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 the gases, 

 oxygen, carbonic acid, and nitrogen, which are held in the blood in 

 a peculiar way, and which are given off from blood when exposed 

 to a vacuum or to an atmosphere of suitable composition ; the 

 relative amounts differ in different kinds of blood, and so serve 

 especially to distinguish arterial from venous blood. 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 



