CHAPTER VI.

DIGESTION.

_The Princ.i.p.al Processes of Nutrition--The General Plan of Digestion--Mastication--The Teeth--Preservation of the Teeth--Insalivation--The Stomach and the Gastric Juice--The Movements of the Stomach--Gastric Digestion--The Intestines--The Bile and Pancreatic Juice--Intestinal Digestion--Absorption by means of Blood-vessels and Lacteals--The Lymphatic or Absorbent System--The Lymph--Conditions which affect Digestion--The Quality, Quant.i.ty, and Temperature of the Food--The Influence of Exercise and Sleep._

[Sidenote: 1. Design of food? How accomplished?]

1. NUTRITION.--The great design of food is to give _nutriment_ or nourishment to the body. But this is not accomplished directly, as the food must first pa.s.s through certain preparatory changes, as follows: (1), _Digestion_, by which the food is reduced to a soluble condition; (2), _Absorption_, by which, when digested, it is imbibed into the blood; (3), _Circulation_, which carries the enriched blood to the various parts of the system; and (4), _a.s.similation_, by which each tissue derives from the blood the materials necessary for its support.

[Sidenote: 2. Sustaining power of food? Simile of the engine? Operation in the human body?]

2. By these means the sustaining power of food is gradually developed and employed, and the vital machinery kept in working order, somewhat after the manner of the steam-engine. To operate the latter, the force imprisoned within the coal and water is set free and converted into motion by the combustion of the fuel and the vaporization of the water. It will be seen, however, when we come to study these operations in the human body, that they are conducted silently and harmoniously, with marvellous delicacy and completeness, and without that friction, and {81} consequent loss of power, which attend the working of the most perfect machinery of man's invention.

[Sidenote: 3. Change of food in digestion? Process of digestion? Describe the alimentary ca.n.a.l.]

3. GENERAL PLAN OF DIGESTION.--The great change which food undergoes in digestion is essentially a reforming process, reducing articles of diet, which are at first more or less solid, crude, and coa.r.s.e, to a liquid and finely comminuted condition, suitable for absorption into the blood. The entire process of digestion takes place in what is called the alimentary ca.n.a.l, a narrow, tortuous tube, about thirty feet in its entire length.

This ca.n.a.l begins in the mouth, extends thence downward through the gullet to the stomach (a receptacle in which the princ.i.p.al work of digestion is performed), and thence onward through the small and large intestines.

[Ill.u.s.tration: FIG. 16.--SECTION OF THE TRUNK SHOWING THE CAVITIES OF THE CHEST AND ABDOMEN.

A, Cavity of Chest; B, Diaphragm; C, Abdomen; D, E, Spinal Column.]

[Sidenote: 4. Situation of the stomach and intestines? Action of the food?

Mechanical action? Chemical?]

4. The stomach and intestines are situated in the cavity of the abdomen (Fig. 16, C, and Fig. 22), and occupy about two-thirds of its s.p.a.ce. The action to which the food is subjected in these organs is of two kinds--mechanical and chemical. By the former it is crushed, agitated, and carried onward from one point to another; by the latter it is changed in form through the solvent power of the various digestive juices. {82}

[Sidenote: 5. Describe the process of mastication? How many and what movements?]

5. MASTICATION.--As soon as solid food is taken into the mouth, it undergoes mastication, or chewing. It is caught between the opposite surfaces of the teeth, and by them is cut and crushed into very small fragments. In the movements of chewing, the lower jaw plays the chief part; the upper jaw, having almost no motion, acts simply as a point of resistance, to meet the action of the former. These movements of the lower jaw are of three sorts: a vertical or cutting, a lateral or grinding, and a _to-and-fro_ or gnawing motion.

[Ill.u.s.tration: FIG. 17.--SECTION OF A TOOTH.

_a_, Enamel; _b_, Cavity; _c c_, Roots; _d_, Body of the Tooth.]

