116 PRINCIPLES OF PHYSIOLOGY 



that the corpuscles, in single file, are 

 carried along by the stream. When shed, 

 however, it quickly coagulates, that is it 

 clots, and the clot, in a suitable vessel, is 

 soon surrounded by and floats in a serum. 

 This power of clotting no doubt is a salutary 

 function, as when vessels have been accident- 

 ally cut, they are, as a rule, soon plugged by 

 the clot, and bleeding ceases. Much investi- 

 gation has been expended on the phenomenon 

 of clotting, a process somewhat analogous to 

 the clotting of milk when it sours, from the 

 formation of lactic acid, or when acted on by 

 the ferment of rennet. The milk separates into 

 curd and whey ; the blood separates into serum 

 and clot. A clot consists of blood corpuscles 

 and a substance called fibrin. If we place 

 a piece of blood clot under a water tap, we can 

 wash out the corpuscles and we obtain fibrin 

 in the form of a yellowish fibrous material. 

 It is evident that the formation of fibrin, which 

 entangles the corpuscles in its fibrous meshes, 

 produces a blood clot. There is, however, no fib- 

 rin in blood, but a substance called fibrinogen. 

 The theory at present in vogue is that when 



