352 A MANUAL OF VETERINARY PHYSIOLOGY 



The sacrifice having been effected, the last remaining body sub- 

 stance, the proteins, has then to bear the full brunt of the 

 system's necessities. A starving animal may therefore be kept 

 alive a longer time by receiving some carbohydrate and fat. 

 It will die because it is not receiving nitrogen, but the mixed 

 carbohydrate and fat will effect economies in the store of body 

 protein, and cause it to last longer. In this respect the carbo- 

 hydrate is of much more value than the fat. 



It was reasonable to suppose, as Liebig did, that the protein 

 substances of the food carried out the mechanical work of the 

 body. We now know better, for we have learned from the facts 

 connected with nitrogenous equilibrium that the body continues 

 to work satisfactorily though the equilibrium is established at 

 different levels. Nevertheless, nitrogenous equilibrium, which 

 we have seen possesses characteristic peculiarities, is effected by 

 excessive work in man and severe work in the dog, though within 

 normal limits it is correct to say it is unaffected by work. 



The views laid down by Liebig were never seriously disputed 

 until Lawes and Gilbert had completed their twenty years' 

 classical work on animal nutrition at Rothamstead. During the 

 years 1849- 1859 these observers gave their results to the world, 

 and their accuracy has never been challenged. Though working 

 at the question of the fattening of animals, and not the ques- 

 tion of energy, they were the first to announce that the value 

 of food for feeding purposes could not be based on the so-called 

 flesh-forming substances it contained, but on the non-nitrogenous 

 elements. It is usual in physiology to attribute the demolition 

 of the Liebig theory to the celebrated ascent of the Faulhorn 

 by Fick and Wislicenus in 1866. We shall see presently the 

 manner in which Lawes and Gilbert arrived at their results. 



We can now understand why it is that nitrogenous equilibrium 

 experimentally produced is accomplished on a smaller amount 

 of protein when non-protein substances enter into the diet. No 

 animal, not even the carnivora, lives entirely on protein. The 

 effect of the non-protein portion is to reduce the amount of 

 protein destroyed, and establish equilibrium at a lower level. 

 Fat, starch, and sugar are protein economisers, while, conversely, 

 a large protein diet is a fat obliterator, and this fact was turned 

 to account by Banting in his treatment of obesity. 



If a fasting animal be brought on to a protein diet equivalent 

 to the amount of nitrogen being daily excreted, it would be reason- 

 able to suppose that nitrogenous equilibrium would at once be 

 established. Experimental inquiry shows this is not so, but 

 that the effect of giving nitrogen is to cause a greater excretion 

 of this substance. It is not, in fact, until an amount of protein 

 is given equivalent to two and a half times the daily excretion 



