BLOOD, (Gr. ap,a. Lat. sanguis. Fr. sang. Germ. Blut. Ital. sangue). This is the title the peculiar fluid which carries into the living tissues of animals the materials necessary to the nutritive processes going on within them.
The physical qualities of this fluid vary extremely ; Among almost all the lower animals it is so far from resembling what we are accus tomed to regard as essential to the blood in man and the vertebrata generally, that its nature is at first sight apt to be mistaken, and we cannot be surprised that the inferiomribes of creation should have been long supposed to be without blood. In the mammalia, birds, reptiles, fishes, and several of the annelida, the blood is of a red colour ; among the whole of the invertebrate, a few of the annelida excepted, it is, on the contrary, nearly colourless ; fre quently it has a decidedly blue tint, and in many instances it is bluish, greenish, or yel lowish. A celebrated chemist (Berzelius) has lately stated that the common fly (one of the insecta) had red blood in the head, and colour less blood in the other parts of its body. It is true, indeed, that if the head of one of these insects be crushed, a reddish fluid is forced out; but this is not blood ; it proceeds from the eyes of the insect, whose blood, in the head as elsewhere, and among all the other species of the genus, as well as among the arach nida, crustacea, and mollusca, is almost co lourless.
From these differences in the appearance of the nutrient fluid, the animal kingdom has been divided into animals having red blood and animals having white blood. But these modifications of colour are not perhaps of so much consequence as has commonly been be lieved, for they are met with among animals having in all other respects the most striking analogy one with another, as has already been seen in our particular article on the ANNE LTDA.
The blood is an opaque, thickish fluid, of a specific gravity greater than that of water. In man its density varies from 1,052 to 1,057.
It has a saline and rather sickly taste, and it diffuses a peculiar odour, which varies somewhat in different tribes, and occasionally in the different sexes of the same species. In all the vertebrata, it is, as we have said, red ; but the shade of this colour varies in different animals, as it is familiarly known to do in the same animal, according as it is ex amined in its course to the tissues which it is destined to supply with nourishment, or after it has already traversed these, and is returning to the centre of the circulation ; the colour, how ever, may be stated to be generally deep.
Examined by the naked eye, the blood ap pears to he perfectly fluid and homogeneous ; but if it be spread in a very thin stratum upon the object plate of a microscope, and viewed under a lens having a magnifying power of between 200 and 300, it will be seen to con sist of two distinct and heterogeneous parts, viz. a transparent yellowish watery fluid, and a number of solid corpuscles, of extreme mi nuteness, suspended in this fluid. To the fluid portion, the name serum is minute corpuscles are spoken of as the gloules of the blood.
The discovery of the globules of the blood is almost contemporaneous with that of the microscope ; it is due to Malpighi and to Leuwenhoeck. A considerable number of ob servers have since engaged in the micro scopical study of the blood ; but it is to Hew son and to the Messrs. Prevost and Dumas that science is indebted for the most important facts and the best connected series of inquiries into the composition and qualities of this fluid.* The form of the globules of the blood varies in different animals, but appears to be at all times essentially the same in individuals of the same species; this at least is the case if we except the first periods of their embryotic existence ; for in the embryo the globules have been found to be different before the formation of the liver, from what they are after the deve lopment of this organ.