CH. XXV.] THE PKOTEIDS 395 



and splits into glycerin and the fatty acid. The following equation 

 represents what occurs in a fat, taking tripalmitin as an example : 



C 3 H 5 (O.C 15 H C0) 3 + 3H 2 = C 3 H 5 (OH) 3 3C 15 H 81 CO.OH. 



[Tripalmitin a fat.] [Glycerin.] [Palmitic acid a 



fatty acid. 



In the process of saponiflcation much the same sort of reaction 

 occurs, the final products being glycerin and a compound of the base 

 with the fatty acid which is oalled a soap. Suppose, for instance, that 

 potassium hydrate is used ; we get 



C 3 H 5 (O.C }5 H 31 CO) 3 + 3KHO = C 3 H 5 (OH) 3 + 3C 15 H 31 CO.OK. 



[Tripalmitin a fat.] [Glycerin.] [Potassium palmitate 



a soap.] 



Emulsification. Another change that fats undergo in the body 

 is very different from saponification. It is a physical rather than a 

 chemical change ; the fat is broken up into very small globules, such 

 as are seen in the natural emulsion milk. 



Lecithin (C 42 H 84 NP0 9 ). This is a very complex fat, which yields 

 on decomposition not only glycerin and fatty acids (stearic and olcic), 

 but phosphoric acid, and an alkaloid [K(CH 3 ) 3 C 2 H 6 2 ] called choline in 

 addition. This substance is found to a great extent in the nervous 

 system (see p. 175), and to a small extent in bile. Together with 

 cholesterin, a crystallisable, monatomic alcohol (C 27 H 45 .HO), which 

 we shall consider more at length in connection with the bile, it is 

 found in small quantities in the protoplasm of all cells. 



The Proteids. 



The proteids are the most important substances that occur in 

 animal and vegetable organisms ; none of the phenomena of life occur 

 without their presence ; and though it is impossible to state positively 

 that they occur as such in living protoplasm, they are invariably 

 obtained by subjecting living structures to analysis. 



Proteids are highly complex compounds of carbon, hydrogen, 

 oxygen, nitrogen, and sulphur occurring in a solid viscous condition 

 or in solution in nearly all the liquids and solids of the body. The 

 different members of the group present differences in chemical and 

 physical properties. They all possess, however, certain common 

 chemical reactions, and are ubited by a close genetic relationship. 



The various proteids differ a good deal in elementary composition. 

 Hoppe-Seyler gives the following percentages : 



C H N S O 



From 51-5 6'9 15*2 0'3 20'9 



To 54-5 7-3 17-0 2-0 23'5 



We are, however, not acquainted with the constitutional formula 

 of proteid substances. There have been many theories on the subject, 



