Physiology and Hygiene for Secondary Schools. Francis M. Walters
Expose the forearm and examine the veins on its surface. With a finger, stroke one of the veins toward the heart, noting that, as the blood is pushed along on one side of the finger the blood follows on the other side. Now stroke the vein toward the hand. Places are found beyond which the blood does not follow the finger. These mark the positions of valves.
To show Effect of Exercise upon the Circulation.—1. With a finger on the "pulse" at the wrist or temple, count the number of heart beats during a period of one minute under the following conditions: (a) when sitting; (b) when standing; (c) after active exercise, as running. What relation, if any, do these observations indicate between the general activity of the body and the work of the heart?
2. Compare the size of the veins on the backs of the hands when they are placed side by side on a table. Then exercise briskly the [pg 064]right hand and arm, clenching and unclenching the fist and flexing the arm at the elbow. Place the hands again side by side and, after waiting a minute, observe the increase in the size of the veins in the hand exercised. How is this accounted for?
To Show the Effect of Gravity on the Circulation.—Hold one hand high above the head, at the same time letting the other hand hang loosely by the side. Observe the difference in the color of the hands and the degree to which the large veins are filled. Repeat the experiment, reversing the position of the hands. What results are observed? In what parts of the body does gravity aid in the return of the blood to the heart? In what parts does it hinder? Where fainting is caused by lack of blood in the brain (the usual cause), is it better to let the patient lie down flat or to force him into a sitting posture?
To study the Circulation in a Frog's Foot (Optional).—A compound microscope is needed in this study and for extended examination it is best to destroy the frog's brain. This is done by inserting some blunt-pointed instrument into the skull cavity from the neck and moving it about. A small frog, on account of the thinness of its webs, gives the best results. It should be attached to a thin board which has an opening in one end over which the web of the foot may be stretched. Threads should extend from two of the toes to pins driven into the board to secure the necessary tension of the web, and the foot and lower leg should be kept moist. Using a two-thirds-inch objective, observe the branching of the small arteries into the capillaries and the union of the capillaries to form the small veins. The appearance is truly wonderful, but allowance must be made for the fact that the motion of the blood is magnified, as well as the different structures, and that it appears to move much faster than it really does. With a still higher power, the movements of the corpuscles through the capillaries may be studied.
Note.—To perform this experiment without destroying the brain, the frog is first carefully wrapped with strips of wet cloth and securely tied to the board. The wrapping, while preventing movements of the frog, must not interfere with the circulation.
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CHAPTER VI - THE LYMPH AND ITS MOVEMENT THROUGH THE BODY
Fig. 27—Diagram showing position of the lymph with reference to the blood and the cells. The central tube is a capillary. The arrows indicate the direction of slight movements in the lymph.
The blood, it will be remembered, moves everywhere through the body in a system of closed tubes. These keep it from coming in contact with any of the cells of the body except those lining the tubes themselves. The capillaries, to be sure, bring the blood very near the cells of the different tissues; still, there is need of a liquid to fill the space between the capillaries and the cells and to transfer materials from one to the other. The lymph occupies this position and does this work. The position of the lymph with reference to the capillaries and the cells is shown in Fig. 27.
Origin of the Lymph.—The chief source of the lymph is the plasma of the blood. As before described, the walls of the capillaries consist of a single layer of flat cells placed edge to edge. Partly on account of the pressure upon the blood and partly on account of the natural tendency of liquids to pass through animal membranes, a considerable portion of the plasma penetrates the thin walls and enters the spaces occupied by the lymph.
[pg 066]The cells themselves also help to form the lymph, since the water and wastes leaving the cells add to its bulk. These mix with the plasma from the blood, forming the resultant liquid which is the lymph. A considerable amount of the material absorbed from the food canal also enters the lymph tubes, but this passes into the blood before reaching the cells.
Composition and Physical Properties of the Lymph.26—As would naturally be expected, the composition of the lymph is similar to that of the blood. In fact, nearly all the important constituents of the blood are found in the lymph, but in different proportions. Food materials for the cells are present in smaller amounts than in the blood, while impurities from the cells are in larger amounts. As a rule the red corpuscles are absent from the lymph, but the white corpuscles are present and in about the same numbers as in the blood.
The physical properties of the lymph are also similar to those of the blood. Like the blood, the lymph is denser than water and also coagulates, but it coagulates more slowly than does the blood. The most noticeable difference between these liquids is that of color, the lymph being colorless. This is due to the absence of red corpuscles. The quantity of lymph is estimated to be considerably greater than that of the blood.
Lymph Vessels.—Most of the lymph lies in minute cavities surrounding the cells and in close relations with the capillaries (Figs. 27 and 30). These are called lymph spaces. Connecting with the lymph spaces on the one[pg 067] hand, and with certain blood vessels on the other, is a system of tubes that return the lymph to the blood stream. The smallest of these, and the ones in greatest abundance, are called lymphatics. They consist of slender, thin-walled tubes, which resemble veins in structure, and, like the veins, have valves. They differ from veins, however, in being more uniform in size and in having thinner walls.
Fig. 28—Diagram of drainage system for the lymph. 1. Thoracic duct. 2. Right lymphatic duct. 3. Left subclavian vein. 4. Right subclavian vein. 5. Superior vena cava. 6. Lacteals. 7. Lymphatic glands. The small tubes connecting with the lymph spaces in all parts of the body are the lymphatics.
The lymphatics in different places gradually converge toward, and empty into, the two main lymph tubes of the body. The smaller of these tubes, called the right lymphatic duct, receives the lymph from the lymphatics in the right arm, the right side of the head, and the region of the right shoulder. It connects with, and empties its contents into, the right subclavian vein at the place where it is joined by the right jugular vein (Fig. 28).
The larger of the lymph tubes is called the thoracic duct. This receives lymph from all parts of the body[pg 068] not drained by the right lymphatic duct, and empties it into the left subclavian vein. Connection is made with the subclavian vein on the upper side at the place where it is joined by the left jugular vein. The thoracic duct has a length of from sixteen to eighteen inches, and is about as large around as a goose quill. The lower end terminates in an enlargement in the abdominal cavity, called the receptacle of the chyle. It is provided with valves throughout its course, in addition to one of considerable size which guards the opening into the blood vessel.
The lymphatics which join the thoracic duct from the small intestine are called the lacteals (Fig. 28). These do not differ in structure from the lymphatics in other parts of the body, but they perform a special work in absorbing the digested fat (Chapter XI).
Lymphatic Glands.—The lymphatic glands, sometimes called lymph nodes, are small and somewhat rounded bodies situated along the course of the lymphatic tubes. They vary in size, some of them being an inch or more in length. The lymph vessels generally open into them on one side and leave them on the other (Figs. 28 and 30). They are not glands in function, but are so called because of their having the general