BLOOD. 489 



4. The Blood-Serum.— Blood-plasma freed from fibrin constitutes 

 blood-serum, and, as already stated, may be obtained by allowing blood 

 to coagulate in a glass cylinder, when the gradual contraction of the 

 fibrin in the clot presses out the serum. 



Serum is an alkaline, transparent fluid, of a specific gravity of from 

 1026 to 1029, amber-yellow in the horse's blood, and almost colorless in 

 that of the other domestic animals. In carnivora and omnivora, as well 

 as in nursing herbivora, the serum is often milky from the contained fat- 

 globules, which gradually rise to the top to form a layer of cream ; this, 

 however, only occurs at, or shortly after, the period of fat absorption. 



In round numbers the composition of serum is as follows : 



"Water, . . ' 90 per cent. 



Proteids, 8 to 9 " " 



Fats, extractives, and salts, . . . . 2 to 1 " " 



The Albuminoids of the Serum. — The following albuminoids are 

 found in serum : Paraglobulin, alkali albuminate, serum-albumen, and fre- 

 quently peptone. 



Paraglobulin has already been considered under blood coagulation ; 

 in coagulation all the fibrinogen becomes solidified, while a considerable 

 amount of paraglobulin still remains in the serum. The amount remain- 

 ing in solution in the serum has been estimated at from 1.7 per cent, in 

 the rabbit to 4.5 per cent, in the horse. 



Alkali albumen (sodium albumen, serum-casein) is precipitated by 

 exact neutralization with acetic acid. It is insoluble in distilled water, 

 but readily soluble in dilute acid and alkalies. 



Serum-albumen exists in larger amount than all the other albumi- 

 noids, the serum containing from 6 to 8 per cent. As already mentioned, 

 it differs from egg-albumen in rotating polarized light 56° to the left, 

 while egg-albumen rotates it but 35.5°, and in its behavior to ether and 

 acids. After removing paraglobulin and alkali albuminate, serum-albu- 

 men may be completely coagulated after acidulation and dilution by heat 

 (70°-75° C). 



The quantitative proportions between serum-albumen and para- 

 globulin vary in different animal species. The proportion, while not 

 even constant in any given species, varies about as follows, paraglobulin 

 being represented by the numerator of the fraction, serum-albumen by 

 the denominator : Horse ^.sVt) O x tt.s?2i Man T .jy T , Dog T .^ 3 , Rabbit 5J5. 



Peptone differs from other albuminoids in that it is not coagulated 

 by heat or acetic acid and potassium ferrocyanide, but is precipitated by 

 tannic acid, corrosive sublimate, absolute alcohol in great excess, phos- 

 phowolframic acid, phosphomolybdic acid, and iodide of mercury and 

 potassium. 



