350 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



general reactions of the native albumins. One of its most useful reactions is 

 its behavior toward magnesium 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 MgSO 4 , the globulins are precipitated com- 

 pletely, 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 MgSO 4 . 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. Precipi- 

 tates 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. 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. indi- 

 cating the possibility that what is called " serum-albumin " may be a mixture 

 of three or more 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 pericardial liquid, hydrocele fluid, etc. The amount 

 of serum-albumin in the blood varies in different animals, ranging among 

 the mammalia from 2.67 per cent, in the horse to 4.52 per cent, in man. In 

 some of the cold-blooded animals it occurs in surprisingly small quantities 

 0.36 to 0.69 per cent. As to the source or origin of serum-albumin, it is 

 generally believed that it comes from the digested proteids of the food. It 

 is known that proteid material in the food is not changed at once to serum- 

 albumin during the act of digestion ; indeed, it is known that the final product 

 of digestion is a proteid or group of proteids of an entirely different character 

 namely, peptones and proteoses; but during the act of absorption into the 

 blood these latter bodies are supposed to undergo transformation into serum- 

 albumin. From a physiological standpoint serum-albumin is considered to be 

 the main source of proteid nourishment for the tissues generally. As will be 

 explained in the section on Nutrition, one of the most important requisites in 

 the nutrition of the cells of the body is an adequate supply of proteid material 

 to replace that used up in the chemical changes, the metabolism, of the tissues. 

 Serum-albumin is supposed to furnish a part, at least, of this supply. As long 

 as the serum- albumin is in the blood-vessels it is of course cut off from the 

 tissues. The cells, however, are bathed directly in lymph, and this in turn is 

 formed from the plasma of the blood which is filtered or, according to some 

 physiologists, secreted through the vessel-walls. Serum-albumin may be 

 looked upon, then, as a supply of proteid nourishment which is replenished, 

 after every meal containing proteids, by absorption from the alimentary canal. 

 Paraglobulin, which belongs to the group of globulins, exhibits the gen- 

 eral reactions characteristic of the group. As stated above, it is completely 

 precipitated from its solutions by saturation with MgSO 4 . It is incompletely 



