20 PROTEIDS OF SERUM. [BOOK i. 



We have some reasons for thinking that more than one proteid 

 is present, but they are all closely allied to each other, and we 

 may for the present speak of them as if they were one, and call 

 the proteid left in serum, after removal of the paraglobulin, by the 

 name of albumin, or, to distinguish it from other albumins found 

 elsewhere, serum-albumin. Serum-albumin is distinguished by 

 being more soluble than the globulins, since it is soluble in 

 distilled water, even in the absence of all neutral salts. Like the 

 globulins, though with much less ease, it is converted by dilute 

 acids and dilute alkalis into acid- or into alkali-albumin. 



The percentage amount of serum-albumin in serum may be 

 put down as 4 or 5, more or less, but it varies and sometimes is 

 less abundant than paraglobulin. In some animals (snakes) it is 

 said to disappear during starvation. 



The more important characters of the three proteids which we 

 have just studied may be stated as follows : 



Soluble in water and in saline solutions of all 



strengths serum-albumin. 



Insoluble in water, readily soluble in dilute saline 

 solutions, insoluble in concentrated saline so- 

 lutions paraglobulin. 



Insoluble in water, hardly soluble at all in dilute 

 saline solutions, and very little soluble in 

 more concentrated saline solutions fibrin. 



Besides paraglobulin and serum-albumin, serum contains a 

 very large number of substances, generally in small quantity, which, 

 since they have to be extracted by special methods, are called 

 extractives ; of these some are nitrogenous, some non-nitrogenous. 

 Serum contains besides important inorganic saline substances; but 

 to these we shall return. 



18. With the knowledge which we have gained of the pro- 

 teids of clotted blood we may go back to the question : Clotting 

 being due to the appearance in blood plasma of a proteid sub- 

 stance, fibrin, which previously did not exist in it as such, what 

 are the causes which lead to the appearance of fibrin ? 



We learn something by studying the most important external 

 circumstances which affect the rapidity with which the blood of 

 the same individual clots when shed. These are as follows : 



A temperature of about 40 C., which is about or slightly above 

 the temperature of the blood of warm-blooded animals, is perhaps 

 the most favourable to clotting. A further rise of a few degrees is 

 apparently also beneficial, or at least not injurious ; but upon a still 

 further rise the effect changes, and when blood is rapidly heated 

 to 56 C. no clotting at all may take place. At this temperature 

 certain proteids of the blood are coagulated and precipitated before 

 clotting can take place, and with this change the power of the blood 

 to clot is wholly lost. If however the heating be not very rapid, 



