Chapter X: Part 10

Digestion of Starch.--If we collect some saliva in a test tube, add to it a little starch paste, place the tube containing the mixture for a few minutes in tepid water, and then test with Fehling's solution, we shall find grape sugar present. Careful tests of the starch paste and of the saliva made separately will usually show no grape sugar in either.

If another test be made for grape sugar, in a test tube containing starch paste, saliva, and a few drops of any weak acid, the starch will be found not to have changed. The digestion or change of starch to grape sugar is caused by the presence in the saliva of an _enzyme_, or _digestive ferment_. You will remember that starch in the growing corn grain was changed to grape sugar by an enzyme called _diastase_. Here a similar action is caused by an enzyme called _ptyalin_. This ferment acts _only_ in an alkaline medium at about the temperature of the body.

Mouth Cavity in Man.--In our study of a frog we find that the mouth cavity has two unpaired and four paired tubes leading from it. These are (_a_) the _gullet_ or food tube, (_b_) the _windpipe_ (in the frog opening through the _glottis_), (_c_) the paired nostril holes (_posterior nares_), (_d_) the paired _Eustachian tubes_, leading to the ear. All of these openings are found in man.

In man the mouth cavity, and all internal surfaces of the food tube, are lined with a _mucous membrane_. The _mucus_ secreted from gland cells in this lining makes a slippery surface so that the food may slip down easily. The roof of the mouth is formed in front by a plate of bone called the _hard palate_, and a softer continuation to the back of the mouth, the _soft palate_. These separate the nose cavity from that of the mouth proper. The part of the space back of the soft palate is called the _pharynx_, or throat cavity. From the pharynx lead off the _gullet_ and _windpipe_, the former back of the latter. The lower part of the mouth cavity is occupied by a muscular tongue. Examination of its surface with a looking-glass shows it to be almost covered in places by tiny projections called _papillae_. These papillae contain organs known as _taste buds_, the sensory endings of which determine the taste of substances. The tongue is used in moving food about in the mouth, and in starting it on its way to the gullet; it also plays an important part in speaking.

The Teeth.--In man the teeth, unlike those of the frog, are used in the mechanical preparation of the food for digestion. Instead of holding prey, they crush, grind, or tear food so that more surface may be given for the action of the digestive fluids. The teeth of man are divided, according to their functions, into four groups. In the center of both the upper and lower jaw in front are found eight teeth with chisel-like edges, four in each jaw; these are the _incisors_, or cutting teeth. Next is found a single tooth on each side (four in all); these have rather sharp points and are called the _canines_. Then come two teeth on each side, eight in all, called _premolars_. Lastly, the _flat-top molars_, or grinding teeth, of which there are six in each jaw. Food is caught between irregular projections on the surface of the molars and crushed to a pulpy mass.

Hygiene of the Mouth.--Food should simply be chewed and relished, with no thought of swallowing. There should be no more effort to prevent than to force swallowing. It will be found that if you attend only to the agreeable task of extracting the flavors of your food, Nature will take care of the swallowing, and this will become, like breathing, involuntary. The instinct by which most people eat is perverted through the "hurry habit" and the use of abnormal foods. Thorough mastication takes time, and therefore one must not feel hurried at meals if the best results are to be secured. The stopping point for eating should be at the _earliest_ moment after one is really satisfied.

Care of the Teeth.--It has been recently found that fruit acids are very beneficial to the teeth. Vinegar diluted to about half strength with water makes an excellent dental wash. Clean your teeth carefully each morning and before going to bed. Use dental silk after meals. We must remember that the bacteria which cause decay of the teeth are washed down into the stomach and may do even more harm there than in the mouth.

How Food is Swallowed.--After food has been chewed and mixed with saliva, it is rolled into little balls and pushed by the tongue into such position that the muscles of the throat cavity may seize it and force it downward. Food, in order to reach the gullet from the mouth cavity, must pass over the opening into the windpipe. When food is in the course of being swallowed, the upper part of this tube forms a trapdoor over the opening. When this trapdoor is not closed, and food "goes down the wrong way," we choke, and the food is expelled by coughing.

The Gullet, or Esophagus.--Like the rest of the food tube the gullet is lined by soft and moist mucous membrane. The wall is made up of two sets of muscles,--the inside ones running around the tube; the outer layer of muscle taking a longitudinal course. After food leaves the mouth cavity, it gets beyond our direct control, and the muscles of the gullet, stimulated to activity by the presence of food in the tube, push the food down to the stomach by a series of contractions until it reaches the stomach. These wavelike movements (called _peristaltic_ movements) are characteristic of other parts of the food tube, food being pushed along in the stomach and the small intestine by a series of slow-moving muscular waves. Peristaltic movement is caused by muscles which are not under voluntary nervous control, although anger, fear, or other unpleasant emotions have the effect of slowing them up or even stopping them entirely.

Stomach of Man.--The stomach is a pear-shaped organ capable of holding about three pints. The end opposite to the gullet, which empties into the small intestine, is provided with a ring of muscle forming a valve called the _pylorus_. There is also another ring of muscle guarding the entrance to the stomach.

