capillary and lymph vessels, and nerve fibers—form the essential parts of all glands. The capillaries and the lymph vessels supply the secreting cells with fluid, and the nerves control their activities.

*Kinds of Glands.*—Glands differ from one another chiefly in the arrangement of their essential parts.(73) The most common plan is that of arranging the parts around a central cavity formed by the folding or pitting of an exposed surface. Many such glands are found in the mucous membrane, especially that lining the alimentary canal, and are most numerous in the stomach, where they supply the gastric juice. If these glands have the general form of tubes, they are called tubular glands; if sac-like in shape, they are called saccular glands. Both the tubular and the saccular glands may, by branching, form a great number of similar divisions which are connected with one another, and which communicate by a common opening with the place where the secretion is used. This forms a compound gland which, depending on the structure of the minute parts, may be either a compound tubular or a compound saccular gland. The larger of the compound saccular glands are also called racemose glands, on account of their having the general form of a cluster, or raceme, similar to that of a bunch of grapes. The general structure of the different kinds of glands is shown in Fig. 85.

[Fig. 85]

Fig. 85—*Diagram illustrating evolution of glands.* A. Simple secreting surface. 1. Gland cells. 2. Basement membrane. 3. Blood vessel. 4. Nerve. B. Simple tubular gland. C. Simple saccular gland. D. Compound tubular gland. E. Compound saccular gland. F. A compound racemose gland with duct passing to a free surface. G. Relation of food canal to different forms of glands. The serous coat has a secreting surface.

*Nature of the Secretory Process.*—At one time the gland was regarded merely as a kind of filter which separated from the blood the ingredients found in its secretions. Recent study, however, of several facts relating to secretion has led to important modifications of this view. The secretions of many glands are known to contain substances that are not found in the blood, or, if present, are there in exceedingly small amounts. Then again the cells of certain glands have been found to undergo marked changes during the process of secretion. If, for example, the cells of the pancreas be examined after a period of rest, they are found to contain small granular bodies. On the other hand, if they are examined after a period of activity, the granules have disappeared and the cells themselves have become smaller (Fig. 86). The granules have no doubt been used up in forming the secretion. These and other facts have led to the conclusion that secretion is, in part, the separation of materials without change from the blood, and, in part, a process by which special substances are prepared and added to the secretion. According to this view the gland plays the double role of a filtering apparatus and of a manufacturing organ.

[Fig. 86]

Fig. 86—*Secreting cells from the pancreas* (after Langley). A. After a period of rest. B. After a short period of activity. C. After a period of prolonged activity. In A and B the nuclei are concealed by the granules that accumulate during the resting period.

*Kinds of Secretion.*—In a general way all the liquids produced by glands may be considered as belonging to one or the other of two classes, known as the useful and the useless secretions. To the first class belong all the secretions that serve some purpose in the body, while the second includes all those liquids that are separated as waste from the blood. The first are usually called true secretions, or secretions proper, while the second are called excretions. The most important glands producing liquids of the first class are those of digestion (Chapter X).

*Excretory Work of Glands.*—The process of removing wastes from the body is called excretion. While in theory excretion may be regarded as a distinct physiological act, it is, in fact, leaving out the work of the lungs, but a phase of the work of glands. From the cells where they are formed, the waste materials pass into the lymph and from the lymph they find their way into the blood. They are removed from the blood by glands and then passed to the exterior of the body.

*The Necessity for Excretion* is found in the results attending oxidation and other chemical changes at the cells (page 107). Through these changes large quantities of materials are produced that can no longer take any part in the vital processes. They correspond to the ashes and gases of ordinary combustion and form wastes that must be removed. The most important of these substances, as already noted (page 110), are carbon dioxide, water, and urea.(74) A number of mineral salts are also to be included with the waste materials. Some of these are formed in the body, while others, like common salt, enter as a part of the food. They are solids, but, like the urea, leave the body dissolved in water.

Waste products, if left in the body, interfere with its work (some of, them being poisons), and if allowed to accumulate, cause death. Their removal, therefore, is as important as the introduction of food and oxygen into the body. The most important of the excretory glands are

*The Kidneys.*—The kidneys are two bean-shaped glands, situated in the back and upper portion of the abdominal cavity, one on each side of the spinal column. They weigh from four to six ounces each, and lie between the abdominal wall and the peritoneum. Two large arteries from the aorta, called the renal arteries, supply them with blood, and they are connected with the inferior vena cava by the renal veins. They remove from the blood an exceedingly complex liquid, called the urine, the principal constituents of which are water, salts of different kinds, coloring matter, and urea. The kidneys pass their secretion by two slender tubes, the ureters, to a reservoir called the bladder (Fig. 87).

