Having been acted upon by the mesenteric glands, and pa.s.sed through them, the chyle flows onward until it is poured into a dilated reservoir for the chyle, known as the receptaculum chyli. This is a sac-like expansion of the lower end of the thoracic duct. Into this receptacle, situated at the level of the upper lumbar vertebrae, in front of the spinal column, are poured, not only the contents of the lacteals, but also of the lymphatic vessels of the lower limbs.

158. The Thoracic Duct. This duct is a tube from fifteen to eighteen inches long, which pa.s.ses upwards in front of the spine to reach the base of the neck, where it opens at the junction of the great veins of the left side of the head with those of the left arm. Thus the thoracic duct acts as a kind of feeding pipe to carry along the nutritive material obtained from the food and to pour it into the blood current. It is to be remembered that the lacteals are in reality lymphatics--the lymphatics of the intestines.

[Ill.u.s.tration: Fig. 61.--Section of a Lymphatic Gland.

A, strong fibrous capsule sending part.i.tions into the gland; B, part.i.tions between the follicles or pouches of the _cortical_ or outer portion; C, part.i.tions of the _medullary_ or central portion; D, E, ma.s.ses of protoplasmic matter in the pouches of the gland; F, lymph-vessels which bring lymph _to_ the gland, pa.s.sing into its center; G, confluence of those leading to the efferent vessel; H, vessel which carries the lymph away _from_ the gland.

159. The Lymphatics. In nearly every tissue and organ of the body there is a marvelous network of vessels, precisely like the lacteals, called the lymphatics. These are busily at work taking up and making over anew waste fluids or surplus materials derived from the blood and tissues generally. It is estimated that the quant.i.ty of fluid picked up from the tissues by the lymphatics and restored daily to the circulation is equal to the bulk of the blood in the body. The lymphatics seem to start out from the part in which they are found, like the rootlets of a plant in the soil. They carry a turbid, slightly yellowish fluid, called lymph, very much like blood without the red corpuscles.

Now, just as the chyle was not fit to be immediately taken up by the blood, but was pa.s.sed through the mesenteric glands to be properly worked over, so the lymph is carried to the lymphatic glands, where it undergoes certain changes to fit it for being poured into the blood.

Nature, like a careful housekeeper, allows nothing to be wasted that can be of any further service in the animal economy (Figs. 63 and 64).

The lymphatics unite to form larger and larger vessels, and at last join the thoracic duct, except the lymphatics of the right side of the head and chest and right arm. These open by the right lymphatic duct into the venous system on the right side of the neck.

The whole lymphatic system may be regarded as a necessary appendage to the vascular system (Chapter VII.). It is convenient, however, to treat it under the general topic of absorption, in order to complete the history of food digestion.

160. The Spleen and Other Ductless Glands. With the lymphatics may be cla.s.sified, for convenience, a number of organs called ductless or blood glands. Although they apparently prepare materials for use in the body, they have no ducts or ca.n.a.ls along which may be carried the result of their work. Again, they are called blood glands because it is supposed they serve some purpose in preparing material for the blood.

The spleen is the largest of these glands. It lies beneath the diaphragm, and upon the left side of the stomach. It is of a deep red color, full of blood, and is about the size and shape of the palm of the hand.

The spleen has a fibrous capsule from which part.i.tions pa.s.s inwards, dividing it into s.p.a.ces by a framework of elastic tissue, with plain muscular fibers. These s.p.a.ces are filled with what is called the spleen pulp, through which the blood filters from its artery, just as a fluid would pa.s.s through a sponge. The functions of the spleen are not known. It appears to take some part in the formation of blood corpuscles. In certain diseases, like malarial fever, it may become remarkably enlarged. It may be wholly removed from an animal without apparent injury. During digestion it seems to act as a muscular pump, drawing the blood onwards with increased vigor along its large vein to the liver.

The thyroid is another ductless gland. It is situated beneath the muscles of the neck on the sides of "Adam's apple" and below it. It undergoes great enlargement in the disease called goitre.

The thymus is also a blood gland. It is situated around the windpipe, behind the upper part of the breastbone. Until about the end of the second year it increases in size, and then it begins gradually to shrivel away.

Like the spleen, the thyroid and thymus glands are supposed to work some change in the blood, but what is not clearly known.