Gastric Glands.--If we open the stomach of the frog, and remove its contents by carefully washing, its wall is seen to be thrown into folds internally. Between the folds in the stomach of man, as well as in the frog, are located a number of tiny pits. These form the mouths of the _gastric glands_, which pour into the stomach a secretion known as the _gastric juice_. The gastric glands are little tubes, the lining of which secretes the fluid. When we think of or see appetizing food, this secretion is given out in considerable quantity. The stomach, like the mouth, "waters" at the sight of food. Gastric juice is slightly acid in its chemical reaction, containing about .2 per cent free _hydrochloric acid_. It also contains two very important enzymes, one called _pepsin_, and another less important one called _rennin_.

Action of Gastric Juice.--If protein is treated with artificial gastric juice at the temperature of the body, it will be found to become swollen and then gradually to change to a substance which is soluble in water. This is like the action of the gastric juice upon proteins in the stomach.

The other enzyme of gastric juice, called _rennin_, curdles or coagulates a protein found in milk; after the milk is curdled, the pepsin is able to act upon it. "Junket" tablets, which contain rennin, are used in the kitchen to cause this change.

The hydrochloric acid found in the gastric juice acts upon lime and some other salts taken into the stomach with food, changing them so that they may pass into the blood and eventually form the mineral part of bone or other tissue. The acid also has a decided antiseptic influence in preventing growth of bacteria which cause decay, and some of which might cause disease.

Movement of Walls of Stomach.--The stomach walls, provided with three layers of muscle which run in an oblique, circular, and longitudinal direction (taken from the inside outward), are well fitted for the constant churning of the food in that organ. Here, as elsewhere in the digestive tract, the muscles are involuntary, muscular action being under the control of the so-called _sympathetic nervous system_. Food material in the stomach makes several complete circuits during the process of digestion in that organ. Contrary to common belief, the greatest amount of food is digested _after_ it leaves the stomach. But this organ keeps the food in it in almost constant motion for a considerable time, a meal of meat and vegetables remaining in the stomach for three or four hours. While movement is taking place, the gastric juice acts upon proteins, softening them, while the constant churning movement tends to separate the bits of food into finer particles. Ultimately the semifluid food, much of it still undigested, is allowed to pass in small amounts through the pyloric valve, into the small intestine. This is allowed by the relaxation of the ringlike muscles of the pylorus.

Experiments on Digestion in the Stomach.--Some very interesting experiments have recently been made by Professor Cannon of Harvard with reference to movements of the stomach contents. Cats were fed with material having in it bismuth, a harmless chemical that would be visible under the X-ray. It was found that shortly after food reached the stomach a series of waves began which sent the food toward the pyloric end of the stomach. If the cat was feeling happy and well, these contractions continued regularly, but if the cat was cross or bad tempered, the movements would stop. This shows the importance of _cheerfulness_ at meals. Other experiments showed that food which was churned into a soft mass was only permitted to leave the stomach when it became thoroughly permeated by the gastric juice. It is the _acid_ in the partly digested food that causes the stomach valve to open and allow its contents to escape little by little into the small intestine.

The partly digested food in the small intestine almost immediately comes in contact with fluids from two glands, the liver and pancreas. We shall first consider the function of the pancreas.

Position and Structure of the Pancreas.--The most important digestive gland in the human body is the pancreas. The gland is a rather diffuse structure; its duct empties by a common opening with the bile duct into the small intestine, a short distance below the pylorus. In internal structure, the pancreas resembles the salivary glands.

Work done by the Pancreas.--Starch paste added to artificial pancreatic fluid and kept at blood heat is soon changed to sugar. Protein, under the same conditions, is changed to a peptone. Fats, which so far have been unchanged except to be melted by the heat of the body, are changed by the action of the pancreas into a form which can pass through the walls of the food tube. If we test pancreatic fluid, we find it strongly _alkaline_ in its reaction. If two test tubes, one containing olive oil and water, the other olive oil and a weak solution of caustic soda, an _alkali_, be shaken violently and then allowed to stand, the oil and water will quickly separate, while the oil, caustic soda, and water will remain for some time in a milky _emulsion_. If this emulsion be examined under the microscope, it will be found to be made of millions of little droplets of fat, floating in the liquid. The presence of the caustic soda helped the forming of the emulsion. Pancreatic fluid similarly emulsifies fats and changes them into soft soaps and fatty acids. Fat in this form may be absorbed. The process of this transformation is not well understood.

Conditions under which the Pancreas does its Work.--The secretion from this gland seems to be influenced by the overflow of acid material from the stomach. This acid, on striking the lining of the small intestine, causes the formation in its walls of a substance known as _secretin_. This secretin reaches the blood and seems to stimulate all the glands pouring fluid into the intestine to do more work. A pint or more of pancreatic fluid is secreted every day.

The Intestinal Fluid.--Three different pancreatic enzymes do the work of digestion, one acting on starch, another on protein, and a third on fats. It has been found that some of these enzymes will not do their work unless aided by the _intestinal_ fluid, a secretion formed in glands in the walls of the small intestine. This fluid, though not much is known about it, is believed to play an important part in the digestion of all kinds of foods left undigested in the small intestine.

Liver.--The liver is the largest gland in the body. In man, it hangs just below the diaphragm, a little to the right side of the body. During life, its color is deep red. It is divided into three lobes, between two of which is found the _gall bladder_, a thin-walled sac which holds the _bile_, a secretion of the liver. Bile is a strongly alkaline fluid of greenish color. It reaches the intestine through the same opening as the pancreatic fluid. Almost one quart of bile is passed daily into the digestive canal. The color of bile is due to certain waste substances which come from the destruction of worn-out red corpuscles of the blood. This destruction takes place in the liver.