[Fig. 87]

Fig. 87—*Relations of the kidneys.* (Back view.) 1. The kidneys. 2. Ureters. 3. Bladder. 4. Aorta. 5. Inferior vena cava. 6. Renal arteries. 7. Renal veins.

*Structure of the Kidneys.*—Each kidney is a compound tubular gland and is composed chiefly of the parts concerned in secretion. The ureter serves as a duct for removing the secretion, while the blood supplies the materials from which the secretion is formed. On making a longitudinal section of the kidney, the upper end of the ureter is found to expand into a basin-like enlargement which is embedded in the concave side of the kidney. The cavity within this enlargement is called the pelvis of the kidney, and into it project a number of cone-shaped elevations from the kidney substance, called the pyramids (Fig. 88).

From the summits of the pyramids extend great numbers of very small tubes which, by branching, penetrate to all parts of the kidneys. These are the uriniferous tubules, and they have their beginnings at the outer margin of the kidney in many small, rounded bodies called the Malpighian capsules (A, Fig. 88). Each capsule incloses a cluster of looped capillaries and connects with a single tubule (Fig. 89). From the capsule the tubule extends toward the concave side of the kidney and, after uniting with similar tubules from other parts, finally terminates at the pyramid. Between its origin and termination, however, are several convolutions and one or more loops or turns. After passing a distance many times greater than from the surface to the center of the kidney, the tubule empties its contents into the expanded portion of the ureter.

[Fig. 88]

Fig. 88—*Sectional view of kidney.* 1. Outer portion or cortex. 2. Medullary portion. 3. Pyramids. 4. Pelvis. 5. Ureter. A. Small section enlarged to show the tubules and their connection with the capsules.

[Fig. 89]

Fig. 89—*Malpighian capsule* highly magnified (Landois). a. Small artery entering capsule and forming cluster of capillaries within. e. Small vein leaving capsule and branching into c, a second set of capillaries, h. Beginning of uriniferous tubule.

The uriniferous tubules are lined with secreting cells. These differ greatly at different places, but they all rest upon a basement membrane and are well supplied with capillaries. These cells provide one means of separating wastes from the blood (Fig. 90).

[Fig. 90]

Fig. 90—*Diagram illustrating renal circulation.* 1. Branch from renal artery. 2. Branch from renal vein. 3. Small artery branches, one of which enters a Malpighian capsule (5). 6. Small vein leaving the capsule and branching into the capillaries (7) which surround the uriniferous tubules. 4. Small veins which receive blood from the second set of capillaries. 8. Tubule showing lining of secreting cells.

*Blood Supply to the Kidneys.*—The method by which the kidneys do their work is suggested by the way in which the blood circulates through them. The renal artery entering each kidney divides into four branches and these send smaller divisions to all parts of the kidney. At the outer margin of the kidney, called the cortex, the blood is passed through two sets of capillaries. The first forms the clusters in the Malpighian capsules and receives the blood directly from the smallest arteries. The second forms a network around the uriniferous tubules and receives the blood which has passed from the capillary clusters into a system of small veins (Fig. 90). From the last set of capillaries the blood is passed into veins which leave the kidneys where the artery branches enter, uniting there to form the main renal veins.

*Work of the Kidneys.*—Why should the blood pass through two systems of capillaries in the kidneys? This is because the separation of waste is done in part by the Malpighian capsules and in part by the uriniferous tubules. Water and salts are removed chiefly at the capsules, while the remaining solid constituents of the urine pass through the secreting cells that line the tubules. It was formerly believed that the kidneys obtained their secretion by a process of filtration from the blood, but this belief has been gradually modified. The prevailing view now is that the processes of filtration and secretion are both carried on by the kidneys,—that the capillary clusters in the Malpighian bodies serve as delicate filters for the separation of water and salts, while the secreting cells of the tubules separate substances by the process of secretion.

On account of the large volume of blood passing through the kidneys this liquid is still a bright red color as it flows into the renal veins (Fig. 90). The kidney cells require oxygen, but the amount which they remove from the blood is not sufficient to affect its color noticeably. The blood in the renal veins, having given up most of its impurities and still retaining its oxygen, is considered the purest blood in the body.

