THE BLOOD. 233 



blood which has been allowed to flow into a tall vessel surrounded by 

 a cooling mixture so as to prevent coagulation and thus permit the 

 red corpuscles to subside. If such plasma be subjected to room-tem- 

 perature, it very shortly undergoes coagulation, exhibiting practically 

 the same phenomena as blood itself. After a variable length of time 

 it also separates into a soft, colorless coagulum or clot consisting of 

 fibrin, and a clear fluid, the serum. /The presence of the red cor- 

 puscles is therefore not essential to the process of coagulation.) 



Rapidity of Coagulation. The rapidity with which the blood 

 coagulates varies in different classes of animals under the same con- 

 ditions: e. g., the blood of the pigeon coagulates immediately; that of 

 the dog, in from one to three minutes ; that of the horse, in from five 

 to thirteen minuteq^hat of man, in from four to seven minutes. 



The time, however, can be lengthened or shortened by either diang^" 



ing the external conditions or by altering temporarily the normal 

 composition of the blood. 



Coagulation is retarded or prevented by the following agents, 

 viz.: (i) A low temperature, especially that of melting ice. (2) The 

 addition of magnesium sulphate (i volume of a 25 per cent, solution 

 1o 3 volumes of blood); of sodium sulphate (i volume of a saturated 

 solution to 7 volumes of blood). (3) The addition of potassium 

 oxalate (i volumes of a i per cent, solution to 3 volumes of blood). 

 (4) The injection into the blood of commercial peptone. (5) The 

 mouth secretion of the leech. 



Coagulation is hastened by the following agents, viz.: (i) A 

 gradually increasing temperature from 38 C. to 50 C. (2) The 

 addition of water in not too large amounts. (3) The presence of 

 foreign bodies. (4) Agitation of the blood e. g., stirring. 



Fibrin and Defibrinated Blood. If freshly drawn blood is 

 stirred with a bundle of twigs or glass rods for a few minutes, the 

 fibrin collects on them in the form of thick bundles or strands; after 

 washing it with water the entangled corpuscles are removed, when 

 the fibrin assumes its natural white appearance. The strands can 

 be resolved into a large number of delicate fibers which possess ex- 

 tensibility and retractibility, and are therefore elastic. The chemic 

 features of fibrin have already been considered (see page 34). The 

 remaining fluid, similar in its physical appearance to the blood, is 

 termed defibrinated blood. When such blood is allowed to remain 

 at rest for a few days, the remaining red corpuscles gradually sink 

 to the bottom of the fluid, above which will be found the clear serum. 



COMPOSITION OF PLASMA AND SERUM. 



Plasma. The plasma obtained by any of the methods previously 

 described is a clear, colorless, transparent, slightly viscid fluid, with 



