4IO THE BLOOD. [CH. xxvl. 



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 : 



(Serum 



Plasma I*,:,. 

 ( Fibrin x 



Blood- 



I Clot 



Corpuscles 



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

 60-65 P ar> ts consist of plasma and 35-40 of corpuscles. 



The bufy 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 neiitral salts like sodium 

 sulphate or magnesium sulphate. 



3. Contact with the living vascular walls. 



4. Contact with oil. 



5. Additionofoxalat.es. These precipitate the calcium necessary 

 for coagulation as insoluble calcium oxalate. 



6. Injection of commercial peptone (which consists chiefly of 

 proteoses) into the circulation of the living animal. 



7. Addition of leech extract. This acts in virtue of a proteose 

 it contains. 



The theory generally received which accounts best for the 

 coagulation of the blood is that of Hammarsten, and it may be 

 briefly stated as follows : 



When blood is in the vessels one of the constituents of the plasma, a 

 proteid of the globulin class called fibrinogen, exists in a soluble form. 



When the blood is shed the fibrinogen molecule is split into two 

 parts : one part is a globulin, which remains in solution ; the other 

 is the insoluble material Jibrin. 



