THE CHEMICAL COMPOSITION OF BLOOD. 



51 



[FiG. 10. 



ular aspect. They are called blood platelets, or blood plaques. They have 



been supposed by some to become developed into, and, indeed, to be early 



stages of, the red corpuscles, arid hence have 



been called ha3inatoblasts ; but this view has 



not been confirmed ; indeed, as we have seen 



( 27), the real hsematoblasts, or developing red 



corpuscles, are of quite a different nature. 



They speedily undergo change after removal 

 from the body, apparently dissolving in the 

 plasma ; they break up, part of their substance 

 disappearing, while the rest becomes granular. 

 Their granular remains are apt to run together, 

 forming in the plasma the shapeless masses 

 which have long been known and described as 

 <4 lumps of protoplasm." By appropriate re- 

 agents, however, these platelets may be fixed 

 and stained in the condition in which they 

 appear after leaving the body. 



The substance composing them is peculiar, 

 and, though we may perhaps speak of them as 

 consisting of living material, their nature is at 

 present obscure. They may be seen within the 

 living bloodvessels [Fig. 10], and therefore 

 must be regarded as real parts of the blood, 

 and not as products of the changes taking vem -3 

 place in blood after it has been shed. 



When a needle or thread or other foreign body is introduced into the 

 interior of a bloodvessel, they are apt to collect upon, and, indeed, are the 

 precursors of the clot which in most cases forms around the needle or thread. 

 They are also found in the thrombi or plugs which sometimes form in the 

 bloodvessels as the result of disease or injury. Indeed, it has been main- 

 tained that what are called white thrombi (to distinguish them from red 

 thrombi, which are plugs of corpuscles and fibrin) are in reality aggrega- 

 tions of blood platelets ; and for various reasons blood platelets have been 

 supposed to play an important part in the clotting of blood, carrying out 

 the work which, in this respect, is by others attributed to the white corpus- 

 cles. But no very definite statement can at present be made about this ; 

 and, indeed, the origin and whole nature of these blood platelets is at pre- 

 sent obscure. 



Fibrin Filaments and Blood 

 Platelets: A, network of fibrin, 

 shown after washing away the 

 corpuscles from a preparation of 

 blood that has been allowed to 

 clot ; many of the filaments radiate 

 from small clumps of blood plate- 

 lets. B (from Osier), blood cor- 

 puscles and elementary particles 

 or blood platelets within a small 



THE CHEMICAL COMPOSITION OF BLOOD. 



34. We may now pass briefly in review the chief chemical characters 

 of blood, remembering always that, as we have already urged, the chief 

 chemical interests of blood are attached to the changes which it undergoes 

 in the several tissues; these will be considered in connection with each 

 tissue at the appropriate place. 



The average specific gravity of human blood is 1055, varying from 1045 

 to 1075 within the limits of health. 



The reaction of blood as it flows from the bloodvessels is found to be 

 distinctly though feebly alkaline. If a drop be placed on a piece of faintly 

 red, highly glazed litmus paper, and then wiped off", a blue stain will be left. 



The whole blood contains, in a certain quantity, gases viz., oxygen, car- 

 bonic acid, and nitrogen, which are held in the blood in a peculiar way 

 which vary in different kinds of blood, and so serve especially to distin- 



