THE BLOOD 235 



takes place more actively in the center, there being here less resistance than 

 at the sides of the coagulum, the upper surface usually becomes depressed 

 or cupped. 



Coagulation of Plasma. Clear plasma may be obtained by means of 

 the centrifuge from blood to which sufficient magnesium sulphate has been 

 added to prevent coagulation, or from horse's 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 

 is subjected to room-temperature, 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 corpuscles 

 is therefore not essential to the process of coagulation. 



Rapidity of Coagulation. The rapidity with which the blood coagu- 

 lates varies in different classes of animals under the same conditions: 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 minutes; that of 

 man, in from four to seven minutes. The time, however, can be lengthened 

 or shortened by either changing 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 to 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 volume of a i per cent, 

 solution to 3 volumes of blood). (4) The injection into the circulating 

 blood of commercial peptone. (5) The mouth secretion of the leech. 



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

 gradually increasing from 38C. to 5oC. (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; on 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 extensibility and retractility, and are therefore elastic. 

 The chemic features of fibrin have already been considered (see page 19). 

 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. 



CHEMIC 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 a specin< 

 gravity of 1.026 to 1.029. It is composed largely of water holding in solution 

 proteins, sugar, fat, inorganic salts, urea, cholestenn, lecithin, etc. In 

 composition it is quite complex, containing as it does not only the nutritive 



