Part 24 (1/2)

362. The following experiments will demonstrate that the blood from every part of the system flows to the heart by the agency of the veins. 1st. Press firmly on one of the veins upon the back of the hand, carrying the pressure toward the fingers; for a moment, the vein will disappear. On removing the pressure of the finger, it will reappear, from the blood rus.h.i.+ng in from below.

2d. If a tape be tied around the arm above the elbow, the veins below will become larger and more prominent, and also a greater number will be brought in view, while the veins above the tape are less distended.

At this time, apply the finger at the wrist, and the pulsation of the arteries still continues, showing that the blood is constantly flowing from the heart through the arteries, into the veins; and the increased size of the veins shows that the pressure of the tape prevents its flowing back to the heart.

361. State the influences that propel the blood from the heart. 362.

Demonstrate by the first experiment that the blood flows to the heart.

By the second experiment.

363. The influences that return the blood to the heart through the veins, are not so easily understood as those that act on the blood in the arteries. Some physiologists have imputed an active propulsive power to the capillary vessels in carrying the blood through the veins. This is not easily explained, and perhaps it is as difficult to understand. An influence upon which others have dwelt, is the suction power of the heart in active dilatation, acting as a _vis a fronte_ (power in front) in drawing blood to it.

364. Another influence that aids the venous circulation is attributed to the propulsive power of the heart. It is not easy to comprehend how this power of the heart can be extended through the capillary vessels to the blood in the veins. Again, an important agency has been found, by some physiologists, in the inspiratory movements, which are supposed to draw the blood of the veins into the chest, in order to supply the vacuum which is created there by the elevation of the ribs and the descent of the diaphragm.

365. One of the most powerful causes which influence the venous circulation, is the frequently-recurring action of the muscles upon the venous trunks. When the muscles are contracted, they compress that portion of the veins which lie beneath the swell, and thus force the blood from one valve to the other, toward the heart. When they are relaxed, the veins refill, and are compressed by the recurring action of the muscles.

_Observation._ The physician, in opening a vein, relies on the energetic contractions and sudden relaxations of the muscles, when he directs the patient to clasp the head of a cane, or the arm of a chair; these alternate motions of the muscles cause an increased flow of blood to the veins of the ligated arm.

363. What is said of the influences that return the blood to the heart? What is said of the propulsive power of the capillaries? Of the suction power of the heart? 364. Give another influence. State another agency. 365. What is one of the most powerful causes which influence venous circulation? Give practical observation.

[Ill.u.s.tration: Fig. 75. An ideal view of the circulation in the lungs and system. From the right ventricle of the heart, (2,) the dark, impure blood is forced into the pulmonary artery, (3,) and its branches (4, 5) carry the blood to the left and right lung. In the capillary vessels (6, 6) of the lungs, the blood becomes pure, or of a red color, and is returned to the left auricle of the heart, (9,) by the veins, (7, 8.) From the left auricle the pure blood pa.s.ses into the left ventricle, (10.) By a forcible contraction of the left ventricle of the heart, the blood is thrown into the aorta, (11.) Its branches (12, 13, 13) carry the pure blood to every organ or part of the body. The divisions and subdivisions of the aorta terminate in capillary vessels, represented by 14, 14. In these hair-like vessels the blood becomes dark colored, and is returned to the right auricle of the heart (1) by the vena cava descendens, (15,) and vena cava ascendens, (16.) The tricuspid valves (17) prevent the reflow of the blood from the right ventricle to the right auricle. The semilunar valves (18) prevent the blood pa.s.sing from the pulmonary artery to the right ventricle. The mitral valves (19) prevent the reflow of blood from the left ventricle to the left auricle.

The semilunar valves (20) prevent the reflow of blood from the aorta to the left ventricle.]

366. The muscles exercise an agency in maintaining the venous circulation at a point above what the heart could perform. As the pulsations are diminished by rest, so they are accelerated by exercise, and very much quickened by violent effort. There can be little doubt that the increased rapidity of the return of blood through the veins, is, of itself, a sufficient cause for the accelerated movements of the heart during active exercise.

_Observation._ The quant.i.ty of blood in different individuals varies.

From twenty-five to thirty-five pounds may be considered an average estimate in a healthy adult of medium size. The time in which the blood courses through the body and returns to the heart, is different in different individuals. Many writers on physiology unconditionally limit the period to three minutes. It is undeniable that the size and health of a person, the condition of the heart, lungs, and brain, the quant.i.ty of the circulating fluid, the amount and character of the inspired air, and the amount of muscular action, exert a modifying influence. The time probably varies from three to eight minutes.

366. What causes the accelerated movements of the heart during active exercise?

_Note._ Let the pupil review the anatomy and physiology of the circulatory organs from fig. 75, or from anatomical outline plates, No. 6 and 7.

CHAPTER XIX.

HYGIENE OF THE CIRCULATORY ORGANS

367. If any part of the system is deprived of blood, its vitality will cease; but, if the blood is lessened in quant.i.ty to a limited extent, only the vigor and health of the part will be impaired. The following conditions, if observed, will favor the free and regular supply of blood to all portions of the system.