[Illustration: Blood Crystals]
USES OF THE BLOOD.—The blood has been called "liquid flesh"; but it is more than that, since it contains the materials for making every organ. The plasma is rich in mineral matter for the bones, and in albumen for the muscles. The red disks are the air cells of the blood. They contain the oxygen so essential to every operation of life. Wherever there is work to be done or repairs to be made, there the oxygen is needed. It stimulates to action, and tears down all that is worn out. In this process, it combines with and actually burns out parts of the muscles and other tissues, as wood is burned in the stove. [Footnote: For the sake of simplicity, perhaps to conceal our own ignorance, we call this process "burning." The simile of a fire is good so far as it goes. But as to the real nature of the change which the physiologist briefly terms "oxidation," we know nothing. This much only can be asserted positively. A stream of oxygen is carried by the blood to the muscles (in fact to every tissue in the body), while, from the muscles the blood carries away a stream of carbonic acid and water. But what takes place in the muscles, when and what chemical change occurs, no one can tell. We see the first and the last stage. We know that contraction of the muscles somehow comes about, oxygen disappears, carbonic acid appears, energy is released, and force is exhibited as motion, heat, and electricity. But the intermediate step is hidden.
There are certain theories advanced, however, that are worth considering. Some physiologists hold that the muscle has the power of taking up the oxygen from the hemoglobin (a body that comprises ninety per cent of the red corpuscles when dried, and is the oxygen carrier of the blood), and fixing it, as well as the raw material (food) furnished by the blood, thus forming a true contractile substance. The breaking down or decomposition of this contractile substance in the muscle, sets free its potential energy. The process is gentle so long as the muscle is at rest, but becomes excessive and violent when contraction occurs. (See "Foster's Physiology," p. 118.) It is also believed by some that the chemical change in the muscle partakes of a fermentive character; that, under the influence of the proper ferments, the substances break up into other and simpler products, thus setting free heat and force; and that this chemical change is followed by a secondary oxidation by the oxygen in the arterial blood, thereby forming carbonic acid and water, as in all putrefactive processes. But these and other views are not as yet fully understood; while they utterly fail to tell us how a collection of simple cells, filled merely with a semifluid mass of matter, can contract and set free muscular power. The commonness of this act hides from us its wonderful nature. But here, hidden in the cell—Nature's tiny laboratory—lies the mystery of life. Before its closed door we ponder in vain, confessing the unskillfulness of our labor, and fearing all the while lest the Secret of the Cell will always elude our search.] The blood, now foul with the burned matter, the refuse of this fire, is caught up by the circulation, and whirled back to the lungs, where it is purified, and again sent bounding on its way.
There are then two different kinds of the blood in the body: the red or arterial, and the dark or venous.
TRANSFUSION.—As the blood is really the "vital fluid" it would seem that feeble persons might be restored to vigor by infusing healthy blood into their veins. This hypothesis, so valuable in its possible results in prolonging human life, has been carefully tested. Animals which have ceased to breathe have thus had their vitality recalled. In the seventeenth century the theory became a subject of special investigation. A maniac was restored to reason by the blood of a calf, and the most extravagant hopes were entertained. But many fatal accidents occurring, experiments upon human beings were forbidden by law, and transfusion soon fell into disuse. It has, however, been successfully practiced in several cases within the last few years, and is a method still in repute for saving lives.
COAGULATION.—When blood is exposed to the air, it coagulates. This is caused by the solidifying of the fibrin, which entangling the disks, forms the "clot." The remaining clear, yellow liquid is the serum. The value of this peculiar property of the blood can hardly be overestimated. The coagulation soon checks all ordinary cases of bleeding. [Footnote: In the case of the lower animals, which have no means of stopping hemorrhages as we have, the coagulation is generally still more rapid. In some species of birds it takes place almost instantaneously.] When a wound is made, and bleeding commences, the fibrin forms a temporary plug, as it were, which is absorbed when the healing process is finished. Thus we see how a Divine foresight has provided not only for the ordinary wants of the body, but also for the accidents to which it is liable. [Footnote: The fibrin is not an essential ingredient of the blood. All the functions of life are regularly performed in people whose blood lacks fibrin; and, in cases of transfusion, where blood deprived of its fibrin was used, the vivifying influence seemed to be the same. Its office, therefore, must mainly be to stanch any hemorrhage which may occur.—FLINT.]
FIG. 37.
[Illustration: The Heart. A, the right ventricle; B, the left ventricle; C, the right auricle; D, the left auricle.]
THE HEART is the engine which propels the blood. It is a hollow, pear- shaped muscle, about the size of the fist. It hangs, point downward, just to the left of the center of the chest. (See Fig. 31.) It is inclosed in a loose sac of serous membrane, [Footnote: The mucous membrane lines the open cavities of the body; the serous, the closed. The pericardium is a sac composed of two layers—a fibrous membrane on the outside, and a serous one on the inside. The latter covers the external surface of the heart, and is reflected back upon itself in order to form, like all the membranes of this nature, a sac without an opening. The heart is thus covered by the pericardial sac, but not contained inside its cavity. A correct idea may be formed of the disposition of the pericardium around the heart by recalling a very common and very convenient, though now discarded headdress, the cotton nightcap. The pericardium incloses the heart exactly as this cap covered the heads of our forefathers.— Wonders of the Human Body.] called the pericardium (peri, about; and kardia, the heart). This secretes a lubricating fluid, and is smooth as satin.
THE MOVEMENTS OF THE HEART consist of an alternate contraction and expansion. The former is called the sys'-to-le, and the latter the di-as'-to-le. During the diastole, the blood flows into the heart, to be expelled by the systole. The alternation of these movements constitutes the beating of the heart which we hear so distinctly between the fifth and sixth ribs. [Footnote: Two sounds are heard if we put our ear over the heart,—the first and longer as the blood is leading the organ, the second as it falls into the pockets of the two arteries, and the valves then striking together cause it. The first sound is mainly the noise made by the muscular tissue. During the first, the two ventricles contract; during the second the two auricles do so. The hand may feel the heart striking the ribs as it contracts,—a feeling called the impulse, or, if quicker and stronger than usual, palpitation. This is not always a sign of disease, but in hypochondriacs is often an effect of the mind on the nerves of the heart.—MAPOTHER]