The suprarenal capsules are two little bodies, one perched on the top of each kidney, in shape not unlike that of a conical hat. Of their functions nothing definite is known.

Experiments.

The action produced by the tendency of fluids to mix, or become equally diffused in contact with each other, is known as _osmosis_, a form of molecular attraction allied to that of adhesion. The various physical processes by which the products of digestion are transferred from the digestive ca.n.a.l to the blood may be ill.u.s.trated in a general way by the following simple experiments.

The student must, however, understand that the necessarily crude experiments of the cla.s.sroom may not conform in certain essentials to these great processes conducted in the living body, which they are intended to ill.u.s.trate and explain.

[Ill.u.s.tration: Fig. 62.]

Experiment 62. _Simple Apparatus for Ill.u.s.trating Endosmotic Action._ "Remove carefully a circular portion, about an inch in diameter, of the sh.e.l.l from one end of an egg, which may be done without injuring the membranes, by cracking the sh.e.l.l in small pieces, which are picked off with forceps. A small gla.s.s tube is then introduced through an opening in the sh.e.l.l and membranes of the other end of the egg, and is secured in a vertical position by wax or plaster of Paris, the tube penetrating the yelk. The egg is then placed in a wine-gla.s.s partly filled with water. In the course of a few minutes, the water will have penetrated the exposed membrane, and the yelk will rise in the tube."--Flint's _Human Physiology_, page 293.

Experiment 63. Stretch a piece of moist bladder across a gla.s.s tube,--a common lamp-chimney will do. Into this put a strong saline solution. Now suspend the tube in a wide mouthed vessel of water. After a short time it will be found that a part of the salt solution has pa.s.sed through into the water, while a larger amount of water has pa.s.sed into the tube and raised the height of the liquid within it.

161. The Quant.i.ty of Food as Affected by Age. The quant.i.ty of food required to keep the body in proper condition is modified to a great extent by circ.u.mstances. Age, occupation, place of residence, climate, and season, as well as individual conditions of health and disease, are always important factors in the problem. In youth the body is not only growing, but the tissue changes are active. The restless energy and necessary growth at this time of life cannot be maintained without an abundance of wholesome food. This food supply for young people should be ample enough to answer the demands of their keen appet.i.te and vigorous digestion.

In adult life, when the processes of digestion and a.s.similation are active, the amount of food may without harm, be in excess of the actual needs of the body. This is true, however, only so long as active muscular exercise is taken.

In advanced life the tissue changes are slow, digestion is less active, and the ability to a.s.similate food is greatly diminished. Growth has ceased, the energy which induced activity is gone, and the proteids are no longer required to build up worn-out tissues. Hence, as old age approaches, the quant.i.ty of nitrogenous foods should be steadily diminished.

Experiment 64. Obtain a sheep's bladder and pour into it a heavy solution of sugar or some colored simple elixir, found at any drug store. Tie the bladder carefully and place it in a vessel containing water. After a while it will be found that an interchange has occurred, water having pa.s.sed into the bladder and the water outside having become sweet.

Experiment 65. Make a hole about as big as a five-cent piece in the large end of an egg. That is, break the sh.e.l.l carefully and snip the outer sh.e.l.l membrane, thus opening the s.p.a.ce between the outer and inner membranes. Now put the egg into a gla.s.s of water, keeping it in an upright position by resting on a napkin-ring. There is only the inner sh.e.l.l membrane between the liquid white of the egg (alb.u.men) and the water.

An interchange takes place, and the water pa.s.ses towards the alb.u.men. As the alb.u.men does not pa.s.s out freely towards the water, the membrane becomes distended, like a little bag at the top of the egg.

162. Ill Effects of a too Generous Diet. A generous diet, even of those who take active muscular exercise, should be indulged in only with vigilance and discretion. Frequent sick or nervous headaches, a sense of fullness, bilious attacks, and dyspepsia are some of the after-effects of eating more food than the body actually requires. The excess of food is not properly acted upon by the digestive juices, and is liable to undergo fermentation, and thus to become a source of irritation to the stomach and the intestines. If too much and too rich food be persistently indulged in, the complexion is apt to become muddy, the skin, especially of the face, pale and sallow, and more or less covered with blotches and pimples; the breath has an unpleasant odor, and the general appearance of the body is unwholesome.

