132 PHYSIOLOGY 



fresh water at the expense of the skeletal muscles, especially those of 

 the back. Here again there is a transformation of a tissue poor in 

 purine bases into a tissue which consists almost exclusively of nucleins 

 and protamines. Whether in this case there is a direct conversion of the 

 mono- amino- acids of the muscle proteins into the diamino-acids and 

 bases typical of protamines, we do not know. It is more probable 

 that only diamino-acids and bases previously existing in the muscle 

 are utilised for the formation of the generative glands, the other amino- 

 acids being oxidised and utilised for the ordinary energy requirements 

 of the animal. 



THE SYNTHESIS OF FATS 



In some plants fat globules have been stated to appear as the first 

 products of the assimilation of carbon dioxide under the influence 

 of sunlight, but there is no doubt that as a rule the formation 

 of fats as reserve material in seeds or fruits occurs at the expense 

 of carbohydrates. In the higher animals, too, although a certain 

 amount of the fat of the body is derived from the fat taken up 

 with the food, the organism can also manufacture neutral fat 

 out of the carbohydrates presented to it in its food. The problem, 

 therefore, of the synthesis of the fats is the problem of the conversion 

 of a sugar such as glucose into glycerin and the fatty acids. Although 

 this conversion is apparently so easily effected by the living organism, 

 it is one which from the chemical standpoint involves considerable 

 difficulties. On account of the fact that the higher fatty acids con- 

 sist largely of oleic and stearic acids, i.e. acids containing eighteen 

 carbon atoms in their chain, it has been thought that the synthesis 

 might be brought about by the linking together of three molecules of 

 a hexose. Such a change would involve a series of difficult chemical 

 transformations. For instance, no less than sixteen out of the eighteen 

 oxygen atoms present in the three glucose molecules would have 

 to be dislodged in order to convert the chain into stearic acid. 

 Moreover, although these two acids contain a multiple of six carbon 

 atoms, a whole array of fats are found both in plants and animals 

 which could not be derived by a simple aggregation of glucose mole- 

 cules, and it is worthy of note that, of all the fatty acids which occur 

 in nature, all those with more than five carbon atoms contain an even 

 number of carbon atoms. Thus in milk, in addition to the three com- 

 mon fats, tristearin, tripalmitin, and triolein, we find the glycerides 

 of caproic, capryllic, capric, lauric, and myristic acids, i.e. acids with 

 6, 8, 10, 12, and 14 carbon atoms. In all cases these acids are the 

 normal acids with straight unbranched chains. It seems probable 

 that in the transformation of carbohydrate into fatty acid the latter 

 is built up, not by six carbon atoms, but by two carbon atoms at a 



