CH. XXIX.] PLASMA AND SERUM 453 



both liquids. The proteins are different, as is shown in the following 

 table : 



Proteins of Plasma. Proteins of Serum. 



Fibrin ogen. Serum globulin. 



Serum globulin. Serum albumin. 



Serum albumin. Fibrin-ferment + nucleo-protein. 



The gases of plasma and serum are small quantities of oxygen, 

 nitrogen, and carbonic acid. The greater part of the oxygen of the 

 blood is combined in the red corpuscles with haemoglobin; the 

 carbonic acid is chiefly combined as carbonates. The gases of the 

 blood have already been considered under Eespiration (see pp. 361- 

 372). 



We may now study one by one the various constituents of the 

 plasma and serum. 



A. Proteins. Fibrinogen, the mother-substance of fibrin, is a 

 globulin. It differs from serum globulin, and may be separated from 

 it by making use of the fact that half-saturation with sodium 

 chloride precipitates it. It coagulates by heat at the low tempera- 

 ture of 56 C. 



Serum globulin and serum albumin. These substances exhibit the 

 usual differences already described between albumins and globulins 

 (p. 428). Both are coagulated by heat at a little over 70 C. They 

 may be separated by dialysis or the use of neutral salts.* The 

 readiest way to separate them is to add to the serum an equal volume 

 of saturated solution of ammonium sulphate. This is equivalent to 

 semi-saturation, and it precipitates the globulin. If magnesium 

 sulphate is used as a precipitant of the globulin it must be added in 

 the form of crystals, and the mixture well shaken to ensure complete 

 saturation. 



Fibrin-ferment or Thrombin. Schmidt's method of preparing it 

 is to take serum and add excess of alcohol. This precipitates all the 

 proteins and the thrombin. After some weeks the alcohol is poured 

 off; the serum globulin and serum albumin have been by this means 

 rendered insoluble in water; an aqueous extract is, however, found 

 to contain fibrin-ferment, which is not so easily coagulated by alcohol 

 as the proteins are. 



B. Extractives. These are non-nitrogenous and nitrogenous. 

 The non-nitrogenous are fats, soaps, cholesterin, and sugar; the 

 nitrogenous are urea (0*02 to 0'04 per cent.), and still smaller 

 quantities of uric acid, creatine, creatinine, xanthine, and hypo- 

 xanthine and ammo-acids. 



* The globulin of the serum precipitated by " salting out" really consists of 

 two proteins, one of which is precipitated by dialysis (euglobulin), and the other is 

 not (pseudo-globulin). 



