*Tests for Nutrients.* _Proteids._-Cover the substance to be tested with strong nitric acid and heat gradually to boiling. If proteid is present it turns yellow and partly dissolves in the acid, forming a yellow solution.

Let cool and then add ammonia. The yellow solid and the solution are turned a deep orange color. Apply this test to foods containing proteid such as white of egg, cheese, lean meat, etc.

_Starch._-_(a)_ Place a small lump of starch in one fourth of a pint of water and heat gradually to boiling, stirring well. Then add enough water to form a thin liquid and fill a test tube half full. Add to this a few drops of a solution of iodine. (Prepare by dissolving a crystal of iodine in 25 cubic centimeters (1/20 pint) of a solution of pota.s.sium iodide in water and add water to this until it is a light amber color.) The starch solution is turned blue, _(b)_ Cut with a razor a thin slice from a potato. Place this in a weak solution of iodine for a few minutes and then examine with the microscope, using first a low and then a high power.

Numerous starch grains inclosed in cellulose walls will be seen (Fig. 60).

_Dextrose, or Grape Sugar._-Place a solution of the substance supposed to contain grape sugar in a test tube and add a few drops of a dilute solution of copper sulphate. Then add sodium hydroxide solution until the precipitate which first forms is redissolved and a clear blue liquid obtained. Heat the upper portion of the liquid slowly to near the boiling point. A little below the boiling point the blue color disappears and a yellow-red precipitate is formed. If the upper layer of the liquid is now boiled, the color deepens and this may be contrasted with the blue color below. Apply this test to the sugar in raisins and in honey.

_Fat._-Fat is recognized by its effect on paper, making a greasy stain which does not disappear on heating and which renders the paper translucent. Try b.u.t.ter, lard, or olive oil. Also show the presence of fat in peanuts by crushing them in a mortar and rubbing the powder on thin paper. If the substance to be tested contains but little fat, this may be dissolved out with ether. If a drop of ether containing the fat is placed on paper, it evaporates, leaving the fat, which then forms the stain.

*To show the Effect of Alcohol upon Proteid.*-Place some of the white of a raw egg in a gla.s.s vessel and cover it with a small amount of alcohol. As the alb.u.min (proteid) hardens, or coagulates, observe that the quant.i.ty of clear liquid increases. This is due to the _withdrawal_ of water from the alb.u.min by the alcohol. Since the tissues are made up chiefly of proteids, a piece of muscle or of liver may be used in the experiment, instead of the egg, with similar results.

*To ill.u.s.trate the Digestive Process.*-To a tumbler two thirds full of water add a little salt. Stir and observe that the salt is dissolved.

Taste the solution to see that the salt has not been changed chemically.

Now add a little powdered limestone to the water and stir as before.

Observe that the limestone does not dissolve. Then add some hydrochloric acid and observe the result. State the part played by the acid and by the water in dissolving the limestone. Apply to the digestion of the different cla.s.ses of foods.

CHAPTER X - ORGANS AND PROCESSES OF DIGESTION

The organs of digestion are adapted to the work of dissolving the foods by both their structure and arrangement. Most of them consist either of tubes or cavities and these are so connected, one with the other, as to form a continuous pa.s.sageway entirely through the body. This pa.s.sageway is known as

*The Alimentary Ca.n.a.l. *-The alimentary ca.n.a.l has a length of about thirty feet and, while it begins at the mouth, all but about eighteen inches of it is found in the abdominal cavity. On account of its length it lies for the most part in coils, the two largest ones being known as the small intestine and the large intestine. Connected with the alimentary ca.n.a.l are the glands that supply the liquids for acting on the food. The divisions of the ca.n.a.l and most of the glands that empty liquids into it are shown in Fig. 63 and named in the table below:

[Table]

*Coats of the Alimentary Ca.n.a.l.*-The walls of the alimentary ca.n.a.l, except at the mouth, are distinct from the surrounding tissues and consist in most places of at least three layers, or coats, as follows:

[Fig. 63]

Fig. 63-*Diagram of the digestive system.* 1. Mouth. 2. Soft palate. 3.