An excess of any one of the different cla.s.ses of foods may lead to serious results. Thus a diet habitually too rich in proteids, as with those who eat meat in excess, often over-taxes the kidneys to get rid of the excess of nitrogenous waste, and the organs of excretion are not able to rid the tissues of waste products which acc.u.mulate in the system. From the blood, thus imperfectly purified, may result kidney troubles and various diseases of the liver and the stomach.

163. Effect of Occupation. Occupation has an important influence upon the quant.i.ty of food demanded for the bodily support. Those who work long and hard at physical labor, need a generous amount of nutritious food. A liberal diet of the cereals and lean meat, especially beef, gives that vigor to the muscles which enables one to undergo laborious and prolonged physical exertion. On the other hand, those who follow a sedentary occupation do not need so large a quant.i.ty of food. Brain-workers who would work well and live long, should not indulge in too generous a diet.

The digestion of heavy meals involves a great expenditure of nervous force. Hence, the forces of the brain-worker, being required for mental exertion, should not be expended to an unwarranted extent on the task of digestion.

164. Effect of Climate. Climate also has a marked influence on the quant.i.ty of food demanded by the system. Much more food of all kinds is consumed in cold than in warm climates. The accounts by travelers of the quant.i.ty of food used by the inhabitants of the frigid zone are almost beyond belief. A Russian admiral gives an instance of a man who, in his presence, ate at a single meal 28 pounds of rice and b.u.t.ter. Dr. Hayes, the Arctic traveler, states from personal observation that the daily ration of the Eskimos is 12 to 15 pounds of meat. With the thermometer ranging from 60 to 70 F. below zero, there was a persistent craving for strong animal diet, especially fatty foods.[24]

[Ill.u.s.tration: Fig. 63.--Lymphatics and Lymphatic Glands of the Axilla.]

The intense cold makes such a drain upon the heat-producing power of the body that only food containing the largest proportion of carbon is capable of making up for the loss. In tropical countries, on the other hand, the natives crave and subsist mainly upon fruits and vegetables.

165. The Kinds of Food Required. An appet.i.te for plain, well-cooked food is a safe guide to follow. Every person in good health, taking a moderate amount of daily exercise, should have a keen appet.i.te for three meals a day and enjoy them. Food should be both nutritious and digestible.

It is nutritious in proportion to the amount of material it furnishes for the nourishment of the tissues. It is digestible in a greater or less degree in respect to the readiness with which it yields to the action of the digestive fluids, and is prepared to be taken up by the blood. This digestibility depends partly upon the nature of the food in its raw state, partly upon the effect produced upon it by cooking, and to some extent upon its admixture with other foods. Certain foods, as the vegetable alb.u.mens, are both nutritious and digestible. A hard-working man may grow strong and maintain vigorous health on most of them, even if deprived of animal food.

While it is true that the vegetable alb.u.mens furnish all that is really needed for the bodily health, animal food of some kind is an economical and useful addition to the diet. Races of men who endure prolonged physical exertion have discovered for themselves, without the teaching of science, the great value of meat. Hence the common custom of eating meat with bread and vegetables is a sound one. It is undoubtedly true that the people of this country, as a rule, eat meat too often and too much at a time. The judicious admixture of different cla.s.ses of foods greatly aids their digestibility.

The great abundance and variety of food in this country, permit this principle to be put into practice. A variety of mixed foods, as milk, eggs, bread, and meat, are almost invariably a.s.sociated to a greater or less extent at every meal.

Oftentimes where there is of necessity a sameness of diet, there arises a craving for special articles of food. Thus on long voyages, and during long campaigns in war, there is an almost universal craving for onions, raw potatoes, and other vegetables.

166. Hints about Meals. On an average, three meals each day, from five to six hours apart, is the proper number for adults. Five hours is by no means too long a time to intervene between consecutive meals, for it is not desirable to introduce new food into the stomach, until the gastric digestion of the preceding meal has been completed, and until the stomach has had time to rest, and is in condition to receive fresh material. The stomach, like other organs, does its work best at regular periods.[25]

Eating out of mealtimes should be strictly avoided, for it robs the stomach of its needed rest. Food eaten when the body and mind are wearied is not well digested. Rest, even for a few minutes, should be taken before eating a full meal. It is well to lie down, or sit quietly and read, fifteen minutes before eating, and directly afterwards, if possible.

