24 THE BLOOD 



In addition to the structures in the blood already described, the 

 plasma often contains minute fatty granules, especially during diges- 

 tion, such as are found in large quantity in chyle and in smaller quan- 

 tity in lymph. The lymph, like the blood, is composed of plasma and 

 corpuscles, the latter being chiefly in the form of lymphocytes ; and the 

 lymph also, like the blood, is capable of coagulation. Blood-platelets, 

 however, have not been observed in lymph. 



PLASMA AND SERUM 



Before coagulation, the blood is composed of a relatively clear 

 plasma holding the corpuscles in suspension. After coagulation, the 

 clot, enclosing the corpuscles, separates and leaves the serum, which 

 is the plasma less the constituents that form fibrin. The plasma is 

 slightly yellowish, alkaline and of a specific gravity of 1026 to IO2Q. It 

 contains proteids, extractives (including fats) and inorganic salts. The 

 proteids consist of fibrinogen, paraglobulin (serum-globulin) and serum- 

 albumin ; and their proportion is about eight parts per thousand. 



Fibniiogen. Fibrinogen (C 108 H 162 N 30 SO 34 ) is a globulin ; but 

 it is distinguished from the other globulins of the plasma by its be- 

 havior in the presence of certain reagents. It is precipitated by half- 

 saturation of the plasma with sodium chloride and is coagulated at a 

 relatively low temperature, 133 Fahr. (56 C.). Under the influence 

 of the so-called fibrin-ferment, its molecule splits into fibrin, which is 

 insoluble, and a soluble globulin. Its proportion in the blood is about 

 three parts per thousand. 



Serum-globulin. Serum -globulin (C&fHraNgoSOgg + | H 2 O) exists in 

 the blood in the proportion of about thirty-one parts per thousand. It 

 coagulates at a temperature of about 158 Fahr. (70 C.). It may be 

 separated from the neutral salts by dialysis. Both fibrinogen and serum- 

 globulin are precipitated by magnesium sulphate. It directly or in- 

 directly nourishes the proteids of the tissues, perhaps undergoing 

 previously a change. into serum-albumin. It may be derived from food, 

 but it is thought by some to be a product of disintegration of leucocytes. 



Serum-albumin. Serum-albumin (C^H^N^SC^) is the most impor- 

 tant nutritive proteid of the blood. It is the only proteid that is not 

 precipitated by magnesium sulphate. It coagulates at a temperature of 

 158 Fahr. (70 C.). Its proportion in the blood is about forty-five parts 

 per thousand. It is probable that it is not a simple albumin but is 

 composed of two or three different albumins. It is highly osmotic, a 

 property that distinguishes it from ordinary egg-albumin. 