Pharynx. 4. Parotid gland. 5. Sublingual gland. 6. Submaxillary gland. 7.

Esophagus. 8. Stomach. 9. Pancreas. 10. Vermiform appendix. 11. Caec.u.m. 12.

Ascending colon. 13. Transverse colon. 14. Descending colon. 15. Sigmoid flexure. 16. r.e.c.t.u.m. 17. Ileo-caecal valve. 18. Duct from liver and pancreas. 19. Liver.

Diagram does not show comparative length of the small intestine.

1. An _inner coat_, or lining, known as the mucous membrane. This membrane is not confined to the alimentary ca.n.a.l, but lines, as we have seen, the different air pa.s.sages. It covers, in fact, all those internal surfaces of the body that connect with the external surface. It derives its name from the substance which it secretes, called _mucus_. In structure it resembles the skin, being continuous with the skin where cavities open to the surface. It is made up of two layers-a thick underlayer which contains blood vessels, nerves, and glands, and a thin surface layer, called the _epithelium._ The epithelium, like the cuticle, is without blood vessels, nerves, or glands.

2. A _middle coat_, which is muscular and which forms a continuous layer throughout the ca.n.a.l, except at the mouth. (Here its place is taken by the strong muscles of mastication which are separate and distinct from each other.) As a rule the muscles of this coat are involuntary. They surround the ca.n.a.l as thin sheets and at most places form two distinct layers. In the inner layer the fibers encircle the ca.n.a.l, but in the outer layer they run longitudinally, or lengthwise, along the ca.n.a.l.(57)

3. An _outer_ or _serous coat_, which is limited to those portions of the ca.n.a.l that occupy the abdominal cavity. This coat is not found above the diaphragm. It is a part of the lining membrane of the cavity of the abdomen, called

[Fig. 64]

Fig. 64-*Diagram of the peritoneum.* 1. Transverse colon. 2. Duodenum. 3.

Small intestine. 4. Pancreas.

*The Peritoneum.*-The peritoneum is to the abdominal cavity what the pleura is to the thoracic cavity. It forms the outer covering for the alimentary ca.n.a.l and other abdominal organs and supplies the inner lining of the cavity itself. It is also the means of holding these organs in place, some of them being suspended by it from the abdominal walls (Fig.

64). By the secretion of a small amount of liquid, it prevents friction of the parts upon one another.

*Digestive Glands.*-The glands which provide the different fluids for acting on the foods derive their const.i.tuents from the blood. They are situated either in the mucous membrane or at convenient places outside of the ca.n.a.l and pa.s.s their liquids into it by means of small tubes, called ducts. In the ca.n.a.l the food and the digestive fluids come in direct contact-a condition which the dissolving processes require. Each kind of fluid is secreted by a special kind of gland and is emptied into the ca.n.a.l at the place where it is needed.

*The Digestive Processes.*-Digestion is accomplished by acting upon the food in different ways, as it is pa.s.sed along the ca.n.a.l, with the final result of reducing it to the form of a solution. Several distinct processes are necessary and they occur in such an order that those preceding are preparatory to those that follow. These processes are known as _mastication, insalivation, deglut.i.tion, stomach digestion_, and _intestinal_ digestion. As the different materials become liquefied they are transferred to the blood, and substances not reduced to the liquid state are pa.s.sed on through the ca.n.a.l as waste. The first two of the digestive processes occur in

*The Mouth.*-This is an oval-shaped cavity situated at the very beginning of the ca.n.a.l. It is surrounded by the lips in front, by the cheeks on the sides, by the hard palate above and the soft palate behind, and by the tissues of the lower jaw below. The mucous membrane lining the mouth is, soft and smooth, being covered with flat epithelial cells. The external opening of the mouth is guarded by the lips, and the soft palate forms a _movable_ part.i.tion between the mouth and the pharynx. In a condition of repose the mouth s.p.a.ce is practically filled by the teeth and the tongue, but the cavity may be enlarged and room provided for food by depressing the lower jaw.

