226 MANUAL OF PHYSIOLOGY. 



much as 1 per cent., commonly varying from 0.1 per cent, to 0.3 

 per cent, of the entire mass of blood. 



SERUM. 



This name is given to the clear fluid which oozes out of the 

 clot of plasma. It only differs from the latter in its chemical 

 composition in so far that fibrin is separated from it. Though 

 chemically this is a slight difference, it signifies the change from 

 a complex living body (blood plasma) into a solution of dead 

 albumins, etc. 



Serum is a clear, straw-colored alkaline fluid of 1028-1030 sp. 

 gr., holding in solution different organic substances and some 

 inorganic salts. After a full meal the serum is said to be more 

 or less milky from the presence of finely divided fat. 



It contains about 9 per cent, of solid matters, of which a large 

 proportion, 7 per cent., are proteids. Of these, the most abundant 

 is (1) serum albumin (about 4 per cent, in man), a solution of 

 which becomes opaque at 60 C., and coagulates at a heat of 73- 

 75 C. The proteid next in importance is (2) serum globulin or 

 paraglobulin (about 3 per cent, in man), which has already baen 

 mentioned. It may be precipitated imperfectly by CO 2 , or com- 

 pletely by magnesium sulphate. (3) Serum casein has been ob- 

 tained from serum by careful neutralization with acetic acid after 

 the removal of the paraglobulin by CO 2 . This is said to be para- 

 globulin which has failed to come down with the CO 2 . (4) Neu- 

 tral fats in a state of fine subdivision are present in a variable 

 quantity ; also (5) lecithin ; (6) traces of sugar ; (7) various 

 products of tissue change kreatine, urea, etc. ; and (8) inorganic 

 salts, viz., sodium chloride, about 5 per cent., and sodium car- 

 bonate, which probably existed in the blood as sodium hydric 

 carbonate. There is also a small quantity of potassium chloride. 

 But it should be remembered that there is about ten times more 

 sodium than potassium salts in the serum, and probably in the 

 blood plasma. 



