The digestion and absorption of food are such fundamental facts in physiology that a thoroughly intelligent comprehension of them is of great importance; at the same time they are so largely merely chemico-physical phenomena that they are readily illustrated by a few simple experiments. These described below take but little time and cost but little money, while they cannot fail to be of value not merely in interesting a class, but in giving its members a much better idea of the way in which food is digested than they can get from merely reading a book.

1. Anatomy Of The Alimentary Canal

Kill a rat by chloroform or drowning. Dissect away the skin from the whole ventral aspect of the body.

Note in the neck region the large salivary glands which meet in the middle line: the posterior gland, close to the middle line, rounded and compact, is the submaxillary; on raising it, its duct will be seen passing forwards to the mouth, into which it may be followed by separating the halves of the lower jaw.

The large gland, composed of several loosely united lobes, and reaching from the neighborhood of the ear to the submaxillary, is the parotid. Its duct will be found passing forwards over the face to the mouth, near the angle of which it passes in through the cheek muscles.

In front of the submaxillary will be found a small gland, the sublingual.

Remove the muscles, etc., covering the larynx and trachea; cut away the front and side walls of the chest and abdomen; remove larynx, trachea, lungs, and heart.

The gullet, a slender muscular tube, will now be exposed in the neck; trace it through the chest; note the relative positions of the abdominal viscera as now exposed, before displacing any of them; then turning the liver up out of the way, follow the gullet in the abdomen until it ends in the stomach.

Note the form of the latter orgau; its projection (fundus) to the left of the entry of the gullet; its great and small curvatures; its narrower pyloric portion on the right, from which the small intestine proceeds. Attached to the stomach, and hanging down over the other abdominal viscera, notice a thin membrane, the omentum.

Follow and unravel the coils of the small intestine, spreading out as far as possible the delicate membrane (mesentery) which slings it. In the mesentery are numerous bauds of fat, running in which will be seen blood-vessels and lacteals.

The termination of the small intestine by opening into the side of the large. Observe the cæcum or blind end of the latter, projecting on one side of the point of entry of the small intestine; on the other side follow the large intestine until it ends at the anal aperture, cutting away the front of the pelvis to follow its terminal portion (rectum). The portion between the cæcum and the rectum is the colon.

Spread out the portion of the mesentery lying in the concavity of the first coil (duodenum) of the small intestine; in it will be seen a thin branched glandular mass, the pancreas.

Observe the portal vein entering the under side of the liver by several branches. Alongside it will be seen the gall-duct, formed by the union of two main branches, and proceeding, as a slender tube, to open into the duodenum about an inch and a half from the pyloric orifice of the stomach.

Note the spleen: an elongated red body lying in the mesentery, behind and to the left of the stomach.

Divide the gullet at the top of the neck, and the rectum close to the anus, and severing mesenteric bands, etc., by which intermediate portions of the alimentary canal are fixed, remove the whole tube; then cutting away the mesentery, spread it out at full length, and note the relative length and diameter of its various parts. The whole is seven or eight times as long as the head and trunk of the animal, and the small intestine forms by far the longest part of it.

Open the stomach; note that the mucous membrane lining the fundus is thin and smooth, and is sharply marked off from the thick corrugated mucous membrane lining the rest of the organ. (This is not the case in the human stomach.) Pass probes through the cardiac orifice into the gullet and through the pyloric orifice into the duodenum.

Remove the liver; note its general form.

Obtain from your butcher an inch or two of the small intestine of a recently killed calf. Place in 50 per cent, alcohol for twenty-four hours. Then open under water and examine with a hand lens to see the villi.

2. The Action Of Saliva On Starch

Make a thin paste of good arrowroot (which is almost pure starch) with boiling water. Let it cool.

a. Add two or three drops Of this starch paste to half a test tubeful of cold water; next add three or four drops of solution of caustic potash and two or three drops of dilute watery solution of blue vitriol (cupric sulphate). Mix thoroughly and boil over a spirit lamp. No orange-red precipitate will result. This shows that there is no grape sugar in the starch paste.

b. Rinse the mouth thoroughly and then collect a small quantity of saliva in a test tube. Dilute with water. Add caustic potash and cupric sulphate solutions as above; mix thoroughly and boil. The mixture will become violet, but give no orange-red precipitate, therefore there is no grape sugar in saliva.

e. Take now three drops of the starch paste and a teaspoonful of saliva; mix with a half test tubeful of water. Place the mixture in a moderately warm place for five minutes. Then add a few drops of the caustic potash and cupric sulphate solutions; mix and boil. An abundant orange or brick-red precipitate will be thrown down, proving the presence of grape sugar, which has been produced by the action of the saliva on the starch.

