78 INTRODUCTION. 



oped in the system, is taken up by the circulating blood and 

 exhaled by the lungs. When the circulation is arrested in any 

 part, of course the blood takes up no more ammonia ; and as 

 that which it contained is gradually exhaled through the 

 tissues, arrest of the circulation in any part for a certain time 

 is followed by coagulation of the fibrin. When blood is 

 drawn from the vessels, the exhalation of ammonia is rapid, 

 and coagulation takes place very readily. Some other chem- 

 ical substances, such as the carbonate of soda, have the 

 power of maintaining the fluidity of the fibrin. 



Fibrin does not coagulate into a homogeneous mass, but 

 forms minute microscopic filaments, or fibrils, which after- 

 wards contract for ten or twelve hours, so that the clot at 

 the end of that time is much smaller than immediately after 

 coagulation. 



We recognize only as fibrin that liquid organic principle 

 which coagulates whenever removed from its natural con- 

 dition. By coagulation its form only is changed, not its 

 weight, and we must consider, therefore, the water which is 

 contained in the coagulated mass as water of composition. 



Pure coagulated fibrin is a grayish-white substance, com- 

 posed of microscopic fibrils, and possessing considerable 

 strength and elasticity. It is insoluble in water and in the 

 serum of the blood, but dissolves slowly in solutions of caustic 

 alkalis. It swells, assumes a jelly-like consistence, and is 

 finally partially dissolved in a very feeble mixture of hydro- 

 chloric acid and water. Like all principles of this class, it 

 decomposes at a moderate temperature in contact with the 

 air and moisture. 



Organization of Fibrin. The question of the organiza- 

 tion of accidentally effused and coagulated fibrin has occupied 

 the attention of pathologists a great deal, and some are of 

 opinion that it is capable of becoming part of the organized 

 living structure. This supposition had its origin in an 

 assumed identity between fibrin and reparative lymph, or, 



