788 PHYSIOLOGY 



with an even number of carbon atoms (v. p. 54). 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. The process would seem to take place more easily 

 through pyruvic acid, as described on p. 121. 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 accounting for 

 its formation from the carbohydrates. By a simple splitting of glucose, 

 we may obtain two molecules of glyceraldehyde, 



CHoOH 



CH 2 OH 



= 2 CHOH 

 I 



CHOH 

 CHOH 



CHOH 



CHO 

 CHOH 



CHO 



which by reduction is readily converted into the corresponding alcohol 

 glycerine, CH 2 OH.CHOH.CH 2 OH. 



We may conclude then that fats are formed by the body with ease 

 from carbohydrates, and that in all probability this change involves a 

 building up of the fatty acid from the lower members by the successive 

 addition of a group containing two atoms of carbon. The whole change, 

 as Leathes has shown (v. p. 121), is an exothermic one. For the formation 

 of one molecule of palmitic acid, four molecules of glucose would be required, 

 and 12-5 per cent, of the total energy of the glucose would be set free as 

 heat. 



THE FORMATION OF FAT FROM PROTEINS. Among the decom- 

 position products of proteins the amino- derivatives of the fatty acids take 

 a prominent part. Of these some may be converted into carbohydrate in 

 the body, while others such as leucine and tyrosine may give rise to aceto- 

 acetic acid. It seems therefore that these latter might in their turn be 

 built up by the process we have just discussed into the higher members 



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

 C 24 H 4S O 2 . 



