2 8o VETERINARY LECTURES 



fibrinogen, when acted upon by fibrin ferment, gives rise to the 

 fibrin found in clotted blood. 



425. Extractives. — These are of two kinds — viz., nitrogenous 

 and non-nitrogenous. The nitrogenous extractives are urea, uric 

 acid, hippuric acid, creatin, creatinine, xanthine, and hypo-xanthine ; 

 the non-nitrogenous are fats, soaps, cholesterine, and sugar. 



426. Mineral Matter. — The salts in solution in the blood are 

 chiefly the salts of potash, soda, lime, and magnesium, phosphates, 

 and iron ; of these, by far the most plentiful is common salt, or 

 chloride of sodium, of which the ash of human blood is said to 

 contain as much as 54 per cent. This substance (salt) must, there- 

 fore, play an exceedingly important part in the body, and, from its 

 presence and great germicidal properties, may, doubtless, assist the 

 opsonin and the phagocytes in their work of protecting the system 

 from the invasion of disease-producing germs. 



427. The Clotting of Blood. — When blood is drawn from the 

 body, it does not remain fluid, but in a short time forms into a jelly- 

 like mass. Then, if left to stand for a few hours longer, it separates 

 into (1) the crassamentum — a firm red clot, consisting almost entirely 

 of red corpuscles, entangled in a network of fibrin — and (2) the serum 

 — a clear, pale, straw-coloured fluid, in which the clot floats. Fibrin 

 is, normally, only produced when the blood is shed, or in some other 

 way deprived of its vitality. Sometimes, under abnormal conditions, 

 fibrin may be produced in the living body, such as in the ante-mortem 

 clots, found in the heart and bloodvessels. 



428. Serum may be considered as plasma from which the fibrin- 

 forming elements have been removed during clotting. It contains 

 proteids, extractives, and salts, just as the plasma does. The 

 extractives and salts are the same in both fluids ; but the proteids of 

 serum are serum albumin, serum globulin, and fibrin ferment. 