*Urea* is the most abundant solid constituent of the urine and is the chief waste product arising from the oxidation of nitrogenous substances in the body. Although secreted by the cells lining the uriniferous tubules, it is not formed in the kidneys. The secreting cells simply separate it from the blood where it already exists. The muscles also have been suggested as a likely source of urea, for here the proteids are broken down in largest quantities; but the muscles produce little if any urea. Its production has been found to be the work of the liver. In the muscular tissue, and in the other tissues as well, the proteids are reduced to a lower order of compounds, such as the compounds of ammonia, which pass into the blood and are then taken up by the liver. By the action of the liver cells these are converted into urea and this is turned back into the blood. From the blood the urea is separated by the secreting cells of the kidneys.

*Work of the Liver.*—The liver, already described as an organ of digestion (page 152), assists in the work of excretion both by changing waste nitrogenous compounds into urea and by removing from the blood the wastes found in the bile. While the chief work of the liver is perhaps not that of excretion, its functions may here be summarized. The liver is, first of all, a manufacturing organ, producing, as we have seen, three distinct products—bile, glycogen, and urea. On account of the nature of the urea and the bile, the liver is properly classed as an excretory organ; but in the formation of the glycogen it plays the part of a storage organ. Then, on account of the use made of the bile after it is passed into the food canal, the liver is also classed as a digestive organ. These different functions make of the liver an organ of the first importance.

*Excretory Work of the Food Canal.*—The glands connected with the food canal, other than the liver, while secreting liquids that aid in digestion, also separate waste materials from the blood. These are passed into the canal, whence they leave the body with the undigested portions of the food and the waste from the liver. Though the nature and quantity of the materials removed by these glands have not been fully determined, recent investigations have tended to enhance the importance attached to this mode of excretion.

*The Perspiratory Glands.*—The perspiratory, or sweat, glands are located in the skin. They belong to the type of simple tubular glands and are very numerous over the entire surface of the body. A typical sweat gland consists of a tube which, starting at the surface of the cuticle, penetrates to the under portion of the true skin and there forms a ball-shaped coil. The coiled extremity, which forms the secreting portion, is lined with secreting cells and surrounded by a network of capillaries. The portion of the tube passing from the coil to the surface serves as a duct (Figs. 91 and 121).

[Fig. 91]

Fig. 91—*Diagram of section through a sweat gland.* a. Outer layer of skin or cuticle. b. Dermis or true skin. d, e. Sections of the tube forming the coiled portion of the gland. c. Duct passing to the surface. The other structures of the skin not shown.

The sweat glands secrete a thin, colorless fluid, called perspiration, or sweat. This consists chiefly of water, but contains a small per cent of salts and of urea. The excretory work of these glands seems not to be so great as was formerly supposed, but they supplement in a practical way the work of the kidneys and, during diseases of these organs, show an increase in excretory function to a marked degree. The perspiration also aids in the regulation of the temperature of the body (Chapter XVI).

*Excretory Work of the Lungs.*—While the lungs cannot be regarded as glands, they do a work in the removal of waste from the body which must be considered in the general process of excretion. They are especially adapted to the removal of gaseous substances from the blood, and it is through them that most of the carbon dioxide leaves the body. The lungs remove also a considerable quantity of water. This is of course in the gaseous form, being known as water vapor.

*Ductless Glands and Internal Secretion.*—Midway in function between the glands that secrete useful liquids and those that remove waste materials from the blood is a class of bodies, found at various places, known as the ductless glands. They are so named from their having the general form of glands and from the fact that they have no external openings or ducts. They prepare special materials which are passed into the blood and which are supposed to exert some beneficial effect either upon the blood or upon the tissues through which the blood circulates. The most important of the ductless glands are the thyroid gland, located in the neck; the suprarenal bodies, situated one just over each kidney; and the thymus gland, a temporary gland in the upper part of the chest. The spleen and the lymphatic glands (page 68) are also classed with the ductless glands. The liver, the pancreas, and (according to some authorities) the kidneys, in addition to their external secretions, produce materials that pass into the blood. They perform in this way a function like that of the ductless glands. The work of glands in preparing substances that enter the blood is known as internal secretion.

*Quantity of Excretory Products.*—If the weight of the normal body be taken at intervals, after growth has been attained, there will be found to be practically no gain or loss from time to time. This shows that materials are leaving the body as fast as they enter and that the tissues are being torn down as fast as they are built up. It also shows that substances do not remain in the body permanently, but only so long perhaps as is necessary for them to give up their energy, or serve some additional purpose in the ever changing protoplasm. The excretory organs then remove from the body a quantity of material that is equal in weight to the materials absorbed by the organs of digestion and respiration. This is estimated for the average individual to be about five pounds daily. The passage of waste from the body is summarized in Table III.