168 CIRCULATORY SYSTEM. 



pose will be found to be rendered red — bloody, as it is termed ; 

 an effect arising from the commixture with it of the part of the 

 crassamentum now under our consideration, viz. the red parti- 

 cles. The colour of the blood is owing to the presence of innume- 

 rable prt/'/iV/cs or o7o6;//es, which are uniformly diffused through 

 it in the fluid state ; and in the coagulated, become entangled 

 within the substance of the fibrin. I have already remarked, 

 however, that blood is not red in all animals, nor even in every 

 part of the same animal ; still (according to Hewson) this white 

 or colourless blood possesses globules similar in form and struc- 

 ture to the coloured ones. The same author likewise informs us, 

 that the globules are of different magnitude in different ani- 

 mals, but that their volume bears no sort of proportion to the 

 size of the animal ; they being as large in the mouse and cat 

 as in the ass and ox ; larger than either in birds ; and largest of 

 all in the skate. 



In making these observations, however, I should remark that 

 no part of the body has afforded a wider field for speculation than 

 these red globules. Being so very minute as only to be discover- 

 able at all though a powerful microscrope, their shape and magni- 

 tude have become variously reported on according to the varying 

 circumstances under which these microscopic examinations have 

 been conducted : to one they have appeared in the form of perfect 

 spheres ; to another, as rings ; to a third, as flattened vesicles. 

 Dr. Young (whose account is recent, and pretty nearly coincides 

 with that of Hewson) found the globules in the skate to resemble 

 an almond in form, only to be less pointed and somewhat flat- 

 tened, and consisting of an external envelope containing a central 

 nucleus; as represented here below : — 



We are further informed by experimenters, that the colouring 

 matter of the globule resides principally or entirely in its external 

 covering, the central nucleus itself being without colour; and, 

 also, that it is only the coloured envelope which is soluble in 

 water; the nucleus still retaining its form while floating, and 

 being obtainable in an entire and separate state. 



Their magnitude has given rise to as much latitude of opinion 

 as their shape. Dr. Young estimates a globule of human blood 

 at s-ioo of a>i inch in diameter: and we may as well sit down 

 contented with this, as spend our time in searching after other 

 evidence, perhaps, after all, less conclusive. 



The colour of the globules is materially aflected by atmospheric 



