CHEMICAL EXAMINATION OF THE BLOOD. 317 



and incinerated. The weight of the ash is deducted from the weight 

 of the precipitate. 



More accurate is the following method : a carefully measured 

 and weighed amount of the plasma is treated with one-half its volume 

 of a saturated solution of sodium chloride and a slight excess of 

 tannic acid. In the resulting precipitate the nitrogen is then esti- 

 mated according to KjeldahPs method. When multiplied by 6.37 

 the corresponding amount of albumin is obtained. 



To remove all albumins from the blood, Cavazzani's method may 

 be employed. To this end, 20-30 c.c. of blood are added to 200 c.c. 

 of distilled water, and treated with five or six drops of a solution 

 consisting of ten parts of acetic acid (sp. gr. 1.040) and one part of 

 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 m.ost conveniently obtained by 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 between 1.026 and 1.029 in man. 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 



