892 PHYSIOLOGY 



owing to its low melting-point. In one experiment, when a cow 

 was fed on linseed oil, the iodine number of the milk fat rose from 

 30, its normal figure, to 704. After the introduction of iodine fat 

 subcutaneously, iodine fats are found in the milk. In another 

 experiment a bitch which had been fed with mutton suet and 

 had deposited in its tissues a fat of high melting-point produced 

 a milk the iodine number of which was the same as that of 

 the mutton suet. In this case the fat of the milk had evidently 

 been derived from the tissues, since during the lactation the animal 

 was being fed on meat which was poor in fat. The same dependence 

 of fatty secretion on diet has been found in geese, where the com- 

 position of the oil secretion of the feather glands has been altered 

 by giving abnormal fats, such as sesame oil, with the food. 



We must conclude that the protein of the food cannot give rise 

 to fat in the body. A nearer consideration of the composition of the 

 proteins, taken in connection with our discussion as to the mechanism 

 by means of which the fat is built up in the body, might help to account 

 for this fact. The fatty acids formed by the disintegration of proteins 

 are chiefly the lower acids of the series, such as acetic and propionic, 

 which would undergo rapid oxidation in the body. Butyric acid 

 has not yet been found among the products of disintegration of 

 the proteins, and the 6-carbon acid, derived from leucin, is not 

 the normal acid, but is a branched chain, viz. isobutyl-acetic acid. 

 The one acid therefore from which the long normal chain of the 

 fatty acids might be built up, namely, butyric acid, is conspicuous 

 by its absence, and there is thus no starting-point among the 

 products of disintegration of the protein molecule which might serve 

 for the synthesis of the fats of the body. 



THE UTILISATION OF FATS IN THE BODY 

 The constant presence of fat, and bodies allied to fat, in proto- 

 plasm, from whatever source obtained, suggests that these substances 

 can enter directly into the chemical changes on which the life of the 

 cell depends and that they play an essential part in vital phenomena. 

 The direct utilisation of fat for the needs of the body is also 

 indicated by the results of experiments on man and the lower animals. 

 After a few days' starvation the body may be regarded as practically 

 free from stored carbohydrate. The sole source of the energy which 

 is evolved under these circumstances must be fats and proteins, 

 and it is possible to determine by an estimation of the nitrogen 

 output the exact fraction of the total energy evolved which is to be 

 ascribed to protein metabolism. Thus in the case of Cetti, the 

 professional faster, it was found that the nitrogenous metabolism 

 per unit of body weight remained fairly "constant between the fifth 