The mouth by its construction is well adapted to carrying on the processes of mastication and insalivation. By the first process the solid food is reduced, by the cutting and grinding action of the teeth, to a finely divided condition. By the second, the saliva becomes mixed with the food and is made to act upon it.

[Fig. 65]

Fig. 65-*The teeth.* _A._ Section of a single molar. 1. Pulp. 2. Dentine.

3. Enamel. 4. Crown. 5. Neck. 6. Root. _B._ Teeth in position in lower jaw. 1. Incisors. 2. Canine. 3. Biscuspids. 4. Molars. _C._ Upper and lower teeth on one side. 1. Incisors. 2. Canines. 3. Biscuspids. 4.

Molars. 5. Wisdom. _D._ Upper and lower incisor, to show gliding contact.

*Accessory Organs of the Mouth.*-The work of mastication and insalivation is accomplished through organs situated in and around the mouth cavity.

These comprise:

1. _The Teeth._-The teeth are set in the upper and lower jaws, one row directly over the other, with their hardened surfaces facing. In reducing the food, the teeth of the lower jaw move against those of the upper, while the food is held by the tongue and cheeks between the grinding surfaces. The front teeth are thin and chisel-shaped. They do not meet so squarely as do the back ones, but their edges glide over each other, like the blades of scissors-a condition that adapts them to cutting off and separating the food (_D_, Fig. 65). The back teeth are broad and irregular, having surfaces that are adapted to crushing and grinding.

Each tooth is composed mainly of a bone-like substance, called _dentine_, which surrounds a central s.p.a.ce, containing blood vessels and nerves, known as the _pulp cavity_. It is set in a depression in the jaw where it is held firmly in place by a bony substance, known as _cement_. The part of the tooth exposed above the gum is the _crown_, the part surrounded by the gum is the _neck_, and the part which penetrates into the jaw is the _root_ (_A_, Fig. 65). A hard, protective material, called _enamel_, covers the exposed surface of the tooth.

The teeth which first appear are known as the _temporary_, or milk, teeth and are twenty in number, ten in each jaw. They usually begin to appear about the sixth month, and they disappear from the mouth at intervals from the sixth to the thirteenth year. As they leave, teeth of the second, or _permanent_, set take their place. This set has thirty-two teeth of four different kinds arranged in the two jaws as follows:

In front, above and below, are four chisel-shaped teeth, known as the _incisors_. Next to these on either side is a tooth longer and thicker than the incisors, called the _canine_. Back of these are two short, rounded and double pointed teeth, the _bicuspids_, and back of the bicuspids are three heavy teeth with irregular grinding surfaces, called the _molars_ (_B_ and _C_, Fig. 65). Since the molar farthest back in each jaw is usually not cut until maturity, it is called a _wisdom_ tooth. The molars are known as the superadded permanent teeth because they do not take the place of milk teeth, but form farther back as the jaw grows in length.

[Fig. 66]

Fig. 66-*Diagram* showing directions of muscular fibers in tongue.

2. _The Tongue._-The tongue is a muscular organ whose fibers extend through it in several directions (Fig. 66). Its structure adapts it to a variety of movements. During mastication the tongue transfers the food from one part of the mouth to another, and, with the aid of the cheeks, holds the food between the rows of teeth. (By an outward pressure from the tongue and an inward pressure from the cheek the food is kept between the grinding surfaces.) The tongue has functions in addition to these and is a most useful organ.

3. _The Muscles of Mastication._-These are attached to the lower jaw and bring about its different movements. The _ma.s.seter_ muscles, which are the heavy muscles in the cheeks, and the _temporal_ muscles, located in the region of the temples, raise the lower jaw and supply the force for grinding the food. Small muscles situated below the chin depress the jaw and open the mouth.

[Fig. 67]

Fig. 67-*Salivary glands* and the ducts connecting them with the mouth.