Functions of Bile.--The action of bile is not very well known. It has the very important faculty of aiding the pancreatic fluid in digestion, though alone it has slight if any digestive power. Certain substances in the bile aid especially in the absorption of fats. Bile seems to be mostly a waste product from the blood and as such incidentally serves to keep the contents of the intestine in a more or less soft condition, thus preventing extreme constipation.

The Liver a Storehouse.--Perhaps the most important function of the liver is the formation within it of a material called _glycogen_, or animal starch. The liver is supplied by blood from two sources. The greater amount of blood received by the liver comes directly from the walls of the stomach and intestine to this organ. It normally contains about one fifth of all the blood in the body. This blood is very rich in food materials, and from it the cells of the liver take out sugars to form glycogen.[43] Glycogen is stored in the liver until such a time as a food is needed that can be quickly oxidized; then it is changed to sugar and carried off by the blood to the tissue which requires it, and there used for this purpose. Glycogen is also stored in the muscles, where it is oxidized to release energy when the muscles are exercised.

Footnote 43: It is known that glycogen _may_ be formed in
the body from protein, and possibly from fatty foods.

The Absorption of Digested Food into the Blood.--The object of digestion is to change foods from an insoluble to a soluble form. This has been seen in the study of the action of the various digestive fluids in the body, each of which is seen to aid in dissolving solid foods, changing them to a fluid, and, in case of the bile, actually assisting them to pass through the wall of the intestine. A small amount of digested food may be absorbed by the blood in the blood vessels of the walls of the stomach. Most of the absorption, however, takes place through the walls of the small intestine.

Structure of the Small Intestine.--The small intestine in man is a slender tube nearly twenty feet in length and about one inch in diameter. If the chief function of the small intestine is that of absorption, we must look for adaptations which increase the absorbing surface of the tube. This is gained in part by the inner surface of the tube being thrown into transverse folds which not only retard the rapidity with which food passes down the intestine, but also give more absorbing surface. But far more important for absorption are millions of little projections which cover the inner surface of the small intestine.

The Villi.--So numerous are these projections that the whole surface presents a velvety appearance. Collectively, these structures are called the _villi_ (singular _villus_). They form the chief organs of absorption in the intestine, several thousand being distributed over every square inch of surface. By means of the folds and villi the small intestine is estimated to have an absorbing surface equal to twice that of the surface of the body. Between the villi are found the openings of the _intestinal glands_.

Internal Structure of a Villus.--The internal structure of a villus is best seen in a longitudinal section. We find the outer wall made up of a thin layer of cells, the _epithelial_ layer. It is the duty of these cells to absorb the semifluid food from within the intestine. Underneath these cells lies a network of very tiny blood vessels, while inside of these, occupying the core of the villus, are found spaces which, because of their white appearance after absorption of fats, have been called _lacteals_. (See figure, page 307.)

Absorption of Foods.--Let us now attempt to find out exactly how foods are passed from the intestines into the blood. Food substances in solution may be soaked up as a sponge would take up water, or they may pass by osmosis into the cells lining the villus. These cells break down the peptones into a substance that will pass into and become part of the blood. Once within the villus, the sugars and digested proteins pass through tiny blood vessels into the larger vessels comprising the _portal circulation_. These pass through the liver, where, as we have seen, sugar is taken from the blood and stored as glycogen. From the liver, the food within the blood is sent to the heart, from there is pumped to the lungs, from there returns to the heart, and is pumped to the tissues of the body. A large amount of water and some salts are also absorbed through the walls of the stomach and intestine as the food passes on its course. The fats in the form of soaps and fatty acids pass into the space in the center of the villus. Later they are changed into fats again, probably in certain groups of gland cells known as _mesenteric_ glands, and eventually reach the blood by way of the thoracic duct without passing through the liver.

Large Intestine.--The large intestine has somewhat the same structure as the small intestine, except that it lacks the villi and has a greater diameter. Considerable absorption, however, takes place through its walls as the mass of food and refuse material is slowly pushed along by the muscles within its walls.

Vermiform Appendix.--At the point where the small intestine widens to form the large intestine, a baglike pouch is formed. From one side of this pouch is given off a small tube about four inches long, closed at the lower end. This tube, the rudiment of what is an important part of the food tube in the lower vertebrates, is called the _vermiform appendix_. It has come to have unpleasant notoriety in late years, as the site of serious inflammation.

Constipation.--In the large intestine live millions of bacteria, some of which make and give off poisonous substances known as toxins. These substances are easily absorbed through the walls of the large intestine, and, when they pass into the blood, cause headaches or sometimes serious trouble. Hence it follows that the lower bowel should be emptied of this matter as frequently as possible, at least once a day. Constipation is one of the most serious evils the American people have to deal with, and it is largely brought about by the artificial life which we lead, with its lack of exercise, fresh air, and sleep. Fruit with meals, especially at breakfast, plenty of water between meals and before breakfast, exercise, particularly of the abdominal muscles, and regular habits will all help to correct this evil.

