THE HISTORY OF FAT IN THE BODY 889 



The change is probably a more complex one, and occurs in separate 

 stages. Glucose easily breaks up under the action of ferments into two 

 molecules of lactic acid, and lactic acid can be equally easily converted 

 into aldehyde and formic acid, thus : 



C 6 H 12 6 = 2C 3 H 6 3 lactic acid, and 

 CH 3 



CH 3 H 

 CHOH = | -f | 



CHO COOH 

 CHOH 



Now aldehydes possess a marked tendency to combine with other 

 molecules of other or the same substance, i.e. to undergo polymerisa- 

 tion. Thus from two molecules of lactic acid we get one molecule 

 of aldol, 



CH 3 



CH 3 CHOH 



CHO CH 2 



CHO 



which by a simple transposition of oxygen would give butyric acid, 

 or by oxidation would give /3-oxybutyric acid, a substance which 

 occurs during various abnormal conditions of metabolism. 



The fats occurring in the body, e.g. in milk, include only the 

 fatty acids with an even number of carbon atoms (v. p. 60). We 

 may probably assume from this fact that the building up. as well 

 as the breaking down, of fatty acids occurs by two carbon atoms 

 at a time. Although heating aldehyde or aldol with potash or any 

 other polymerising agent gives rise to a mixture of many substances, 

 it is probable that under the catalytic agencies at the disposal of 

 the living cell these synthetic changes are directed entirely in one 

 direction, so that from butyric acid we shall have hexoic, caprylic, 

 capric acid, and so on. Why the process comes to an end with the 

 formation of the 16 and 18 carbon atoms it is difficult to see.* 

 Possibly with the formation of acids whose melting-point is higher 

 than that of the body temperature, a certain stability is imparted 

 to them which prevents their further circulation and ready synthesis 

 to the still higher acids. 



With regard to the glycerine which is a necessary constituent 

 of the neutral fats laid down in the body, there is no difficulty in 



* From the fats extracted from the kidney Dunham has isolated carnaubic 

 acid, C 24 H 48 O 2 . 



