A Civic Biology, Presented in Problems

by George W. Hunter

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.)

Work done by the Pancreas.—Starch paste added to artificial pancreatic fluid and kept at blood heat is soon changed to sugar. Protein, under the same conditions, is changed to a peptone. Fats, which so far have been unchanged except to be melted by the heat of the body, are changed by the action of the pancreas into a form which can pass through the walls of the food tube. If we test pancreatic fluid, we find it strongly alkaline in its reaction. If two test tubes, one containing olive oil and water, the other olive oil and a weak solution of caustic soda, an alkali, be shaken violently and then allowed to stand, the oil and water will quickly separate, while the oil, caustic soda, and water will remain for some time in a milky emulsion. If this emulsion be examined under the microscope, it will be found to be made of millions of little droplets of fat, floating in the liquid. The presence of the caustic soda helped the forming of the emulsion. Pancreatic fluid similarly emulsifies fats and changes them into soft soaps and fatty acids. Fat in this form may be absorbed. The process of this transformation is not well understood.

Conditions under which the Pancreas does its Work.—The secretion from this gland seems to be influenced by the overflow of acid material from the stomach. This acid, on striking the lining of the small intestine, causes the formation in its walls of a substance known as secretin. This secretin reaches the blood and seems to stimulate all the glands pouring fluid into the intestine to do more work. A pint or more of pancreatic fluid is secreted every day.

The Intestinal Fluid.—Three different pancreatic enzymes do the work of digestion, one acting on starch, another on protein, and a third on fats. It has been found that some of these enzymes will not do their work unless aided by the intestinal fluid, a secretion formed in glands in the walls of the small intestine. This fluid, though not much is known about it, is believed to play an important part in the digestion of all kinds of foods left undigested in the small intestine.

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 the same opening as the pancreatic fluid. Almost one quart of bile is passed daily into the digestive canal. The color of bile is due to certain waste substances which come from the destruction of worn-out red corpuscles of the blood. This destruction takes place in the liver.

Diagram of a bit of the wall of the small intestine, greatly magnified, a, mouths of intestinal glands; b, villus cut lengthwise to show blood vessels and lacteal (in center); e, lacteal sending branches to other villi; i, intestinal glands; m, artery; v, vein; l, t, muscular coats of intestine wall.

Functions of Bile.—The action of bile is not very well known. It has the very important faculty of aiding the pancreatic fluid in digestion, though alone it has slight if any digestive power. Certain substances in the bile aid especially in the absorption of fats. Bile seems to be mostly a waste product from the blood and as such incidentally serves to keep the contents of the intestine in a more or less soft condition, thus preventing extreme constipation.

The Liver a Storehouse.—Perhaps the most important function of the liver is the formation within it of a material called glycogen, or animal starch. 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. It normally contains about one fifth of all the blood in the body. This blood is very rich in food materials, and from it the cells of the liver take out sugars to form glycogen.[43] Glycogen is stored in the liver until such a time as a food is needed that can be quickly oxidized; then it is changed to sugar and carried off by the blood to the tissue which requires it, and there used for this purpose. Glycogen is also stored in the muscles, where it is oxidized to release energy when the muscles are exercised.

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. A small amount of digested food may be absorbed by the blood in the blood vessels of the walls of the stomach. Most of the absorption, however, takes place through the walls of the small intestine.