334 THE BLOOD. 



lactic acid. The mixture is boiled for about ten minutes, filtered, 

 and the precipitate washed separately with hot water, and finally 

 pressed in a piece of muslin. The filtrate and washings, which are 

 practically colorless, are then concentrated to a small volume. Any 

 traces of albumin which may still be present thus separate out and 

 are filtered off. If too much of the acid solution has been added, the 

 mixture may not clear on boiling. In that event a few crystals of 

 sodium carbonate are added, when coagulation promptly occurs. On 

 the other hand, it may at times be necessary to add a few drops 

 more of the acid solution. 



The remaining constituents of the plasma are also found in the 

 serum, and will be considered in that connection. 



The Serum. 



The serum results from the blood-plasma during the process of 

 coagulation. It is most conveniently obtained bv whipping blood 

 immediately after being shed, whereby the greater portion of the 

 fibrin is removed and the formation of large clots prevented. The 

 corpuscles and smaller pieces of fibrin are separated by centrifuga- 

 tion or by allowing the fluid to stand in the cold until sedimen- 

 tation has occurred. The serum is then siphoned off and filtered. 

 It thus appears as a slightly viscid, fairly transparent fluid of a light 

 straw color, which presents a feebly alkaline reaction and a specific 

 gravity varying in man between 1.026 and 1.029. In its chemical 

 composition serum differs from plasma principally in the presence 

 of the fibrin ferment and in the absence of fibrinogen. In its place, 

 however, traces of two other globulins, which are not present in the 

 plasma, are found. One of these is termed fibrinoglobulin, and is 

 thought to result during the formation of fibrin from fibrinogen. 

 The other is the so-called cell-globulin, and is supposedly referable 

 to the decomposition of leucocytes during the process of coagulation. 

 The remaining constituents are qualitatively the same in both fluids. 

 Slight quantitative differences, however, exist. A portion of the 

 calcium, magnesium, and phosphoric acid is thus eliminated 

 together with the fibrin, and accordingly lower values are found 

 in the serum than in the plasma. 



An idea of the mineral constituents of the serum, and their 

 quantitative relations, may be had from the accompanying table : 



Man. 



Potassium oxide 0.3870.401 pro mille. 



Sodium oxide 4.290 



Chlorine 3.5653.659 



Calcium oxide 0.155 



Magnesium oxide 0.101 



From this it will be seen that sodium in the form of the chloride 

 largely predominates in the serunr, while potassium occurs only in 



