894 PHYSIOLOGY 



We may conclude then that the tissues of the body are able to 

 obtain their energy by the direct utilisation of the fats which they 

 contain. The changes in the fat molecules which are involved in the 

 utilisation of their energy are still to be determined. The energy of fat 

 is only available on its oxidation. The transformation of fats into fatty 

 acids or glycerine, or the synthesis of fats from aldehydes or from carbo- 

 hydrates,, which we have discussed in the previous section, do not 

 involve any large changes of energy. Weight for weight, butyric acid 

 with its 4 carbon atoms has practically the same heat- value as stearic 

 acid with its 18 carbon atoms, or stearine with its 57 carbon atoms. We 

 have therefore to determine what changes the great fat molecule 

 undergoes before it is brought into a condition in which it may 

 undergo oxidation and set free the energy required for the purposes of 

 the body. The general tendency of metabolic research of recent years 

 is to show that the living cell is in a position to effect all changes 

 which do not involve a large evolution or absorption of energy in 

 either direction. In the plant cell, at any rate, the fatty acids may be 

 converted into amino-acids, or the latter may be deaminised, as occurs 

 in the walls of the gut, into fatty or oxyacids. Dextrose may pass into 

 maltose, and glycogen into starch, or starch may be converted into 

 maltose or dextrose. If therefore fats are constantly being made from 

 carbohydrates, or from the lower molecules such as aldehyde, by a 

 process of repeated addition of a group containing two carbon atoms, 

 it is probable that the same change will go on in a reverse direction 

 when fats are broken down previous to oxidation. 



In the germination of oily seeds the utilisation of the fat is pre- 

 ceded by the splitting of the higher fatty acids into acids of lower 

 molecular weight. Although we cannot trace out in the animal 

 body the stages in the breakdown of a large fatty acid, such as stearic 

 acid, we can, by a certain artifice much used in metabolic experi- 

 mentation, bring forward evidence in favour of the view that the 

 breakdown, like the building up of fats, occurs by two carbon atoms 

 at a time. When, in the process of breaking down, a fat finally 

 arrives at the four- or two-carbon stage, it is quickly oxidised and is 

 therefore not traceable in the excretions or in the fluids of the body. 

 This end stage may, however, be preserved from oxidation by hanging 

 it, so to speak, on to an aromatic ring. If acetic acid or ethyl alcohol 

 be administered in small quantities, it is entirely oxidised. If, 

 however, these bodies be attached to a benzene ring and be adminis- 

 tered as a phenacetic acid or phenyletbyl alcohol, they are excreted 

 in the oxidised form of phenaceturic acid, which is simply a combina- 

 tion of phenacetic acid with glycine. In the same way benzoic acid 

 and benzyl alcohol are excreted in the form of hippuric acid, thus : 



Phenacetic acid, C 6 H 5 . CH 2 COOH, is excreted as C 6 H 5 .CH 2 .- 



