THE BLOOD 83 



After blood is shed it rapidly becomes viscous and then sets into 

 a solid 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 plasma, many radiating from small clumps of 

 blood tablets. 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 hav- 

 ing 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 





Fig. 28. — Fibrin filaments and blood tablets : A, network of fibrin shown after washing away the 

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

 radiate from smaU clamps of blood tablets ; B (from Osier), blood corpuscles and blooil tablets 

 within a small vein. 



washing with water. Semm is plasma mimes fibrin. The relation 

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

 scheme of the constituents of the blood : — 



( Plasma i^®^^°^ 

 Blooil I IFibnn) 



^^^^1 I Clot 



( Corpuscles ] 



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 eoat) consists mainly of fibrin 

 and white corpuscles. 



Coagulation is hastened by — 



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



2. Contact with foreign matter. 



3. Injury to the vessel walls. 



4. Agitation. 



5. Addition of calcium salts. 



62 