Hygienic Habits of Eating; the Causes and Prevention of Dyspepsia.--From the contents of the foregoing chapter it is evident that the object of the process of digestion is to break up solid food so that it may be absorbed to form part of the blood. Any habits we may form of thoroughly chewing our food will evidently aid in this process. Undoubtedly much of the distress known as dyspepsia is due to too hasty meals with consequent lack of proper mastication of food. The message of Mr. Horace Fletcher in bringing before us the need of proper mastication of food and the attendant evils of overeating is one which we cannot afford to ignore. It is a good rule to go away from the table feeling a little hungry. Eating too much overtaxes the digestive organs and prevents their working to the best advantage. Still another cause of dyspepsia is eating when in a _fatigued_ condition. It is always a good plan to rest a short time before eating, especially after any hard manual work. We have seen how great a part unpleasant emotions play in preventing peristaltic movements of the food tube. Conversely, pleasant conversation, laughter, and fun will help you to digest your meal. Eating between meals is condemned by physicians because it calls the blood to the digestive organs at a time when it should be more active in other parts of the body.

Effect of Alcohol on Digestion.--It is a well-known fact that alcohol extracts water from tissues with which it is in contact. This fact works much harm to the interior surface of the food tube, especially the walls of the stomach, which in the case of a hard drinker are likely to become irritated and much toughened. In very small amounts alcohol stimulates the secretion of the salivary and gastric glands, and thus appears to aid in digestion.

The following results of experiments on dogs, published in the _American Journal of Physiology_, Vol. I, Professor Chittenden of Yale University gives as "strictly comparable," because "they were carried out in succession on the same day." They show that alcohol retards rather than aids in digestion:--

==========================================================================
NUMBER OF EXPERIMENT |1/16 LB. MEAT WITH WATER| 1/16 LB. MEAT WITH
| | DILUTE ALCOHOL
------------------------+------------------------+------------------------
XVII [alpha] 9:15 A.M. | Digested in 3 hours |
XVII [beta] 3:00 P.M. | | Digested in 3:15 hours
XVIII [alpha] 8:30 A.M. | Digested in 2:30 hours |
XVIII [beta] 2:10 P.M. | | Digested in 3:00 hours
XIX [alpha] 9:00 A.M. | Digested in 2:30 hours |
XIX [beta] 2:30 P.M. | | Digested in 3:00 hours
XX [alpha] 9:15 A.M. | | Digested in 2:45 hours
XX [beta] 2:30 P.M. | Digested in 2:15 hours |
VI [alpha] 9:15 A.M. | | Digested in 3:45 hours
VI [beta] 1:00 P.M. | Digested in 3:15 hours |
------------------------+------------------------+------------------------
Average | 2:42 hours | 3:09 hours
------------------------+------------------------+------------------------

As a result of his experiments, Professor Chittenden remarks: "We believe that the results obtained justify the conclusion that gastric digestion as a whole is not materially modified by the introduction of alcoholic fluids with the food. In other words, the unquestionable acceleration of gastric secretion which follows the ingestion of alcoholic beverages is, as a rule, counterbalanced by the inhibitory effect of the alcoholic fluids upon the chemical process of gastric digestion, with perhaps at times a tendency towards preponderance of inhibitory action." Others have come to the same or stronger conclusions as to the undesirable action of alcohol on digestion, as a result of their own experiments.

Effect of Alcohol on the Liver.--The effect of heavy drinking upon the liver is graphically shown in the following table prepared by the Scientific Temperance Federation of Boston, Mass.:--

]

Footnote 44: Does not include deaths from general alcoholic
paralysis or other organic diseases due to alcohol. Liver
cirrhosis due to alcohol conservatively estimated at 75 per
cent of total cases.

"Alcoholic indulgence stands almost if not altogether in the
front rank of the enemies to be combated in the battle for
health."--PROFESSOR WILLIAM T. SEDGWICK.

XXI. THE BLOOD AND ITS CIRCULATION

_Problems.--To discover the composition and uses of the different parts of
the blood._
_To find out the means by which the blood is circulated about the
body._

LABORATORY SUGGESTIONS

_Demonstration._--Structure of blood, fresh frog's blood and
human blood. Drawings.
_Demonstration._--Clotting of blood.
_Demonstration._--Use of models to demonstrate that the
heart is a force pump.
_Demonstration._--Capillary circulation in web of frog's
foot or tadpole's tail. Drawing.
_Home or laboratory exercise._--On relation of exercise on
rate of heart beat.

Function of the Blood.--The chief function of the digestive tract is to change foods to such form that they can be absorbed through the walls of the food tube and become part of the blood.[45]

Footnote 45: This change is due to the action of certain
enzymes upon the nutrients in various foods. But we also
find that peptones are changed back again to proteins when
once in the blood. This appears to be due to the
_reversible_ action of the enzymes acting upon them. (See
page 307.)

If we examine under the microscope a drop of blood taken from the frog or man, we find it made up of a fluid called _plasma_ and two kinds of bodies, the so-called _red corpuscles_ and _colorless corpuscles_, floating in this plasma.

