350 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



2. Plasma. — The liquid in which the globules swim 

 (plasma) is a solution of fibrin, albumen, and salts, with 

 often a trace of sugar and fats. 



Coagulation of Blood. — If blood be drawn and 

 allowed to stand it coagulates. This is produced by the 

 solidification of the fibrin. All albuminoids pass more 

 or less easily from the liquid to the solid state. Albu- 

 men, or white of egg, is solidified by heat at 160 , or by 

 alcohol ; casein of milk is solidified by acid and heat at 

 70° to 80° or less. Fibrin solidifies much more easily; 

 exposed to air or touching any foreign body, it solidifies 

 at once. The causes and conditions of the solidification 

 of fibrin are still under discussion. 



If blood be drawn and allowed to stand, as already 

 said, it coagulates first into a tremulous jelly by the 

 solidification of the fibrin. If it continues to stand, 

 gradually the fibrin contracts more and more firmly, and 

 a liquid is squeezed out. The whole mass thus separates 

 into two parts, clot and serum. The clot consists of 

 the solidified fibrin and the entangled globules. The 

 serum consists of a solution of albumen and salts, with 

 perhaps a trace of sugar. This is shown in the follow- 

 ing schedule : 



Blood 



Globules . 



Fresh. Coagulated. 



Red. 



Plasma, solution of. 



< Keel. 

 1 White, 

 f Fibrin. 



Albumen. 



Salts. 



Sugar, a trace 



Fat, a trace. 



Clot. 



Serum. 



In fever blood coagulates more slowly. In coagu- 

 lation the globules have time to settle a little before 

 the solidification of the fibrin, leaving thus the char- 

 acteristic buff coat of fevers on the top. If salt be added 



