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these spaces leads into a system of thin-walled vessels which, because of their milky appearance after their absorption of fats, are collectively called the lacteals (Lat. lac milk).
Mesenteric Glands. The entire digestive tract hangs in the body cavity within a double fold of the mesentery or membrane which lines the body cavity. In this double fold are found, besides the organs of the digestive tract, blood vessels leading to and from them, nerves, connective tissue, and fat, numerous small collections of gland cells called the mesenteric glands. These glands receive the fatty contents of the lacteals and, in some way, change this fat so that it may become part of the blood. Eventually, the fats reach the blood through the thoracic duct of the lymphatic system (which we shall study later). Fats reach the blood, then, without passing through the liver with the other foods absorbed by the villi.
Large Intestine. The large intestine has somewhat the same structure as the small intestine except that the diameter is much greater. It also contains no villi nor transverse folds on its inner surface. Considerable absorption of food, however, takes place through its walls as the food mass is slowly pushed along by the muscles within its walls.
VERMIFORM APPENDIX. At the point where the small intestine widens to form the large intestine a baglike pouch is formed. From one side of this pouch is given off a small tube about four inches long, closed at the lower end. This tube, the function of which in man is unknown, is called the vermiform appendix. It has come to have unpleasant notoriety in late years, as the site of serious inflammation. It often becomes necessary to remove the appendix in order to prevent this inflammation spreading to the surrounding tissues. In some of the lower vertebrates (for example, fishes), the vermiform appendix is extremely large, and appears to be used as an organ of digestion and absorption. In man it has become reduced in size (perhaps through disuse) so that it is a mere vestige of what it is in lower vertebrates.
Hygienic Habits of Eating; the Causes and Prevention of Dyspepsia. From the contents of the foregoing chapter it is evident that the object of the process of digestion is to break up solid food so that it may be absorbed to form part of the blood. Any habits we may form of thoroughly chewing our food will evidently aid in this process. A lump of white of egg will not be digested by pepsin in the experiment just performed; minced egg, on the other hand, is quickly changed to a peptone. Undoubtedly much of the
distress known as dyspepsia is due to too hasty meals with consequent lack of proper mastication of food. Another cause is overeating. It is a good rule to go away from the table feeling hungry. Eating too much overtaxes the digestive organs and prevents their working to the best advantage. Still another cause of dyspepsia is eating when in a fatigued condition. It is always a good plan to rest a short time before eating, especially after any hard manual work. Eating between meals is also condemned by physicians because it calls the blood to the digestive organs at a time when it should be in other parts of the body.
Effect of Alcohol on Digestion. - It is a well-known fact that alcohol extracts water from tissues with which it is in contact. This fact works much harm to the interior surface of the food tube, especially the walls of the stomach, which in the case of a hard drinker are likely to become irritated and much toughened. In small amounts alcohol is believed to stimulate the secretion of the salivaric and gastric glands, and thus it seems to aid in digestion. It is doubtful, however, if this aid is real.
The following results of experiments on dogs, published in the American Journal of Physiology, Vol. I, Professor Chittenden gives as "strictly comparable," because "they were carried out in succession on the same day":
XXIX. THE BLOOD
Function of the Blood. We have seen in the preceding chapter that the chief function of the digestive tract is to change foods to such form that they can be absorbed through the walls of the food tube. The food, after it has passed through the intestine walls, ultimately reaches the blood and becomes, as we shall see, a part of this tissue. By means of a system of closed tubes, this fluid tissue circulates to all parts of the body, depositing its burden of food at the places where it is most needed and where it will be used, either in the repair or building of tissues or in the release of energy.
Laboratory Exercise. Examine a prepared slide of the blood of a frog. Note that three constituents are found: (1) Ovoid cells, each containing a nucleus. What color do these bodies have? They are called the red corpuscles. (2) Other colorless corpuscles of irregular form may be seen. What can you say of the number of colorless corpuscles as compared with the red corpuscles? Notice that the colorless corpuscles have the power to change their shape. They are said to be amaboid. Like the amoeba, they also have the power to take up particles of food and other materials and ingest them. (3) The colorless fluid in which the corpuscles float is known as the plasma.