Composition of Plasma.--The plasma of blood is found to be largely (about 90 per cent) water. It also contains a considerable amount of protein, some sugar, fat, and mineral material. It is, then, the medium which holds the fluid food that has been absorbed from within the intestine. This food is pumped to the body cells where, as work is performed, oxidation takes place and heat is given off as a form of energy. The almost constant temperature of the body is also due to the blood, which brings to the surface of the body much of the heat given off by oxidation of food in the muscles and other tissues. When the blood returns from the tissues where the food is oxidized, the plasma brings back with it to the lungs part of the carbon dioxide liberated where oxidation has taken place. Some waste products, to be spoken of later, are also found in the plasma.

The Red Blood Corpuscle; its Structure and Functions.--The red corpuscle in the blood of the frog is a true cell of disklike form, containing a nucleus. The red corpuscle of man is made in the red marrow of bones and in its young stages has a nucleus. In its adult form, however, it lacks a nucleus. Its form is that of a biconcave disk. So small and so numerous are these corpuscles that over five million are found in a cubic centimeter of normal blood. They make up almost one half the total volume of the blood. The color, which is found to be a dirty yellow when separate corpuscles are viewed under the microscope, is due to a protein material called _haemoglobin_. Haemoglobin contains a large amount of iron. It has the power of uniting very readily with oxygen whenever that gas is abundant, and, after having absorbed it, of giving it up to the surrounding media, when oxygen is there present in smaller amounts than in the corpuscle. This function of carrying oxygen is the most important function of the red corpuscle, although the red corpuscle also removes part of the carbon dioxide from the tissues on their return to the lungs. The taking up of oxygen is accompanied by a change in color of the mass of corpuscles from a dull red to a bright scarlet.

Clotting of Blood.--If fresh beef blood is allowed to stand overnight, it will be found to have separated into two parts, a dark red, almost solid _clot_ and a thin, straw-colored liquid called _serum_. Serum is found to be made up of about 90 per cent water, 8 per cent protein, 1 per cent other organic foods, and 1 per cent mineral substances. In these respects it very closely resembles the fluid food that is absorbed from the intestines.

If another jar of fresh beef blood is poured into a pan and briskly whipped with a bundle of little rods (or with an egg beater), a stringy substance will be found to stick to the rods. This, if washed carefully, is seen to be almost colorless. Tested with nitric acid and ammonia, it is found to contain a protein substance which is called _fibrin_.

Blood plasma, then, is made up of a fluid portion of serum, and fibrin, which, although in a fluid state in the blood vessels within the body, coagulates when blood is removed from the blood vessels. This coagulation aids in making a blood clot. A clot is simply a mass of fibrin threads with a large number of corpuscles tangled within. The clotting of blood is of great physiological importance, for otherwise we might bleed to death even from a small wound.

Blood Plates.--In blood within the circulatory system of the body, the fibrin is held in a fluid state called _fibrinogen_. An enzyme, acting upon this fibrinogen, the soluble protein in the blood, causes it to change to an insoluble form, the fibrin of the clot. This change seems to be due to the action of minute bodies in the blood known as _blood plates_. Under abnormal conditions these blood plates break down, releasing some substances which eventually cause this enzyme to do its work.

The Colorless Corpuscle; Structure and Functions.--A colorless corpuscle is a cell irregular in outline, the shape of which is constantly changing. These corpuscles are somewhat larger than the red corpuscles, but less numerous, there being about one colorless corpuscle to every three hundred red ones. They have the power of movement, for they are found not only inside but outside the blood vessels, showing that they have worked their way between the cells that form the walls of the blood tubes.

A Russian zoologist, Metchnikoff, after studying a number of simple animals, such as medusae and sponges, found that in such animals some of the cells lining the inside of the food cavity take up or engulf minute bits of food. Later, this food is changed into the protoplasm of the cell. Metchnikoff believed that the colorless corpuscles of the blood have somewhat the same function. This he later proved to be true. Like the amoeba, they feed by engulfing their prey. This fact has a very important bearing on the relation of colorless corpuscles to certain diseases caused by bacteria within the body. If, for example, a cut becomes infected by bacteria, inflammation may set in. Colorless corpuscles at once surround the spot and attack the bacteria which cause the inflammation. If the bacteria are few in number, they are quickly eaten by certain of the colorless corpuscles, which are known as _phagocytes_. If bacteria are present in great quantities, they may prevail and kill the phagocytes by poisoning them. The dead bodies of the phagocytes thus killed are found in the pus, or matter, which accumulates in infected wounds. In such an event, we must come to the aid of nature by washing the wound with some antiseptic, as weak carbolic acid or hydrogen peroxide.

Antibodies and their Uses.--In case of disease where, for example, fever is caused by poison given off from bacteria we find the cells of the body manufacture and pour into the blood a substance known as an _antibody_. This substance does not of necessity kill the harmful germs or even stop their growth. It does, however, unite with the toxin or poison given off by the germs and renders it entirely harmless.

Function of Lymph.--The tissues and organs of the body are traversed by a network of tubes which carry the blood. Inside these tubes is the blood proper, consisting of a fluid plasma, the colorless corpuscles, and the red corpuscles. Outside the blood tubes, in spaces between the cells which form tissues, is found another fluid, which is in chemical composition very much like plasma of the blood. This is the _lymph_. It is, in fact, fluid food in which some colorless amoeboid corpuscles are found. Blood gives up its food material to the lymph. This it does by passing it through the walls of the capillaries. The food is in turn given up to the tissue cells, which are bathed by the lymph.

