THE BLOOD. . 161 



but faint alkaline reaction, owing chiefly to the presence of 

 sodium salts, a saline taste, and a faint odor characteristic of 

 the animal group to which it belongs, owing probably to volatile 

 fatty acids. The specific gravity of human blood varies between 

 1045 and 1075, with a mean of 1Q55 ; the specific gravity of the 

 corpuscles being about 1105 and of the plasma 1027. This dif- 

 ference explains the sinking of the corpuscles in blood with- 

 drawn from the vessels and kept quiet. Much the same diffi- 

 culties are encountered in attempts at the exact determination 

 of the chemical composition of the blood, as in the case of other 

 living tissues. Plasma alters its physical and its chemical com- 

 position, to what extent is not exactly known, when removed 

 from the body. 



Composition of Serum, — The fluid remaining after coagula- 

 tion of the blood can, of course, be examined chemically with 

 considerable thoroughness and confidence. 



By far the greater part of serum consists of water; thus, it 

 has been estimated that of 100 parts the following statement will 

 represent fairly well the proportional composition : 



Water 90 parts; 



Proteids 8 to 9 " 



Salines, fats, and extractives (small in 

 quantity and not readily obtained 

 free) lto3 parts. 



The proteids are made up of two substances which can be 

 distinguished by solubility, temperature at which coagulation 

 occurs, etc., known as paraglobulin and serum-aUmmen, and 

 which may exist in equal amount. 



It is not possible, of course, to say whether these substances 

 exist as such in the living blood-plasma or not. 



The fats are very variable in quantity in serum, depend- 

 ing on a corresponding variability in the plasma, in which 

 they would be naturally found in greatest abundance after a 

 meal. They exist as neutral stearin, palmitin, olein, and as 

 soaps. 



The principal extractives found are urea, creatin and allied 

 bodies, sugar, and lactic acid. Serum in most animals contains 

 more of sodium salts than the corpuscles, while the latter in 

 man and some other mammals contain a preponderating quan- 

 tity of potassium compounds. 



The principal salts of serum are sodium chloride, sodium bi- 

 carbonate, sodium sulphate and phosphate; in smaller quantity, 

 11 



