CHEMICAL COMPOSITION OF THE HUMAN BODY 25 



The physiological functions of fat are : 



(a) By their physiological combustion they are a source 

 of heat and force. 



(b) Being poor conductors of heat, they shield the body 

 from rapid cooling. 



(r) They serve as protective coverings for delicate organs 

 (eyes, kidneys). 



Cholesterins are isomeric monatomic alcohols of unknown 

 constitution, having the empirical formula C 26 H 47 (OH). 

 They crystallize in rhomboid tables, are insoluble in water, 

 but readily soluble in hot alcohol and ether. Moistened 

 with concentrated sulphuric acid and a little iodine solution, 

 the cholesterin crystals become blue, green, and red. A 

 solution of it in chloroform is colored blood-red by concen- 

 trated sulphuric acid. Cholesterins are found in all parts of 

 the organism, chiefly in the brain and nerves, also in the 

 bile. With fatty acids they form esters, capable of saponifi- 

 cation with alkali and found in small quantities throughout 

 the whole body. The physiological function of cholesterin 

 is not known; its ester (lanolin) protects the skin and hairs, 

 for which, as it does not become rancid, it is well adapted. 



Lecithins are ester-like compounds of glycero-phosphoric 

 acid with two fatty acid radicles on the one hand and of an 

 ammonia base, cholin, on the other. Cholin is trimethyl- 

 oxyethyl-ammonium-hydroxide. From cholin we, obtain, 

 by reduction, neurin, and by oxidation, muscarin. Neurin 

 and muscarin are poisons, cholin is not. 



The most common lecithin, stearic acid lecithin, is di- 

 stearyl-glycero - phosphoric acid trimethyl - oxyethyl - am- 

 monium-hydroxide : 



/0-C W H M 



c^-o-cXo 



\0 /OH 

 \p=O 



\ \ 



\O C H N 



W.^ 2 n 4 .lM x 



\ 

 OH. 



Lecithin is insoluble but swells up in water, forming the 