[Sidenote: 6. Composition of the teeth? Enamel of the teeth? Interior of teeth?]

6. The teeth are composed of a bone-like material, and are held in place by roots running deeply into the jaw. The exposed portion, or "crown," is protected by a thin layer of enamel (Fig. 17, _a_), the hardest substance in the body, and, like flint, is capable of striking fire with steel. In the interior of each tooth is a cavity, containing blood-vessels and a nerve, which enter it through a minute opening at the point of the root (Fig. 19).

[Sidenote: 7. The milk teeth? The permanent teeth?]

7. There are two sets of teeth; first, those belonging to the earlier years of childhood, called the milk teeth, which are twenty in number and small.

At six or eight years of age, when the jaw expands, and when the growing body requires a more powerful and numerous set, the roots of {83} the milk teeth are absorbed, and the latter are "shed," or fall out, one after another (Fig. 18), to make room for the permanent set.

[Ill.u.s.tration: FIG. 18.--SECTION OF THE JAWS.

1' 2' 3' 4' 5', The Milk Teeth; 1" to 8", The Germs of the Permanent Set.]

[Sidenote: 8, 9. Number of teeth? How distributed?]

8. There are thirty-two teeth in the permanent set, as many being in one jaw as the other. Each half-jaw has eight teeth, similarly shaped and arranged in the same order: thus, two incisors, one canine, two bicuspids, and three molars. The front teeth are small, sharp, and chisel-edged, and are well adapted for cutting purposes; hence their name incisors. The canines stand next, one on each side of the jaw; these receive their name from their resemblance to the long, pointed tusks of the dog (Fig. 19).

{84}

[Ill.u.s.tration: FIG. 19.--SECTION OF THE JAWS--RIGHT SIDE.

V, A, N, Veins, Arteries, and Nerves of the Teeth. The root of one tooth in each jaw is cut vertically to show the cavity and the blood-vessels, etc., within it. 1 to 8, Permanent Teeth.]

9. The bicuspids, next in order, are larger and have a broader crown than the former; while behind them are the molars, the largest and most powerful of the entire set. These large back teeth, or "grinders," present a broad, rough surface, suitable for holding and crushing the food. The third molar, or "wisdom tooth," is the last to be cut, and does not appear until about the twenty-first year. {85} The order of arrangement of the teeth is indicated by the following dental formula:--

[Ill.u.s.tration]

[Sidenote: 10. Different forms of teeth? Human teeth? The inference?]

10. It is interesting, at this point, to notice the different forms of teeth in different animals, and observe how admirably their teeth are suited to the respective kinds of food upon which they subsist. In the _carnivora_, or flesh-feeders, the teeth are sharp and pointed, enabling them both to seize their prey, and tear it in pieces; while the _herbivora_, or vegetable-feeders, have broad, blunt teeth, with rough crowns, suitable for grinding the tough gra.s.ses and grains upon which they feed. Human teeth partake of both forms; some of them are sharp, and others are blunt; they are therefore well adapted for the mastication of both flesh and vegetables. Hence we argue that, although man may live exclusively upon either vegetable or animal food, he should, when possible, choose a diet made up of both varieties.

[Sidenote: 11. Cleaning of teeth? Effects of not cleaning?]

11. PRESERVATION OF THE TEETH.--In order that the teeth shall remain in a sound and serviceable condition, some care is of course requisite. In the first place, they require frequent cleansing; for every time we take food, some particles of it remain in the mouth; and these, on account of the heat and moisture present, soon begin to putrefy. This not only renders the breath very offensive, but promotes decay of the teeth. {86}

[Sidenote: 12. Effects upon the saliva? Formation of tartar? How prevented?

How destroyed?]

12. The saliva, or moisture of the mouth, undergoes a putrefactive change, and becomes the fertile soil in which a certain minute fungus has its growth. This fluid, too, if allowed to dry in the mouth, collects upon the teeth in the form of an unsightly, yellow concretion, called tartar. To prevent this formation, and to remove other offensive substances, the teeth should be frequently cleaned with water, applied by means of a soft tooth-brush. The destruction of the tartar fungus is best effected by the use of a weak solution of carbolic acid.