Composition of Plasma. - The plasma of blood (when chemically examined in man) is found to be largely (about 90 per cent) water. It also contains a considerable amount of proteid, some sugar, fat, and mineral material. It is, then, the medium which holds the fluid food (or at least part of it) that has been absorbed from the food within the intestine. When the blood returns from the tis
sues where the food is oxidized, the plasma brings back with it to the lungs the carbon dioxide liberated from the tissues of the body where oxidation has taken place. Blood returning from the tissues of the body has from 45 to 50 per cent of carbon dioxide in its composition. (See Respiration, page 380.) Some waste products, to be spoken of later, are also found in the plasma.
Demonstration. - Get some fresh beef blood. Let it stand overnight in a jar. In the morning it will be found to have separated into two parts, a dark red clot and a thin straw-colored liquid, called serum. Serum is found to be made up of about 90 per cent water, 8 to 9 per cent proteid, and from 1 to 2 per cent sugars, fats, and mineral matter. In these respects it rather closely resembles the fluid food that is absorbed from the intestines.
Clotting of Blood.-Pour another jar of fresh beef blood into a pan and briskly whip it with a bundle of little rods (or with an egg beater). A stringy substance will be found to stick to the rods. This, if washed carefully, is seen to be almost colorless. Test with nitric acid and ammonia. Note the deep orange color. It is a proteid substance called fibrin.
Blood plasma, then, is made up of serum, a fluid portion, and fibrin, which, although in a fluid state in the blood vessels within the body, coagulates on exposure to air.
This fibrin, when exposed to air, coagulates or thickens. It is this coagulation which aids in the formation of a blood clot. A clot is simply a mass of fibrin with a large number of corpuscles tangled within. The clotting of blood is of great physiological importance, for otherwise we might bleed to death from the smallest wound.
In blood within the circulatory system of the body the fibrin is held in a fluid state called fibrinogen. It is believed that an enzyme, acting upon this fibrinogen, causes the change to take place in blood which is exposed to the air.
The Red Blood Corpuscle; its Structure and Functions. In the blood of the frog we have seen that the red corpuscle is a true cell of disklike form. The red corpuscle of man, however, lacks a nucleus. Its form is that of a biconcave disk. So small and so numerous are these corpuscles that over five million are found in a drop of normal blood. The color, which is found to be a dirty yellow when separate corpus
cles are viewed under the microscope, is due to a proteid material called haemoglobin. Haemoglobin, which constitutes about 35 per cent of the corpuscle, contains a large amount of iron. It has the power of uniting very readily with oxygen whenever that gas is abundant, and after having absorbed it, of giving it up to the surrounding media, when oxygen is there present in smaller amounts than in the corpuscle. This function of carrying oxygen is one of the most important functions of the blood. The taking up of oxygen is accompanied by a change in color of the mass of corpuscles from a dull red to a bright scarlet.
LENGTH OF LIFE OF RED CORPUSCLES.—It is difficult to say just how long the red corpuscles live in the body. We know, however, that large numbers are destroyed every day in the spleen. (See the body cavity of a frog. The spleen is a small reddish body found lying in the mesentery, just ventral to the kidneys. It does not appear to be connected by any duct or tube with any part of the body; hence it is called a ductless gland.)
The coloring matter of the bile, and possibly other body excretions, is due to the color obtained from the worn-out blood corpuscles. To make up for the loss of the red corpuscles, new ones are manufactured in the red marrow of bone. The red marrow cells are in a continual state of division, forming new red corpuscles which at first are nucleated cells. These later lose their nuclei and become disk-shaped.
The Colorless Corpuscle; Structure and Functions. A colorless corpuscle is a cell irregular in outline, the shape of which is constantly changing. These corpuscles are somewhat larger than the red corpuscles but less numerous, there being about one colorless corpuscle to every three hundred red ones. They seem to have the power of movement, for they are found not only inside blood vessels, but outside the blood tubes, showing that they have worked their way between the cells that form the walls of the blood vessels.
A Russian zoölogist, Metchnikoff, after studying a number of simple animals, such as medusæ and sponges, found that in such animals some of the cells lining the inside of the food cavity take up or engulf minute bits of food. Later, this food is changed into the protoplasm of the cell. Metchnikoff believed that the colorless corpuscles of the blood have somewhat the same function. This he later proved to be true. Like the amoeba,