39 2 THE CHEMICAL COMPOSITION OF THE BODY. [CH.XXV. 



Triacetin is a type of a neutral fat ; stearin, palmitin, and olein 

 ought more properly to be called tristearin, tripalmitin, and tri- 

 olein respectively. Each consists of glycerin in which the three 

 atoms of hydrogen in the hydroxyls are replaced by radicles of 

 the fatty acid. This is represented in the following formulae : 



Acid. Radicle. Fat. 



Palmitic acid C 15 H 31 .COOH Palmityl C 13 H 31 .CO Palmitin C 3 H 5 (OC 15 H 31 .CO), 

 Stearic acid C^H^.COOH Stearyl C^H^.CO Stearin C 3 H 5 (OC 17 H 3 ..CO) 8 

 Oleicacid C 17 H S3 .COOH Oleyl C IT H 33 .CO Olein C 3 H 5 (OC 17 H 33 .CO), 



Decomposition Products of the Pats. The fats split up 

 into the substances out of which they are built up. 



Under the influence of superheated steam, mineral acids, and 

 in the body by means of certain ferments (for instance, the fat- 

 splitting ferment, steapsin, of the pancreatic juice), a fat combines 

 with water and splits into glycerin and the fatty acid. The 

 following equation represents what occurs in a fat, taking 

 tripalmitin as an example : 



C S H 5 (O.C ]5 H 31 CO) 3 + 3 H 2 = C 3 H 5 (OH) 3 + 3 C 18 H SI 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 com- 

 pound of the base with the fatty acid which is called 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 S H S (OH) S + 3C 15 H 31 CO.OK 

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



Emulsiflcation. 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 ^aH^NPOg). This is a very complex fat, which 

 yields on decomposition not only glycerin and a fatty (stearic) 

 acid, but phosphoric acid, and an alkaloid [N^CH^CaHgOg] 

 called choline in addition. This substance is found to a great 

 extent in the nervous system (see p. 1 7-4), and to a small extent 

 in bile. Together with cholesterin, a crystallisable, monatomic 

 alcohol (C 2 7H 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 