Some of the amoeboid corpuscles from the blood make their way between the cells forming the walls of the capillaries. _Lymph, then, is practically blood plasma plus some colorless corpuscles. It acts as the medium of exchange between the blood proper and the cells in the tissues of the body._ By means of the food supply thus brought, the cells of the body are able to grow, the fluid food being changed to the protoplasm of the cells. By means of the oxygen passed over by the lymph, oxidation may take place within the cells. Lymph not only gives food to the cells of the body, but also takes away carbon dioxide and other waste materials, which are ultimately passed out of the body by means of the lungs, skin, and kidneys.

Internal Secretions.--In addition to all the functions given above, the blood has recently been shown to carry the secretions of a number of glands through which it passes, although these glands have no ducts to carry off their secretions. These internal secretions seem _absolutely necessary_ for the health of the body. Several glands, the thyroid, adrenal bodies, the testes, and ovaries, as well as the pancreas, give off these remarkable substances.

The Amount of Blood and its Distribution.--Blood forms, by weight, about one sixteenth of the body. This would be about four quarts to a body weight of 130 pounds. Normally, about one half of the blood of the body is found in or near the organs lying in the body cavity below the diaphragm, about one fourth in the muscles, and the rest in the head, heart, lungs, large arteries, and veins.

Blood Temperature.--The temperature of blood in the human body is normally about 98.6 deg. Fahrenheit when tested under the tongue by a thermometer, although the temperature drops almost two degrees after we have gone to sleep at night. It is highest about 5 P.M. and lowest about 4 A.M. In fevers, the temperature of the body sometimes rises to 107 deg.; but unless this temperature is soon reduced, death follows. Any considerable drop in temperature below the normal also means death. Body heat results from the oxidation of food, and the circulation of blood keeps the temperature nearly uniform in all parts of the body.

Cold-blooded Animals.--In animals which are called cold-blooded, the blood has no fixed temperature, but varies with the temperature of the medium in which the animal lives. Frogs, in the summer, may sit for hours in water with a temperature of almost 100 deg. In winter, they often endure freezing so that the blood and lymph within the spaces under the loose skin are frozen into ice crystals. This change in body temperature is evidently an adaptation to the mode of life.

Circulation of the Blood in Man.--The blood is the carrying agent of the body. Like a railroad or express company, it takes materials from one part of the human organism to another. This it does by means of the organs of circulation,--the heart and blood vessels. These blood vessels are called _arteries_ where they carry blood away from the heart, _veins_ where they bring blood back to the heart, and _capillaries_ where they connect the larger blood vessels. The organs of circulation thus form a system of connected tubes through which the blood flows.

The Heart; Position, Size, Protection.--The heart is a cone-shaped muscular organ about the size of a man's fist. It is located immediately above the diaphragm, and lies so that the muscular apex, which points downward, moves while beating against the fifth and sixth ribs, just a little to the left of the midline of the body. This fact gives rise to the notion that the heart is on the left side of the body. The heart is surrounded by a loose membranous bag called the _pericardium_, the inner lining of which secretes a fluid in which the heart lies. When, for any reason, the pericardial fluid is not secreted, inflammation arises in that region.

Internal Structure of Heart.--If we should cut open the heart of a mammal down the midline, we could divide it into a right and a left side, _each of which would have no internal connection with the other_. Each side is made up of an upper thin-walled portion with a rather large internal cavity, the _auricle_, which opens into a lower smaller portion with heavy muscular walls, the _ventricle_. Communication between auricles and ventricles is guarded by little flaps or _valves_. The auricles receive blood from the veins. The ventricles pump the blood into the arteries.

The Heart in Action.--The heart is constructed on the same plan as a force pump, the valves preventing the reflux of blood into the auricle when it is forced out of the ventricle. Blood enters the auricles from the veins because the muscles of that part of the heart relax; this allows the space within the auricles to fill. Almost immediately the muscles of the ventricles relax, thus allowing blood to pass into the chambers within the ventricles. Then, after a short pause, during which time the muscles of the heart are resting, a wave of muscular contraction begins in the auricles and ends in the ventricles, with a sudden strong contraction which forces the blood out into the arteries. Blood is kept on its course by the valves, which act in the same manner as do the valves in a pump. The blood is thus made to pass into the arteries upon the contraction of the ventricle walls.

I. Circulation in a fish. _G_, gills; _C_, capillaries of the body. Notice the two-chambered heart.

II. The circulation in a frog. _L_, the lungs; _C_, the capillaries. Notice the heart has three chambers. What is the condition of blood leaving the ventricle to go to the cells of the body?

III. The circulation in man. _H_, head; _A_, arms; _L_, lungs; _S_, stomach; _Li_, liver; _K_, kidney; _S.I._, small intestine; _L.I._, large intestine; _Le_, legs; _1_, right auricle; _2_, right ventricle; _3_, left ventricle; _4_, left auricle; _5_, dorsal aorta; _6_, vein to lungs. ]

The Course of the Blood in the Body.--Although the two sides of the heart are separate and distinct from each other, yet every drop of blood that passes through the right heart likewise passes later through the left heart. There are two distinct systems of circulation in the body. The _pulmonary circulation_ takes the blood through the right auricle and ventricle, to the lungs, and passes it back to the left auricle. This is a relatively short circulation, the blood receiving in the lungs its supply of oxygen, and there giving up some of its carbon dioxide. The greater circulation is known as the _systemic circulation_; in this system, the blood leaves the left ventricle through the great dorsal _aorta_. A large part of the blood passes directly to the muscles; some of it goes to the nervous system, kidneys, skin, and other organs of the body. It gives up its supply of food and oxygen in these tissues, receives the waste products of oxidation while passing through the capillaries, and returns to the right auricle through two large vessels known as the _venae cavae_. It requires only from twenty to thirty seconds for the blood to make the complete circulation from the ventricle back again to the starting point. This means that the entire volume of blood in the human body passes three or four thousand times a day through the various organs of the body.[46]

Footnote 46: See Hough and Sedgwick, _The Human Mechanism_,
page 136.

