16 ORGANISED FLUIDS. 



tractility, forms a compact mass or clot, the diameter of 

 which is less than that of the vessel in which it is contained ; 

 while the lighter serum floats on the top and in the space 

 around the clot. 



Now the only active agent in this change in the arrange- 

 ment of the different constituents of the blood, is the fibrin ; 

 and although the globules of the blood constitute a portion 

 of the clot, yet they take no direct part in its formation, and 

 their presence in it is thus accounted for; the fibrin, in 

 coagulating, assumes a filamentous and reticular structure, 

 in the meshes of which the globules become entangled, and 

 thus are made to contribute to the composition of the clot, the 

 bulk of which they increase, and to which they impart the 

 red colour. 



It was an ancient theory that the clot was formed solely 

 by the union of the globules with each other. The fallacy 

 of this opinion is easily demonstrated by the two following 

 decisive experiments : 



The first is that of Miiller, on the blood of the frog, who 

 separated, by means of a filter, the globules from the fibrin, 

 the latter still forming a clot, although deprived of the glo- 

 bules. This experiment is not, however, applicable to the 

 blood of man, or of mammalia in general, the globules in 

 these being too small to be retained by the filter. The second 

 expedient is, however, perfectly suited to the human blood. 

 It is well known that if blood, immediately after its removal 

 from the body, be stirred with a stick, the fibrin will adhere 

 to it in the form of shreds : the blood being defibrinated by 

 this means, the globules fall to the bottom of the basin in 

 which the blood is contained, on account of their gravity ; 

 but they do not cohere so as to form a clot, remaining dis- 

 connected and loose. 



It is difficult to determine the exact time which the blood 

 takes to coagulate, because this coagulation is not the work of 

 a moment ; but, from its commencement to its completion, the 

 process occupies usually several minutes. The first evidence 

 of the formation of the clot, is the appearance of a thin and 

 greenish serum on the surface of the blood, in which may be 



