133 HANDBOOK OF PHYSIOLOGY. 



the same volume as the previously liquid blood, and adheres so closely 

 to the sides of the containing vessel that if the latter be inverted none 

 of its contents escape. The solid mass is the crassamentum or clot. If 

 the clot be watched for a few minutes, drops of a light, straw-colored 

 fluid, the serum, may be seen to make their appearance on the surface, 

 and, as they become more and more numerous, to run together, form- 

 ing a complete superficial stratum above the solid clot. At the same 

 time the fluid begins to transude at the sides and at the under-surface 

 of the clot, which in the course of an hour or two floats in the liquid. 

 The first drops of serum appear on the surface about eleven or twelve 

 minutes after the blood has been drawn; and the fluid continues to 

 transude for from thirty-six to forty-eight hours. 



The clotting of blood is due to the development in it of a substance 

 called, fibrin, which appears as a meshwork (fig. 117) of fine fibrils. This 

 meshwork entangles and incloses within itself the blood corpuscles. 

 The first clot formed, therefore, includes the whole of the constituents 

 of the blood in an apparently solid mass, but soon the fibrinous mesh- 

 work begins to contract and the serum which does not belong to the 

 clot is squeezed out. When the whole of the serum has transuded the 

 clot is found to be smaller, but firmer and harder, as it is now made up 

 of fibrin and blood corpuscles only. Thus coagulation rearranges the 

 constituents of the blood; liquid blood being made up of plasma and 

 blood corpuscles, and clotted blood of serum and clot. 



Liquid Blood. 



Clotted Blood. 



Under ordinary circumstances coagulation occurs before the red cor- 

 puscles have had time to subside; and thus from their being entangled 

 in the meshes of the fibrin, the clot is of a deep red color throughout, 

 probably slightly darker at the most dependent part, from greater accu- 

 mulation of red corpuscles there than elsewhere. When, however, coag- 

 ulation is from any cause delayed, as when blood is kept at a tempera- 

 ture slightly above C. (32 F.), or when clotting is naturally slow, as 

 is the case with horse's blood, or, lastly, in certain diseased conditions, 

 particularly in inflammatory states, time is allowed for the colored cor- 

 puscles to sink to the bottom of the fluid. When clotting after a time 

 occurs, the upper layers of the blood are free of colored corpuscles and 



