526 A MANUAL OF PHYSIOLOGY 



fat into the fat of the organs. This ' organized ' intracellular fat 

 differs in various ways from the fats of adipose tissue. Its ' iodine 

 value ' (p. 4) is higher (Leathes), and a large proportion of it consists 

 of phosphatide lipoids (p. 337). 



The most convincing evidence that fat is not produced in increased 

 amount under the influence of phosphorus has been obtained by 

 determining by actual analysis the total fat in animals, and then 

 poisoning similar animals with phosphorus and again estimating 

 the total fat. Far from being increased, the fat may even be de- 

 creased in the poisoned animals (Taylor, etc.). There is no ground, 

 then, for the assumption that phosphorus and other substances, 

 like arsenic, antimony, etc., which bring about so-called ' fatty 

 degeneration ' of the organs, act by causing or accelerating the 

 transformation of protein into fat. Yet there is good evidence that 

 they do accelerate the decomposition of protein, or at least inter- 

 fere with its normal metabolism, for after phosphorus poisoning 

 amino-acids (leucin, tyrosin, glycin) appear in the urine. The 

 observations of Lusk and his pupils indicate that phosphorus does 

 not directly increase the amount of protein broken down, but does 

 so indirectly, by favouring the conversion of the carbo-hydrate -like 

 radicle of the protein molecule into leucin, tyrosin, and perhaps 

 fat, and thereby necessitating an increased consumption of protein. 



A celebrated experiment, performed nearly forty years ago, was 

 long supposed to furnish an absolute proof of the formation of fat 

 from protein, under strictly physiological conditions, although in a 

 humble form of animal life. Maggots were allowed to develop from 

 the egg on blood containing a known amount of fat. The quantity 

 of fat in the eggs was also known. After the maggots had grown, 

 ten times as much fat was found in them as had been contained in 

 the blood and eggs together. The trifling quantity of sugar in 

 the blood was utterly inadequate to account for the fat, which, it 

 was concluded, must therefore have come from the proteins of the 

 blood (Hofmann). It can be objected to this experiment that no 

 precautions were taken to prevent the growth of micro-organisms 

 on the blood, and fat might have been formed by them from the 

 proteins. Further, the fat estimations would scarcely pass muster 

 according to the present standards. 



The experiments of Pettenkofer and Voit, which were supposed 

 to have demonstrated that in the higher animals also fat is formed 

 from proteins under normal conditions, are in the same position. 

 According to them, a dog fed for a time on a liberal diet of lean 

 meat may go on excreting a quantity of nitrogen equal to that in the 

 food, while there is a deficiency in the carbon given off. Or if the 

 dog is not in nitrogenous equilibrium (p. 529), but putting on 

 nitrogen in the form of ' flesh,' the deficiency in the carbon given off 

 may be too great in proportion to the nitrogen deficit to warrant the 

 assumption that all the retained carbon has been put on in the form 

 of protein. In either case, carbon in large amount can only come 

 from the proteins of the food, and can only be stored up in the body 

 in the form of fat. For lean meat contains but a trifling quantity of 

 carbon in any other proximate principle than protein, and the non- 

 protein carbon of the animal body is only to a very small extent 

 contained in carbo-hydrates or other substances than fat. 



Pfliiger has criticised these experiments, and has shown that 

 lean meat contains more fat than was supposed, and this is now 

 generally admitted. He has endeavoured to show that the fat and 



