74 THE CHEMICAL CONSTITUENTS OF THE BODY 



Albumins occur chiefly in the animal fluids. To this group belong serum 

 albumin, egg albumin, albumin of milk, etc. 



2. Globulins: insoluble in water; soluble in dilute salt solutions, from which 

 they are precipitated by further dilution. Solutions are coagulated by boiling. 

 Soluble in water on addition of very small quantities of acid or alkali, whence 

 they are precipitated by neutralization. Likewise when solution is effected by 

 minimal quantities of alkali, they are precipitated by carbon dioxide and are 

 redissolved by excess of the same. Complete precipitation by saturation with 

 MgSO 4 , partial, by saturation with NaCl. They are salted out by half -satura- 

 tion with ammonium sulphate. 



This characterization of the globulins however is no longer sufficient, for 

 it appears from several recent researches that among the compounds which are 

 precipitated by fractional salting out of the globulins in the blood, there occur 

 substances which are neither insoluble in water nor precipitated from their 

 solutions with carbon dioxide. The only positive distinction between albumins 

 and globulins therefore consists in their relation to neutralization, and especially 

 to ammonium sulphate: the globulins are precipitated by half-saturation, the 

 albumins are not. 



The globulins also occur chiefly in the fluids of the animal body; but they 

 may be obtained from the tissues. To this group belong fibrinogen and serum 

 globulin of the blood, myosin and myogen of the muscles, etc. 



3. The true mucins are substances insoluble in water and in solutions of 

 neutral salts, but soluble with very little alkali. The solutions are viscous, and 

 form with acetic acid a precipitate not soluble in an excess of the acid. Chem- 

 ically the mucins are characterized by their high content of the carbohydrate 

 groups. The true mucins occur in the submaxillary saliva, in the umbilical cord 

 (Wharton's jelly), etc. 



The so-called mucoids are distinguished from the true mucins in certain 

 respects which are not yet definitely understood. They are obtained from the 

 ovarial fluids, from the cornea, the vitreous body, the urine, etc. 



It is also asserted that the mucins contain fat. If the submaxillary mucin 

 be first extracted with ether and be then digested with pepsin-HCl, one can 

 later extract with ether more than three per cent fat from the substance 

 (Nerking). 



B. Simple proteids which can be split off from compound proteids. Here 

 belong globin from haemoglobin, the proteids from the mucins,, etc. 



C. The native proteids are changed by the action of alkalies and acids in 

 sufficient concentration into alkali and acid albuminates (syntonin). In the 

 formation of alkali albuminates some nitrogen is split off from the proteid, 

 and with stronger action of the alkali some sulphur also is separated. 



In spite of their different modes of formation, and in spite of different 

 chemical constitutions, the alkali and acid albuminates are very closely related 

 to each other. In water or dilute NaCl solution they are almost insoluble, but 

 are soluble on addition of small quantities of acid or alkali. Such a solution 

 (even if neutral) is not coagulated on boiling, without the addition of sufficient 

 quantity of neutral salts. Solutions of albuminates are precipitated at room 

 temperature by neutralization, by excess of mineral acids, by many metallic 

 salts. An acid solution of albuminate is readily precipitated by NaCl, an alka- 

 line solution only with difficulty. 



D. Under the influence of the digestive fluids there are formed by hydro- 

 lytic cleavage of the proteids a number of new substances, the so-called albu- 