Portal Circulation.--Some of the blood, on its way back to the heart, passes to the walls of the food tube and to its glands. From there it is sent with its load of absorbed food to the liver. Here the vein which carries the blood (called the portal vein) breaks up into capillaries around the cells of the liver, when it gives up sugar to be stored as glycogen. From the liver, blood passes directly to the right auricle. The _portal circulation_, as it is called, is the only part of the circulation where the blood passes through two sets of capillaries on its way from auricle to auricle.

Circulation in the Web of a Frog's Foot.--If the web of the foot of a live frog or the tail of a tadpole is examined under the compound microscope, a network of blood vessels will be seen. In some of the larger vessels the corpuscles are moving rapidly and in spurts; these are _arteries_. The arteries lead into smaller vessels hardly greater in diameter than the width of a single corpuscle. This network of _capillaries_ may be followed into larger _veins_ in which the blood moves regularly. This illustrates the condition in any tissue of man where the arteries break up into capillaries, and these in turn unite to form veins.

Structure of the Arteries.--A distinct difference in structure exists between the arteries and the veins in the human body. The arteries, because of the greater strain received from the blood which is pumped from the heart, have thicker muscular walls, and in addition are very elastic.

Cause of the Pulse.--The _pulse_, which can easily be detected by pressing the large artery in the wrist or the small one in front of and above the external ear, is caused by the gushing of blood through the arteries after each pulsation of the heart. As the large arteries pass away from the heart, the diameter of each individual artery becomes smaller. At the very end of their course, these arteries are so small as to be almost microscopic in size and are very numerous. There are so many that if they were placed together, side by side, their united diameter would be much greater than the diameter of the large artery (_aorta_) which passes blood from the left side of the heart. This fact is of very great importance, for the force of the blood as it gushes through the arteries becomes very much less when it reaches the smaller vessels. This gushing movement is quite lost when the capillaries are reached, first, because there is so much more space for the blood to fill, and second, because there is considerable friction caused by the very tiny diameter of the capillaries.

Capillaries.--The capillaries form a network of minute tubes everywhere in the body, but especially near the surface and in the lungs. It is through their walls that the food and oxygen pass to the tissues, and carbon dioxide is given up to the plasma. They form the connection that completes the system of circulation of blood in the body.

Function and Structure of the Veins.--If the arteries are supply pipes which convey fluid food to the tissues, then the veins may be likened to drain pipes which carry away waste material from the tissues. Extremely numerous in the extremities and in the muscles and among other tissues of the body, they, like the branches of a tree, become larger and unite with each other as they approach the heart.

If the wall of a vein is carefully examined, it will be found to be neither so thick nor so tough as an artery wall. When empty, a vein collapses; the wall of an artery holds its shape. If you hold your hand downward for a little time and then examine it, you will find that the veins, which are relatively much nearer the surface than are the arteries, appear to be very much knotted. This appearance is due to the presence of tiny valves within. These valves open in the direction of the blood current, but would close if the direction of the blood flow should be reversed (as in case a deep cut severed a vein). As the pressure of blood in the veins is much less than in the arteries, the valves thus aid in keeping the flow of blood in the veins toward the heart. The higher pressure in arteries and the suction in the veins (caused by the enlargement of the chest cavity in breathing) are the chief factors which cause a steady flow of blood through the veins in the body.

Lymph Vessels.--The lymph is collected from the various tissues of the body by means of a number of very thin-walled tubes, which are at first very tiny, but after repeated connection with other tubes ultimately unite to form large ducts. These lymph ducts are provided, like the veins, with valves. The pressure of the blood within the blood vessels forces continually more plasma into the lymph; thus a slow current is maintained. On its course the lymph passes through many collections of gland cells, the _lymph glands_. In these glands some impurities appear to be removed and colorless corpuscles made. The lymph ultimately passes into a large tube, the _thoracic duct_, which flows upward near the ventral side of the spinal column, and empties into the large subclavian vein in the left side of the neck. Another smaller lymph duct enters the right subclavian vein.

The Lacteals.--We have already found that part of the digested food (chiefly carbohydrates, proteins, salts, and water) is absorbed directly into the blood through the walls of the villi and carried to the liver. Fat, however, is passed into the spaces in the central part of the villi, and from there into other spaces between the tissues, known as the _lacteals_. The lacteals carry the fats into the blood by way of the thoracic duct. The lacteals and lymph vessels have in part the same course. It will be thus seen that lymph at different parts of its course would have a very different composition.