[Sidenote: 13. Destruction of the enamel? How guarded against?]

13. Again, it should be borne in mind that the enamel, Nature's protection for the teeth, when once destroyed, is not formed anew; and the body of the tooth thus exposed, is liable to rapid decay. On this account, certain articles are to be guarded against; such as sharply acid substances that corrode the enamel, and hard substances that break or scratch it--as gritty tooth powders, metal tooth picks, and the sh.e.l.ls of hard nuts. Sudden alternations from heat to cold, when eating or drinking, also tend to crack the enamel.

[Sidenote: 14. Mixing of food with the saliva? What is the saliva? How secreted? The salivary glands?]

14. INSALIVATION.--When the morsel of food is cut and ground by the teeth, it is at the same time also intimately mixed with the saliva, or fluids of the mouth. This const.i.tutes the second step of digestion, and is called insalivation. The saliva, the first of the digestive solvents, is a colorless, watery, and frothy fluid. It is secreted (_i. e._ separated from the blood) partly by the mucous membrane which lines the mouth; but chiefly by the salivary glands, of which there are three pairs situated near the mouth.

[Sidenote: 15. The flow of saliva? The thought of food? Anxiety and grief?

Animals fed upon dry and coa.r.s.e food?]

{87} 15. These glands consist of cl.u.s.ters of very small pouches, around which a delicate network of blood-vessels is arranged: they empty into the mouth by means of little tubes, or ducts. The flow from these glands is at all times sufficient to maintain a soft and moist condition of the tongue and mouth; but when they are excited by the presence and taste of food, they pour forth the saliva more freely. Even the mere thought of food will at times cause the saliva to flow, as when the appet.i.te is stimulated by the sight or smell of some savory article; so that the common expression is correct that "the mouth waters" for the favorite articles of food. Anxiety and grief prevent its flow, and cause "the tongue to cleave to the roof of the mouth." In the horse and other animals, that feed upon dry and coa.r.s.e fodder, and require an abundant supply of saliva, we find large salivary glands, as well as powerful muscles of mastication.

[Ill.u.s.tration: FIG. 20.--STRUCTURE OF A SALIVARY GLAND.]

[Ill.u.s.tration: FIG. 21.--THE HEAD OF A HORSE, showing the large salivary gland (_a_), its duct (_b_), the muscles of mastication (_c_, _d_, _e_, _f_, and _g_).]

[Sidenote: 16. Importance of the process? The first place? The second? The third?]

16. The mingling of the saliva with the food seems a simple process, but it is one that plays an important part {88} in digestion. In the first place, it facilitates the motions of mastication, by moistening the food and lubricating the various organs of the mouth. Secondly, it prepares the way for other digestive acts: by the action of the teeth, the saliva is forced into the solid food, softens the harder substances, and a.s.sists in converting the whole morsel into a semi-solid, pulpy ma.s.s, that can be easily swallowed, and readily permeated by other digestive fluids. The saliva also, by dissolving certain substances, as sugar and salt, develops the peculiar taste of each; whereas, if the tongue be dry and coated, they are tasteless. Hence, if substances are insoluble, they are devoid of taste.

[Sidenote: 17. Its final importance? Starch? How effected? Ptyalin?]

17. Finally, the saliva has the property of acting chemically upon the food. As we have before stated (Chap. IV.), starch, as starch, cannot enter the tissues of the body; but, in order to become nutriment, must first be changed to grape sugar. This change is, in part, effected by the saliva, and takes place almost instantly, whenever it comes in contact with cooked starch. This important function is due to an organic ingredient of the saliva called _ptyalin_. This substance has been extracted from the saliva by the chemist, and has been found, by experiment, to convert into sugar two thousand times its own weight of starch.