CII. XXVI.] COAGULATION OF THE BLOOD 413 



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, as it is now 

 made up chiefly of fibrin and blood corpuscles. Thus coagulation 

 re-arranges the constituents of the blood ; liquid blood is made up of 

 plasma and blood corpuscles, and clotted blood of serum and clot. 



Fibrin is formed from the flasma, and may be obtained free from 

 corpuscles when blood -plasma is allowed to clot, the corpuscles having 

 previously been removed. It may be also obtained from blood by 

 whipping it with a bunch of twigs ; the fibrin adheres to the twigs 

 and entangles but few corpuscles. These may be removed by washing 

 with water. Serum is plasma minus fibrin. The relation of plasma, 

 serum, and clot can be seen at a glance in the following scheme of the 

 constituents of the blood : 



fSerum 

 Plasma | p 



1 

 ICormiscles 



It may be roughly stated that in 100 parts by weight of blood 60-65 

 parts consist of plasma and 35-40 of corpuscles. 



The "buffy coat is seen when blood coagulates slowly, as in horse's 

 blood. The red corpuscles sink more rapidly than the white, and 

 the upper stratum of the clot (buffy coat) consists mainly of fibrin 

 and white corpuscles. 



Coagulation is hastened by 



1. A temperature a little over that of the body. 



2. Contact with foreign matter. 



3. Injury to the vessel walls. 



4. Agitation. 



5. Addition of calcium salts. 

 Coagulation is hindered or prevented by 



1. A low temperature. In a vessel cooled by ice, coagulation may 

 be prevented for an hour or more. 



2. The addition of a large quantity of neutral salts like sodium 

 sulphate or magnesium sulphate. 



3. Contact with the living vascular walls. 



4. Contact with oil. 



5. Addition of soluble oxalates. These precipitate the calcium 

 necessary for coagulation as insoluble calcium oxalate. Sodium 

 fluoride or citrate may be used instead of the oxalate. 



6. Injection of commercial peptone (which consists chiefly of 

 proteoses) into the circulation of the living animal. 



