Test the contents of the different tubes for peptone with the biuret test. (See below.)1

Action of Gastric Juice.—Most proteid substances are insoluble in water. They belong to the class of substances known as "colloids" - substances that do not easily pass through a membrane by osmosis. Protein is changed by the pepsin of the gastric fluid to a substance readily soluble. After protein is digested it is known as a peptone. Digestion of proteid results in a change of a colloid substance to one which will diffuse readily through a membrane, or a crystalloid. Peptones are crystalloid substances.

Gastric juice acts most perfectly at the temperature of the normal body. The enzyme pepsin will not act in an alkaline medium; boiling or freezing prevents its action as well. A slightly acid medium is necessary for proteid digestion in the stomach.

The other enzyme of gastric juice, called rennin, curdles or coagulates the proteid found in milk; after the milk is curdled, then the enzyme, pepsin, is able to act upon it and change it to a soluble crystalloid substance. The hydrochloric acid found in the gastric juice acts upon lime and some other salts taken into the stomach with food.

EXPERIMENT. - Add, drop by drop, very dilute hydrochloric acid to a test tube containing limewater. Notice the change that takes place. By this means lime is changed from an insoluble to a soluble form and can be absorbed into the blood, to be later used in the building of bone.

MOVEMENT OF Walls of StomaCH. — The stomach walls, provided with three layers of muscle which run in an oblique, circular, and longitudinal direction (taken from the inside outward), are well fitted for the constant churning of the food in that organ. Here, as elsewhere in the digestive tract, the muscles are involuntary, muscular action being under the control of the so-called sympathetic nervous system. Food material in the stomach makes several complete circuits during the process of digestion in that organ. While this is taking place, the gastric juice acts upon proteins, softening them, while the constant churning movement tends to separate the bits of food into finer particles. Finally, some of the partly digested food is allowed to pass in small amounts through the pyloric valve, into the small intestines. This is done by the expansion of the ringlike muscles of the pylorus.

The food test for proteid already given (nitric acid followed by ammonium hydrate) is known as the xanthoproteic.

Another test is made in the following manner: Place in a test tube containing oteid some concentrated caustic soda solution. To this add, drop by drop, a little weak copper sulphate solution. Note the resulting color (violet). Heat the material and the color deepens.

If the proteid has been digested to peptone the above test will show a rose pink. This test, known as the biuret test, is used for the detection of peptones.

HUNTER'S BIOL.-22

Absorption in Stomach. - Fluids leave the stomach more rapidly than do solids, milk, for example, taking about two hours to digest, while a meal of meat and vegetables will not leave the stomach for three to four hours. It is not thought that any great amount of absorption of digested food occurs through the wall of the stomach. As soon as food reaches the small intestine, however, sugars and peptones are slowly absorbed, and pass into the blood.

The Intestine and Glands connected with it. Laboratory Work on the Frog.1 The liver is the most prominent structure found in the body cavity. Note its position, its red color, the number of lobes into which it is divided. If the liver be pushed to one side, the small intestine is found to occupy part of the remaining space in the body cavity. Between the stomach and a coil of the small intestine lies a pinkish gland, the pancreas. The duct or tube which carries its secretion empties, together with the bile duct from the liver, into the small intestine, a short distance posterior to where the latter leaves the stomach. Find the greenish gall bladder which holds the secretion of the liver, the bile. Try to trace the course of the bile duct to the small intestine. Notice the abrupt change in the diameter of the food tube near its posterior end where it forms the large intestine; the latter empties into a space called the cloaca. This forms a common outlet for the food tube, kidneys, and reproductive organs.

POSITION AND STRUCTURE OF THE PANCREAS.-In man, the pancreas occupies the same relative position that it does in the frog. The gland is a rather diffuse structure; its duct empties in a common opening with the bile duct, a short distance below the pylorus. In internal structure, the pancreas resembles the salivary glands. The fluid, as we shall see, has some functions possessed by the saliva.

The following experiments may be performed to illustrate the process of digestion by pancreatic fluid. The several enzymes of pancreatic fluid are sold in a powdered form as a substance called pancreatin.2 An artificial pancreatic fluid may be prepared by adding to some pancreatin enough water to dissolve it. To show saponification of fats add ten volumes of 1.5 per cent sodium carbonate.

Experiments to show the Properties of Pancreatic Fluid. - Prepare (1) test tube containing starch and artificial pancreatic fluid; (2) test tube containing proteid and artificial pancreatic fluid; (3) test tube containing oil and artificial pancreatic fluid + ten volumes of 1.5 per cent sodium carbonate; (4) test tube containing oil and water. Place tubes 1 and 2 in a pan containing warm water. Leave at a temperature of 98° Fahrenheit, if possible, for twelve hours. Then test No. 1 with Fehling's solution; No. 2 for peptone with the biuret test. Test the contents of tube No. 1 or No. 2 with red litmus paper. What reaction do you obtain? Shake up No. 3 and No. 4 an equal number of times; compare the color and appearance of the two tubes one minute after shaking. Note that the milky condition existing

In the female frog it will be necessary to remove the ovary, filled with tiny black and white eggs, and the oviducts, twisted tubes through which the eggs are passed to the outside of the body, before working on the rest of the digestive tract.

