SEC. 3. 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 blood vessels 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 a certain quantity of gases, viz. oxygen, 

 carbonic acid and nitrogen, which are held in the blood in a pecu- 

 liar way, which vary in different kinds of blood, and so serve 

 especially to distinguish arterial from venous blood, and which may 

 be given off from blood when exposed to an atmosphere, according 

 to the composition of that atmosphere. These gases of blood we 

 shall study in connection with respiration. 



The normal blood consists of corpuscles and plasma. 



If the corpuscles be 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, the rest being plasma. As we have already 

 seen, 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. 



The plasma, is resolved by the clotting of the blood into serum 

 and fibrin. 



35. The serum contains in 100 parts 



Proteid substances about 8 or 9 parts. 



Fats, various extractives, and saline matters 2 or 1 



Water 90 



F. 4 



