410 BLOOD AND LYMPH. 



Proteids of the Blood-plasma. The general properties and 

 reactions of proteids and the related compounds, as well as a classi- 

 fication of those occurring in the animal body, are described briefly 

 in the appendix. This description should be read before attempt- 

 ing to study the proteids of the plasma and the part they take in 

 coagulation. Three proteids are usually described as existing in the 

 plasma of circulating blood, namely, fibrinogen, paraglobulin, 

 or, as it is sometimes called, " serum-globulin," and serum-albumin. 

 The first two of these proteids, fibrinogen and paraglobulin, belong to 

 the group of globulins, and hence have many properties in common. 

 Serum-albumin belongs to the group of so-called "native albumins" 

 of which egg-albumin constitutes another member. 



Serum-albumin. This substance is a typical proteid. It can be 

 obtained readily in crystalline form from the horse's blood. Its 

 percentage composition, according to Michel, is as follows: C, 53.08; 

 H, 7.10; N, 15.93; S, 1.90; O, 21.96. 



Its molecular composition, according to Schmiedeberg,* may be 

 represented by C 78 H 122 N 20 SO 24 or some multiple of this formula. 

 Serum-albumin shows the general reactions of the native albumins. 

 One of its most useful reactions is its behavior toward magnesium 

 sulphate and ammonium sulphate. Serum-albumin usually occurs in 

 liquids together with the globulins, as is the case in blood. If such 

 a liquid is thoroughly saturated with solid magnesium sulphate or 

 half saturated with ammonium sulphate, the globulins are precipi- 

 tated completely, while the albumin is not affected. So far as the 

 blood and similar liquids are concerned, a definition of serum- 

 albumin might be given by saying that it comprises all the proteids 

 not precipitated by saturation with magnesium sulphate or by half 

 saturation with ammonium sulphate. When its solutions have a 

 neutral or an acid reaction, serum-albumin is precipitated in an 

 insoluble form by heating the solution above a certain degree. 

 Precipitates produced in this way by heating solutions of proteids 

 are spoken of as coagulations heat coagulations and the exact 

 temperature at which coagulation occurs is to a certain extent 

 characteristic for each proteid. The temperature of coagulation of 

 serum-albumin is usually given at from 70 to 75 C, but it varies 

 greatly with the conditions, for instance, with the reaction of the 

 solution or its concentration in salts. It has been asserted, in fact, 

 that careful heating under proper conditions gives separate coagula- 

 tions at three* different temperatures, namely, 73, 77, and 84 

 C, indicating the possibility that what is called "serum-albumin" 

 may be a mixture of three proteids. Serum-albumin occurs in 

 blood-plasma and blood-serum, in lymph, and in the different 

 normal and pathological exudations found in the body, such as per- 

 * " Archiv f. exper. Pathol, u. Fharmakol.," 39, 1, 1897. 



