BLOOD is the

From Agepedia

BLOOD is the red fluid contained in the bloodvessels (q. v.) of animal bodies. It is found in the mammalia, in birds, in reptiles and in fishes. In the last two classes of animals, the temperature of the blood is much lower than in the former, for which reason they are distinguished by the name coldblooded, while the others are termed ivarmblooded animals. Insects and worms, instead of red blood, have a juice of a whitish color, whicjfe hj cidltA white blood. In the blood, two different substances are contained, which are separated by coagulationthe serum, a fluid like the white of an egg, and a thick matter, to which the red color properly belongs, which is much heavier than the former, and is called the coagidum. The last may be divided again into two different partsinto the cruor, or that part of the blood which is intrinsically reel, and coagulable, and lymph orfhine, to which the coagulation of the blood must be ascribed. The fibrine, in young animals, is much whiter than in older and stronger ones. The blood of the latter contains much more azote than that of the former. If the nourishment of animals is changed, we also find an alteration in the constituent parts of their blood. It is also changed by diseases. In animals that are hunted to death, or killed by lightning, the blood does not coagulate. The blood, of birds is more highly colored, and warmer, than that of viviparous animals, and coagulates more easily in the air. That of reptiles and fishes coagulates with difficulty. Aided by magnifying glasses of a strong power, one may observe, in examining the blood of the living animal, or in blood which is newly drawn, that it consists, especially the cruor, of little globular bubbles, the globides of the blood, as they are called, the diameter of which amounts to about the three hundredth part of a line. In blood that has been drawn some time, although this time may be very short, they are not to be discovered. They are the effect of the life that pervades the blood. The more robust and healthy an animal is, the more globules are perceived. They show, as it were, the transition from the formless liquid to the original form of the first organized matter. The blood is of the greatest importance to the life of an animal, and may be considered as the source of life. As long as the body is living, the blood is in perpetual motion. When it is taken out of the body, a remarkable change soon follows: it begins to coagulate, and then undergoes, first an acetous, and, after a few days, a putrid fermentation. All the blood takes its origin from the chyle, and deposits, by degrees, the nourishing particles requisite to the preservation and growth of the body, by a multitude of vessels adapted thereto. This is done while it is driven from the heart into the remotest parts of the body, and from thence back. The circulation of the blood is, as it were, the principle and first condition of life. With it, except in cases of fainting, suffocation, &c, life ceases. The heart, the centre of the circulation of the blood, has a twofold motion, of contraction and dilatation, which constantly alternate. With the heart two kinds of vessels are connectedthe arteries and the veins. (See BloodVessels.) The circulation of the blood proceeds with an astonishing rapidity: did it flow at an equal rate in a straight line, it would run, in the space of one minute, through 149 feet. This swiftness, however, exists only in the larger vessels near the heart; the farther the blood recedes from the heart, the slower its motion becomes. In a grownup person, in good health, we may reckon the mass of blood at 24-30 pounds. Blood Vessels are the tubes or vessels in which the blood circulates. They are divided into two classes,arteries and veins,which have two points of union or connexionthe first in the heart, from which they both originate, and the other in the minute vessels or network, in which they terminate. The arteries arise from the heart, and convey the blood to all parts of the foody; the veins return it to the heart. The arteries distribute throughout the body a pure, red blood, for the purposes of nourishment; while the veins return to the heart a darkcolored blood, more or less loaded with impurities, and deprived of some of its valuable properties. But this is not returned again to the body in the same state. For the heart is wisely divided into two portions or sides, a right and left, one of which receives the impure blood from the veins, and sends it to the lungs to be defecated and freshly supplied with oxygen or vital air, while the other receives the pure red blood from the lungs, and circulates it anew through the arteries. The arteries arise from the left ventricle of the heart by one large trunk, nearly an inch in diameter, which is gradually subdivided into smaller ones, as it proceeds towards the limbs, till they terminate, at last, in vessels so small as to be almost invisible, and in a fine network of cells, extending through the whole body, in which the blood is poured out, and nutrition or the increase of the body takes place, and from which the residue is taken up by the small veins, to be returned to the heart. The arteries and veins are widely different in their structure, as well as their uses. The former are composed of very strong, firm, elastic coats or membranes, which are four in number. The external covering and the internal lining of the arteries, although belonging to different classes of membranes, are both very thin and soft. The second coat is very thick, tough and elastic, being that which chiefly gives their peculiar appearance to the arteries. The third is formed of fibres, apparently muscular, arranged in circular rings around the tube of the vessels. It is well known that the pulse of the heart is felt in the arteries alone, although, in the bleeding of a vein, we sometimes see the blood start as if in unison with the beating of the heart. The pulse is produced by the wave or stream of blood, which is driven by the heart through the arteries, distending and slightly elevating them, after which they instantly contract from their elasticity, and thus force the blood into the smaller vessels. The pulse varies in its character with the general state of the health. (See Pulse.) When arteries are cut or wounded, the firmness of their coats prevents their closing, and hence arises the fatal nature of wounds of large vessels, wiiich will remain open till they are tied up, or till death is produced. The veins commence in small capillary tubes in every part of the body, and, by their gradual union, form large trunks, till they at last terminate in two (one ascending from the lower parts of the body, the other descending from the head and arms), which pour their contents into the heart. Their structure is much less firm than that of the arteries. They are very thin and soft, consisting of only two thin coats or membranes. The inner, or lining membrane, is frequently doubled into folds, forming valves, which nearly close the passage in the veins, and thus give very material support to the blood as it is moving up in them towards the heart. These valves are not found in the veins of the bowels, the lungs or the head. The number of the veins is much greater than that of the arteries, an artery being often accompanied by two veins. They differ also in this, that, while the arteries are deeply seated in the flesh, to guard them from injury, the veins are very frequently superficial, and covered only by the skin. The veins, it is well known, are the vessels commonly opened in bloodletting, although, in cases which render it necessary, a small artery is sometimes divided.There are two portions of the venous system, which do not correspond exactly with our general de scription ; these are the veins of the bowels and of the lungs. The former circulate their blood through the liver beforeit returns to the heart, and the latter, the pulmonary veins, convey red blood from the lungs to the heart. (For an account of the circulation of the blood, see Heart.) It should also lie mentioned, that the large vein, which brings back the blood from the lower part of the body, receives from the lymphatic and lacteal vessels the chyle from the bowels, which supplies the waste of the blood and nourishes the body, and the serous and other watery fluids which are taken up by the absorbents in all parts of the body.