Severe exercise and hard study just after a full meal, are very apt to delay or actually arrest digestion, for after eating heartily, the vital forces of the body are called upon to help the stomach digest its food. If our bodily energies are compelled, in addition to this, to help the muscles or brain, digestion is r.e.t.a.r.ded, and a feeling of dullness and heaviness follows. Fermentative changes, instead of the normal digestive changes, are apt to take place in the food.

167. Practical Points about Eating. We should not eat for at least two or three hours before going to bed. When we are asleep, the vital forces are at a low ebb, the process of digestion is for the time nearly suspended, and the retention of incompletely digested food in the stomach may cause bad dreams and troubled sleep. But in many cases of sleeplessness, a trifle of some simple food, especially if the stomach seems to feel exhausted, often appears to promote sleep and rest.

[NOTE. The table on the next page shows the results of many experiments to ill.u.s.trate the time taken for the gastric digestion of a number of the more common solid foods. There are a good many factors of which the table takes no account, such as the interval since the last meal, state of the appet.i.te, amount of work and exercise, method of cooking, and especially the quant.i.ty of food.]

Table Showing the Digestibility of the More Common Solid Foods.

Food How Time in Cooked Stomach, Hours ------------------------------------------------- Apples, sweet and mellow Raw 1 Apples, sour and hard " 2 Apple Dumpling Boiled 3 Ba.s.s, striped, fresh Broiled 3 Beans, pod Boiled 2 Beef, with salt only " 2 " fresh, lean Raw 3 " " " Fried 4 " " " Roasted 3 " old, hard, salted Boiled 4 Beefsteak Broiled 3 Beets Boiled 3 Bread, corn Baked 3 " wheat, fresh " 3 b.u.t.ter Melted 3 Cabbage, with vinegar Raw 2 " " " Boiled 4 " heads Raw 2 Carrots Boiled 3 Cheese, old, strong Raw 3 Chicken, full-grown Frica.s.see 2 " soup Boiled 3 Codfish, cured, dried " 2 Corncake Baked 2 Custard " 2 Duck, domestic Roasted 4 " wild " 4 Eggs, fresh, whipped Raw 1 " " 2 " soft-boiled Boiled 3 " hard-boiled " 3 " Fried 3 Fowl, domestic Boiled 4 " " Roasted 4 Gelatin Boiled 2 Goose Roasted 2 Green corn and beans Boiled 3 Hash, meat and vegetables Warmed 2 Lamb Broiled 2 Liver " 2 Milk Boiled 2 " Raw 2 Mutton, fresh Broiled 3 " " Boiled 3 " " Roasted 3 Oysters, fresh Raw 2 " " Roasted 3 " " Stewed 3 Parsnips Boiled 2 Pig Roasted 2 Pig's feet, soused Boiled 1 Pork, recently salted " 4 " Fried 4 " Raw 3 " steaks Fried 3 " Stewed 3 " fat or lean Roasted 5 Potatoes Baked 2 " Boiled 3 " Roasted 2 Rice Boiled 1 Sago " 1 Salmon, salted " 4 Soup, barley " 1 " beans " 3 " beef, vegetables, bread " 4 " marrow bone " 4 " mutton " 3 Sponge Cake Baked 2 Suet, beef, fresh Boiled 5?

" mutton " 4 Tapioca " 2 Tripe, soused " 1 Trout, salmon, fresh " 1 " " " Fried 1 Turkey, wild Roasted 2 " domestic Boiled 2 " " Roasted 2 Turnips Boiled 3 Veal Roasted 4 " Fried 4 Venison, steaks Broiled 1

The state of mind has much to do with digestion. Sudden fear or joy, or unexpected news, may destroy the appet.i.te at once. Let a hungry person be anxiously awaiting a hearty meal, when suddenly a disastrous telegram is brought him; all appet.i.te instantly disappears, and the tempting food is refused. Hence we should laugh and talk at our meals, and drive away anxious thoughts and unpleasant topics of discussion.

The proper chewing of the food is an important element in digestion.

Hence, eat slowly, and do not "bolt" large fragments of food. If imperfectly chewed, it is not readily acted upon by the gastric juice, and often undergoes fermentative changes which result in sour stomach, gastric pain, and other digestive disturbance.