The Nervous Control of the Heart and Blood Vessels.--Although the muscles of the heart contract and relax without our being able to stop them or force them to go faster, yet in cases of sudden fright, or after a sudden blow, the heart may stop beating for a short interval. This shows that the heart is under the control of the nervous system. Two sets of nerve fibers, both of which are connected with the central nervous system, pass to the heart. One set of fibers accelerates, the other slows or inhibits, the heart beat. The arteries and veins are also under the control of the sympathetic nervous system. This allows of a change in the diameter of the blood vessels. Thus, blushing is due to a sudden rush of blood to the surface of the body caused by an expansion of the blood vessels at the surface. The blood vessels of the body are always full of blood. This results from an automatic regulation of the diameter of the blood tubes by a part of the nervous system called the _vasomotor nerves_. These nerves act upon the muscles in the walls of the blood vessels. In this way, each vessel adapts itself to the amount of blood in it at a given time. After a hearty meal, a large supply of blood is needed in the walls of the stomach and intestines. At this time, the arteries going to this region are dilated so as to receive an extra supply. When the brain performs hard work, blood is supplied in the same manner to that region. Hence, one should not study or do mental work immediately after a hearty meal, for blood will be drawn away to the brain, leaving the digestive tract with an insufficient supply. Indigestion may follow as a result.

The Effect of Exercise on the Circulation.--It is a fact familiar to all that the heart beats more violently and quickly when we are doing hard work than when we are resting. Count your own pulse when sitting quietly, and then again after some brisk exercise in the gymnasium. Exercise in moderation is of undoubted value, because it sends the increased amount of blood to such parts of the body where increased oxidation has been taking place as the result of the exercise. The best forms of exercise are those which give as many muscles as possible work--walking, out-of-door sports, any exercise that is not violent. Exercise should not be attempted immediately after eating, as this causes a withdrawal of blood from the digestive tract to the muscles of the body. Neither should exercise be continued after becoming tired, as poisons are then formed in the muscles, which cause the feeling we call _fatigue_. Remember that extra work given to the heart by extreme exercise may injure it, causing possible trouble with the valves.

Treatment of Cuts and Bruises.--Blood which oozes slowly from a cut will usually stop flowing by the natural means of the formation of a clot. A cut or bruise should, however, be washed in a weak solution of carbolic acid or some other antiseptic in order to prevent bacteria from obtaining a foothold on the exposed flesh. If blood, issuing from a wound, gushes in distinct pulsations, then we know that an artery has been severed. To prevent the flow of blood, a tight bandage known as a _tourniquet_ must be tied between the cut and the heart. A handkerchief with a knot placed over the artery may stop bleeding if the cut is on one of the limbs. If this does not serve, then insert a stick in the handkerchief and twist it so as to make the pressure around the limb still greater. Thus we may close the artery until the doctor is called, who may sew up the injured blood vessel.

The Effect of Alcohol upon the Blood.--It has recently been discovered that alcohol has an extremely injurious effect upon the colorless corpuscles of the blood, lowering their ability to fight disease germs to a marked degree. This is well seen in a comparison of deaths from certain infectious diseases in drinkers and abstainers, the percentage of mortality being much greater in the former.

Dr. T. Alexander MacNichol, in a recent address, said:--

"Massart and Bordet, Metchnikoff and Sims Woodhead, have
proved that alcohol, even in very dilute solution, prevents
the white blood corpuscles from attacking invading germs,
thus depriving the system of the cooperation of these
important defenders, and reducing the powers of resisting
disease. The experiments of Richardson, Harley, Kales, and
others have demonstrated the fact that one to five per cent
of alcohol in the blood of the living human body in a
notable degree alters the appearance of the corpuscular
elements, reduces the oxygen bearing elements, and prevents
their reoxygenation."

Alcohol weakens Resistance to Disease.--In acute illnesses, grippe, fevers, blood poisoning, etc., substances formed in the blood termed "antibodies" antagonize the action of bacteria, facilitating their destruction by the white blood cells and neutralizing their poisonous influence. In a person with good "resistance" this protective machinery, which we do not yet thoroughly understand, works with beautiful precision, and the patient "gets well." Experiments by scientific experts have demonstrated that alcohol restrains the formation of these marvelous antibodies. Alcohol puts to sleep the sentinels that guard your body from disease.

The Effect of Alcohol on the Circulation.--Alcoholic drinks affect the very delicate adjustment of the nervous center's controlling the blood vessels and heart. Even very dilute alcohol acts upon the muscles of the tiny blood vessels; consequently, more blood is allowed to enter them, and, as the small vessels are usually near the surface of the body, the habitual redness seen in the face of hard drinkers is the ultimate result.

"The first effect of diluted alcohol is to make the heart
beat faster. This fills the small vessels near the surface.
A feeling of warmth is produced which causes the drinker to
feel that he was warmed by the drink. This feeling, however,
soon passes away, and is succeeded by one of chilliness. The
body temperature, at first raised by the rather rapid
oxidation of the alcohol, is soon lowered by the increased
radiation from the surface.

"The immediate stimulation to the heart's action soon passes
away and, like other muscles, the muscles of the heart lose
power and contract with less force after having been excited
by alcohol."--MACY, _Physiology_.

Alcohol, when brought to act directly on heart muscle, lessens the force of the beat. It may even cause changes in the tissues, which eventually result in the breaking of the walls of a blood vessel or the plugging of a vessel with a blood clot. This condition may cause the disease known as _apoplexy_.