3. Gastric Digestion

a. Obtain a pig's stomach. Cut it open and wash away its contents with a gentle stream of water. Then dissect off the mucous membrane from its middle part, mince and put aside for a couple of days in three or four ounces of glycerine. The glycerine dissolves the pepsin. Then strain off the glycerine through muslin.

b. Get a butcher to " whip" some fresh drawn blood for you with a bunch of wire or twigs. The blood fibrin will collect on these (p. 181), and when thoroughly washed with water, forms a good proteid for digestion experiments. One lot of it thus obtained and washed may be put aside in 50 per cent, alcohol, and will provide material for digestion experiments for years.

c. Add a teaspoonful of muriatic acid to a pint of water.

d. Dilute a teaspoonful of the pepsin solution a with two table-spoonfuls of water. Fill a test tube with the mixture; add a few shreds of washed fibrin, and set aside in a warm but not hot place for twenty-four hours. No change will occur, showing that pepsin alone will not dissolve proteids.

e. Put some shreds of fibrin in a test tube of the mixture c in a warm place for twenty-four hours. The fibrin will swell up and become translucent, but will not dissolve. This shows that dilute acids will not in a short time dissolve proteids.

f. Half fill a test tube with the mixture c, add a teaspoonful of the pepsin solution a, and then a few shreds of fibrin. Place in a warm place for twenty-four hours. The fibrin will be more or less completely dissolved at the end of that time. We thus find that pepsin alone and dilute acid alone (at least in a moderate time) will not dissolve proteids, but that acting together they quickly effect a solution.

4. The Action Of Bile On Fatty Substances

a. Shake up some olive oil with water in a test tube. The two liquids soon separate when the shaking ceases.

b. Obtain an ox gall from the butcher. Cut it open and collect the bile. (The bile of herbivorous animals differs from human bile in being green in color.) Shake up some oil with bile instead of water. A creamy emulsion is formed from which the oil only slowly floats up to the top.

5. The Action Of The Pancreatic Secretion On Fats

a. Obtain a pig's pancreas; mince, and extract with about its own bulk of water for two or three hours. Strain off the watery infusion. Add to it half its bulk of oil in a test tube and shake thoroughly. The oil will be very thoroughly emulsified; and separate very slowly on standing.

6. Action Of Pancreatic Secretion On Starch

With some of the watery extract of pancreas perform the experiments described above under heading 2; substituting pancreatic extract for saliva.

7. Action Of Pancreatic Secretion On Proteids

a. Obtain a fresh pig's pancreas. Lay aside in a cool place for twenty-four hours. Mince, and extract for two days with twice its bulk of glycerine. Strain off the glycerine extract.

b. Dilute the glycerine extract with ten times its bulk of water. Place part of this mixture in a test tube together with some fibrin shreds, and put aside in a warm place. After twenty-four hours none of the fibrin will have been dissolved.

c. To the diluted glycerine extract as above add a teaspoonful of dilute acid (3 c). The fibrin will swell but not dissolve.

d. To another portion of the diluted glycerine extract add just sufficient bicarbonate of soda to make it distinctly alkaline, as tested by litmus paper. Then put in some fibrin and set aside in a warm place for a day. The fibrin will be more or less completely dissolved. We thus find that the pancreas affords a substance which, in the presence of weak alkalies,dissolves proteids.

The fat-absorbing power of the lymphatics of the small intestine is very readily demonstrable, without giving pain to an animal or any unnecessary destruction of life. In most families superfluous kittens or puppies have to be killed soon after birth. Feed a kitten or puppy on rich milk, and three hours after place it in a box or under a bell-jar with a sponge soaked with ether or chloroform. When the animal is completely insensible cut off its head, and then rapidly open the abdomen and spread out the mesentery (the thin membrane which slings the small intestine). In it will be seen a beautiful network of lacteal vessels filled with milk-white liquid, some of which can be collected if one of the lacteals be cut open. For comparison a kitten or puppy may be used which has had no food for eight or ten hours. The lacteals being then filled with clear, watery-looking lymph, will be recognized with difficulty.