FATE OF THE FATTY CONSTITUENTS OF FOOD. 665 



If a lean dog be fed on a diet of meat and spermaceti, a large amount 

 of adipose tissue may be accumulated, in which, however, but a trace of 

 spermaceti may be found. Again, fat alone, as we know, is incapable, as 

 a food, of sustaining life, although it, to a certain extent, saves the wast- 

 ing of tissue; so, as a consequence, when fat alone is given as a food, 

 the amount of urea excreted by the kidneys is less than would be 

 excreted by a starving animal. This, evidently, is to be explained by the 

 fact that fat, being readily oxidized, is rapidly converted into carbon 

 dioxide, which is, of course, eliminated in the expired air and serves to 

 a certain extent to spare the elimination of the proteids in oxidation. 



, It is, then, clear that in the animal body fats are made from some- 

 thing which is not fat. Two possible sources of fat suggest themselves. 

 It is known that the nitrogen of urea represents the total amount of 

 nitrogen passing through the body, and that a certain quantity of urea 

 (one hundred grammes) represents a certain quantity of proteids (three 

 hundred grammes). If, however, we estimate the quantity of carbon in 

 one hundred grammes of urea, we will find that a large amount of carbon 

 remains unaccounted for. Part of this evidently goes off as carbon 

 dioxide. It is probable that the remainder is fixed in the body as fat. 

 It may, therefore, be assumed that proteids split up into non-nitrogenous 

 and nitrogenous compounds, the former, when not completely oxidized 

 into carbon dioxide and water, being deposited as fat, the latter leaving 

 the body oxidized as urea. Other illustrations as to the development of 

 fats from proteids may be readily given. Thus, in the pancreatic diges- 

 tion of proteids fatty acids may be developed. The fatty degeneration 

 of muscle is evidently due to the decomposition of proteids, while 

 animals fed on a pure diet of lean meat with a small amount of fat will 

 deposit in their tissues more fat than is contained in the food. 



Still other lines of study point to the development of fat from 

 proteids. It is known that in poisoning with phosphorus various organs 

 rich in proteids undergo fatty degeneration, and that the fat so formed 

 is produced at the expense of the tissue-albumen, the fat being formed 

 from the non-nitrogenous residue of the proteids after the formation of 

 urea. The following experiment proves this : — 



A large dog was allowed to fast for twelve days, so freeing 

 its tissues almost entirely from fat, and was then slowly poisoned 

 with phosphorus. Death occurred on the. twentieth day of fasting. 

 Before poisoning, from the fifth to' the twelfth day of fasting, the 

 elimination of urea in the urine was about constant, and amounted 

 to 7.8 grammes daily; as a consequence of the phosphorus poisoning 

 .the elimination of urea was greatly increased, amounting at last to 23.9 

 grammes daily, or more than three times the normal amount removed 

 during fasting. The post-mortem examination showed extensive fatty 



