178 



when acted upon by fibrin ferment, gives rise to the fibrin found in 

 clotted blood. 



423. Extractives. — These are of two kinds, viz., nitrogenous and 

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

 hippuric acid, creatine, creatinine, xanthine, and hypo-xanthine ; the 

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



424. 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, therefore, play an exceedingly 

 important part in the body, and, from its presence and great 

 germicidal properties, may, doubtless, assist the phagocytes in their 

 work of protecting the system from the invasion of disease-producing 

 germs. 



425. 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 (i) the Crassamentum — a firm red clot, consisting almost entirely 

 of red corpuscles, entangled in a network of fihvin — 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. 



426. 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. 



