THE BLOOD 



103 



a firm red jelly. The jelly soon contracts and squeezes out a straw- 

 coloured fluid called the serum, in which the shrunken clot ultimately 

 floats. 



With the microscope, filaments of fibrin are seen forming a net- 

 work throughout the fluid, many radiating from small clumps of 

 blood platelets. It is the formation of fibrin which is the essential 

 act of coagulation : this entangles the corpuscles and forms the clot. 

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

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

 previously been removed. It may also be 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 



FIG. 28. Fibrin filaments and blood platelets : A, network of fibrin shown after washing away the 

 corpuscles from a preparation of blood that lias been allowed to clot. Many of the filaments 

 radiate from small clumps of blood platelets. B (from Osier), blood corpuscles and blood platelets 

 within a small vein. 



washing with water. Serum is plasma minus the fibrin which it 

 yields. The relation of plasma, serum, and clot can be seen at a 

 glance in the following scheme of the constituents of the blood : 



Blood.. 



f Plasma 



Serum 

 Fibrin] 



Corpuscles 



Clot 



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. 



6. Injection of nucleo-protein produces intravascular clotting 



