8o THE BLOOD. 



two principal forms, the red and the white corpuscles. 

 "When coagulation has taken place quickly, both kinds of 

 corpuscles may be uniformly diffused through the clot; 

 but, when it has been slow, the red corpuscles, being the 

 heaviest constituent of the blood, tend by gravitation to 

 accumulate at the bottom of the clot ; and the white cor- 

 puscles, being among the lightest constituents, collect in 

 the upper part, and contribute to the formation of the 

 buffy coat. 



The human red Wood-corpuscles (fig. 26) are circular flattened 

 cells of different sizes, the majority varying in diameter 

 from -g-^L _ to -j-oVo- f an i ncn > an( l a ^ out T O <jo~s- f an i nc k 

 in thickness. Their borders are rounded ; their surfaces, 

 in the most perfect and usual state, slightly concave ; but 

 they readily acquire flat, or convex surfaces when, the 

 liquor sanguinis being diluted, they are swollen by absorb- 

 ing more fluid into their cavity. They are composed of a 

 colourless, structureless, and transparent envelope, enclos- 

 ing a peculiar matter named cruor, or, as it may be termed, 

 cruoro-globulin. The cell-wall is tough and elastic, so 

 that, as the cells circulate, they admit of elongation and 

 other changes of form, in adaptation to the vessels, yet 

 recover their natural shape as soon as they escape from 

 compression. They have no nuclei, and their contents are 

 probably homogeneous ; at least they appear so, when 

 their surfaces are flat or slightly convex ; it is only when 



the case of the oval cells, only the long diameter is here given. It is 

 remarkable, that although the size of the red blood-cells varies so much 

 in the different classes of the vertebrate kingdom, that of the white 

 corpuscles remains comparatively uniform, and thus they are, in some 

 animals, much greater, in others much less than the red corpuscles 

 existing side by side with them. 



It may be here remarked, that the appearance of a nucleus in the red 

 blood-cells of birds, reptiles, amphibia and fish, has been lately shown 

 by Mr. Savory to be the result of post-mortem change; no nucleus 

 being visible in the cells as they circulate in the living body, or in those 

 which have just escaped from the blood-vessels. 



