476 VETERINARY PHYSIOLOGY 



Clotting is due to changes in the plasma, since this fluid 

 ivill coagulate in the absence of corpuscles. 

 The change may be represented thus : — 



Blood 



Plasma 



Serum Clot Corpuscles 



The change consists in the formation of a series of fine 

 elastic threads of fibrin throughout the plasma, and, if red 

 corpuscles are present, they are entangled in the meshes of 

 the network and give the clot its red colour. 



These threads may be readily collected in mass upon a 

 stick with which the blood is whipped as it is shed. The 

 red fluid blood which is left, consisting of blood cells and 

 serum, is said to be defibrinated. 



Blood 



Plasma 



Fibrin Serum Corpuscles 



Defibrinated Blood 



A study of clotting blood by means of the ultra- 

 microscope shows that the fibrin first separates as small 

 acicular particles which run together to form threads. 



Fibrin is a protein substance. It is slowly dissolved in 

 solutions of neutral salts. It is coagulated by heat, and is 

 precipitated when an excess of a neutral salt is added. It 

 therefore belongs to the group of globulins. 



The plasma, before clotting, and the serum, squeezed out 

 from the clot, both contain in the same proportions an 

 albumin (serum albumin) and a globulin, or series of globulins, 

 which may be classed together as serum globulin. But the 



