/ 



SEC. 2. THE CHEMICAL COMPOSITION OF BLOOD. 



As we have already urged, the chief chemical interests of blood 

 are attached to the changes which it undergoes in the several 

 tissues, and which will be considered in connection with each tissue 

 at the appropriate place. Nevertheless a brief summary of the 

 main characters of blood as a whole may be introduced here. 

 ( 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 blood-vessels is found to be distinctly 

 though feebly alkaline. 



If the corpusclelTbe supposed to retain the amount of water 

 proper to them, blood may, in general terms, be considered as 

 consisting by weight of from about one-third to somewhat less than 

 one-half of corpuscles, ;bhe rest being plasma. As will be insisted 

 on presently, the number of corpuscles in a specimen of blood 

 is found to vary considerably, not only in different animals and in 

 different individuals, but in the same individual at different times. 



Conspicuous and striking as are the results of coagulation, 

 massive as appears to be the clot which is formed, it must be remem- 

 bered that by far the greater part of the clot consists of corpuscles. 

 The amount by weight of fibrin required to bind together a number 

 of corpuscles in order to form even a large firm clot is exceedingly 

 small. Thus the average quantity by weight of fibrin in human 

 blood is said to be '2 p. c., but the amount which can be obtained 

 from a given quantity of plasma varies extremely; the variation 

 being due not only to circumstances affecting the blood, but also to 

 the method employed. 



The difficulties indeed of acquiring an exact knowledge of the 

 chemical constitution of the plasma, which as we have seen from the 



