BLOOD, the animal fluid by which the tissues of the body are nourished, and which is contained in the tubes called from their office Blood-Vessels.
On first flowing from the vessel in which it is contained the blood is a thick, viscid, and tenacious fluid. In all the more highly organised animals it is of a red colour : but redness is not one of its essential properties. In several tribes of animals which possess true and proper blood, this fluid is not of a red colour, and there is no animal whose blood is visibly red in all the parts of the body. The blood of the insect is colourless and transparent; that of the reptile is of a yellowish colour; in the main part of the body of the fish, that is, in the whole of its muscular system, the blood is without colour ; hence the whiteness of the general substance of the body of the fish : but in the more important organs, and especially in those which constitute the circle of nutrition, called the organs of organic life, the blood is of a red colour, as in the heart, the branchhe or gills, and so on. In the bird the blood is of a deep red ; but it is the deepest of all in the mamm'alia. In some species of mammalia it is deeper than in others; in the hare, for example, it is much deeper than in the rabbit. It is deeper in some varieties of the same species than in others, and more especially in different varieties of the human family.
In man and all the higher animals the body contains two kinds of blood, each of which is distinguished by a striking difference of colour. Each kind of blood is contained in its own peculiar set of vessels: the one in the vessel called a vein, hence called venous blood; the other in the vessel called an artery, arterial blood. Venous blood is of a dark or Modena-red colour ; arterial blood is of a bright scarlet colour. Venous differs from arterial blood in its most essential properties no less than in its colour : venous blood is incapable of nourishing the body and of stimulating the organs; arterial blood is the proper nutrient and stimulant of the system.
The specific gravity of human blood (water being 1000) may be stated to be about 1055 or 1056, from which standard it is capable of increasing to 1120, and of siuking to 1026, this being the extreme range of variation hitherto observed. Venous is heavier than arterial
blood, the former being commonly estimated at 1062, and the latter at 1049. The higher the organisation of the blood the greater is its specific gravity : hence the specific gravity of the blood of the higher is greater than that of the lower animals.
There is a remarkable difference in different classes of animals in the temperature of the blood. In some it is only a degree or two above that of the surrounding medium. Creatures with blood of this low temperature are called cold-blooded, in contradistinction to warm blooded animals, whose temperature is maintained under whatever variety of circumstances they may be placed considerably above that of the surrounding air.
The following table of the temperature of the blood of different animals, is compiled from the researches of Tiedemann and Rudolphi on this subject.
Arterial is warmer by one degree than venous blood.
Disease is capable of effecting a considerable change in the tempera ture of the blood. In almost every case of fever the temperature of the blood differs from the natural standard. In the cold fit of iutermittent fever (ague) it sometimes sinks as low as 94° ; in some types of continued fever it rises as high as 102°. In cholera it sinks to 90°. In inflammation of moderate severity it exceeds the natural standard by 4 degrees ; in intense inflammation it is capable of rising above it as high as 7 degrees.
The blood, whilst circulating in the body is composed of two parts, a liquid and a solid. The liquid is called liquor sanguinis, and the solid, on account of its cellular character, blood-globules or corpuscles. When blood is allowed to stand, after it is taken from the body, it separates into two distinct parts, a solid mass, and a fluid matter in which the solid mass swims. The solid portion of the blood, which includes the blood-corpuscles and a portion of the liquor sanguinis called the fibrin, is termed the Clot, or the Crassamentum ; the fluid portion is called the Serum; and the process by which the separation takes place is denominated Coagulation.