2 This substance must be bought from a reliable firm, as it is frequently adulterated.

in No. 3 after shaking continues for a longer period than the same condition in No. 4. Examine, under the compound microscope, a drop of the fluid taken from tube No. 3. Notice the fluid appears to contain thousands of tiny droplets of fat which float in the water surrounding them. Such a mass of finely separated particles of oil and water is called an emulsion. FUNCTIONS OF PANCREATIC FLUID. - From these experiments we see that pancreatic fluid is alkaline in its reaction. It has the power, by means of an enzyme called amolypsin, to change starches to sugars. A second enzyme, called trypsin, changes proteids to peptones. Oils and fats, with the aid of a third enzyme (lipase) are emulsified and in part changed to soap. It is estimated that half an ounce of soap is formed daily in the small intestine by this means. In such a form fats are enabled to pass through

Appearance of milk under the microscope, showing the natural grouping of the fat globules. In the circle a single group is highly magnified. Milk is one form of an emulsion. (S. M. Babcock, Wis. Bul. No. 61.)

the walls of the intestine. This is, then, a form of digestion. The pancreatic fluid is thus seen to play a very important part in digestion, as it acts upon all three nutrients, starches, proteins, and fats.

LIVER. - The liver is the largest gland in the body. In man, it hangs just below the diaphragm, a little to the right side of the body. During life, its color is deep red. It is divided into three lobes, between two of which is found the gall bladder, a thin-walled sac which holds the bile, a secretion of the liver. Bile is a strongly alkaline fluid of greenish color. It reaches the intestine through a common opening with the pancreatic fluid. Almost one quart of bile is passed daily into the digestive canal.

FUNCTIONS OF BILE. - The action of bile on foods is not very well known. It does have slight action in emulsifying fats. It is slightly antiseptic, and is thus valuable in preventing fermentation within the intestine. Its greatest importance, however, is the peculiar faculty it has of aiding the passage of fats through the walls of the intestine.

EXPERIMENT.

Place filter paper within each of two funnels. In ore funnel, wet the filter paper with water. In the second, wet the paper with ox gall dissolved in water (equivalent to bile). Place olive oil in each funnel. Through which filter does the olive oil pass with greater rapidity?

FORMATION OF GLYCOGEN. - Another important function of the liver (which may be taken up in connection with the circulation of the blood) is the formation within it of a material called glycogen or animal sugar. The liver is supplied by blood from two sources. The greater amount of blood received by the liver comes directly from the walls of the stomach and intestine to this organ. This blood is very rich in food materials, and from it the cells of the liver take out sugars to form glycogen. Glycogen is stored in the liver until such a time as a food is needed that can be quickly oxidized;

D

then the glycogen is carried off by the blood to the tissue which requires it and there used for this purpose.

The Absorption of Digested Food into the Blood. The object of digestion is to change foods from an insoluble to a soluble form. This has been seen in the study of the action of the various digestive fluids in the body, each of which is seen to aid in dissolving solid foods, changing them to a fluid, and, in case of the bile, actually assisting them to pass through the wall of the intestine by osmosis. A very small amount of digested food may be absorbed by the blood in the E blood vessels of the walls of the stomach. Most of the absorption, however, takes place through the walls of the small intestine. Let us examine this structure somewhat closely to see how it is adapted to absorb liquid food.

A much magnified section through the wall of the small intestine (after Benda). A, B, transverse folds of intestine covered with the fingerlike villi; note the very great absorbing surface thus gained; C, connective tissue; D, E, circular and longitudinally running muscle fibers.

STRUCTURE OF THE SMALL INTESTINE. The small intestine in man is a slender

tube nearly twenty feet in length and about one inch in diameter. Its walls contain muscles which, by a series of slow waves of contraction, force

the fluid food gradually toward the posterior end of the tube. If the chief function of the small intestine is that of absorption, we must look for adaptations which increase the absorbing surface of the tube. This is gained in part by the inner surface of the tube being thrown into transverse folds which not only retard the rapidity with which food passes down the intestine, but also give more absorbing surface. But far more important for absorption are millions of little projections which cover the inner surface of the small intestine. So numerous are these projections that the whole surface presents a velvety appearance. Collectively, these structures are called the villi (singular villus). They form the chief organs of absorption in the intestine, several thousand being

distributed over every square inch of surface. Between the villi are found the openings of many small tubelike glands, the intestinal glands. These glands manufacture a digestive fluid, the function of which is believed to be somewhat like that of the pancreatic fluid.

Internal Structure of a Villus. — The internal structure of a villus is best seen in a longitudinal section. We find the outer wall made up of a thin layer of cells, the epithelial layer. It is the duty of these cells to absorb, by osmosis, the semifluid food from within the intestine. Sugars and peptones are passed through the cells to a number of tiny capillaries or blood vessels found immediately under the epithelial layer. From here they pass (through what is known as the portal circulation) into the liver, where, as we have seen, sugar is taken from the blood and stored as glycogen.

Diagram of a longitudinal section through a villus; a, epithelium which takes up food and transports it to the tubes within; b, an artery; c, capillaries; d, a lacteal.

From the liver, this food

Course of Food after Absorption. within the blood is sent to the heart, from there pumped to the lungs, then some of it returns to the heart and is pumped to the tissues of the body, where as nitrogenous food it may build protoplasm or, if carbohydrate, may be used to furnish energy. A large amount of water and some salts are absorbed through the walls of the stomach and intestine as food passes on its course. The fats, in the form of an emulsion, are also taken up through the cells lining the villi and pass, not into the blood vessels, but into a space which occupies part of the interior of the villus. Each of