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

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

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

*To ill.u.s.trate Acid and Alkaline Reactions.*-To a tumbler half full of water add a teaspoonful of hydrochloric or other acid, as vinegar. To a second tumbler half full of water add an equal amount of cooking soda.

Taste each liquid, noting the sour taste of the acid, and the alkaline taste of the soda. Hold a piece of red litmus paper in the soda solution, noting that it is turned blue. Then hold a piece of blue litmus paper in the acid solution, noting that it is turned red. Add acid to the soda solution, and soda to the acid solution, until the conditions are reversed, testing with the red and blue litmus papers.

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

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

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

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

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

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

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

CHAPTER XI - ABSORPTION, STORAGE, AND a.s.sIMILATION

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

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

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

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

2. A small lymph tube, called a _lacteal_, which occupies the center of the villus and connects at the base with other lymph tubes, also called lacteals (_B_, Fig. _76_).

3. A network of capillaries.

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

[Fig. 76]

Fig. 76-*The villi.* _A._ Diagram of a small section of mucous membrane of small intestine. 1. Villi. 2. Small glands, called _crypts_.

_B._ Diagram showing structure of villi. 1. Small artery. 2. Lacteal. 3.

Villus showing termination of the lacteal. 4. Villus showing capillaries.

5. Villus showing both the lacteal and the capillaries. 6. Small vein. 7.

Layer of epithelial cells.

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

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

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

[Fig. 77]

Fig. 77-*Diagram of routes* from food ca.n.a.l to general circulation. See text.

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

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

While considerable doubt exists as to the cause of these changes and as to the places also where some of them occur, their purpose is quite apparent.

The materials forming the dissolved foods, although adapted to absorption, are not suited to the needs of the body, and if introduced in this form are likely to interfere with its work.(67) They are changed, therefore, into the forms which the body can use.

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

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

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

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

[Fig. 78]

Fig. 78-*Liver cells* where is stored the glycogen. _C._ Capillaries.

[Fig. 79]

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

Connective tissue fibers.

2. _The fat_ is stored for the most part in the connective tissue. Certain of the connective tissue cells have the property of taking fat from the blood and of depositing it within their inclosing membranes (Fig. 79).

When this is done to excess, and the cells become filled with fat, they form the so-called _adipose tissue_. Most of this tissue is found under the skin, between the muscles, and among the organs occupying the abdominal cavity. If one readily takes on fat, it may also collect in the connective tissue around the heart. The stored-up fat is redissolved as needed, and enters the blood, where it again becomes available to the active cells.

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