48 GRAHAM LUSK 



that this excess of carbon or of carbon and hydrogen is expended in the pro- 

 duction of animal heat and serves to protect the organism from being attacked 

 by atmospheric oxygen. 



' Further on he remarks : 



In their final forms meat and blood which are consumed yield the greater 

 part of their carbon to the respiration, their nitrogen *is recovered as urea, and 

 their sulphur as sulphuric acid. Before this occurs the dead meat and blood 

 must be converted into living flesh and blood. The food of carnivora is con- 

 verted into blood which is destined for the reproduction of organized tissue. 



We know that the nitrogen-containing products of metabolism are not sus- 

 ceptible of further change and are eliminated from the blood by the kidney. 



Differences in the quantity of urea secreted in these and similar experiments 

 are explained by the condition of the animal in regard to the amount of the 

 natural movement permitted. Every movement increases the amount of organ- 

 ized tissue which undergoes metamorphosis. Thus, after a walk, the secretion 

 of urine in man is invariably increased. 



In the animal body the components of fat are used for the respiration 

 process and hence for the production of animal heat. 



If the condition and the weight of all parts of a carnivorous animal are 

 to be maintained it must daily receive a certain definite measure of sulphur and 

 nitrogen-containing food substances as well as of fat. 



The difficulties of calculating the metabolism are discussed. 



The weight of the ingested materials must be the same as those eliminated 

 in the forms of uric acid, urea, carbonic acid and water. The weight of the 

 ingested fat must be the equivalent of the fat eliminated in the form of carbonic 

 acid and water. From this it follows that the quantity of oxygen absorbed 

 cannot be a measure of the amount of the living substance destroyed in a given 

 time. 



The oxygen absorption expresses the sum of two factors; one the destruction 

 of nitrogen-free substances and the other the destruction of nitrogen-containing 

 substances. It has already been frequently stated that the measure of the latter 

 can be determined from the nitrogen content of the urine. 



He later considers the metabolism of a horse: "A horse preserves 

 itself in a state of health if he be given 7^ kg. hay and 2 1 / 4 kg. oats. 

 Hay contains 1.5 per cent and oats 2.2 per cent of nitrogen. Assuming 

 that all the protein in the food is transformed into the fibrin and serum 

 albumin of the blood, there would be produced daily 4 kg. of blood, con- 

 taining 20 per cent of water and 140 gm. of nitrogen. The quantity of 

 carbon combined with the protein and ingested at the same time would 

 have been 448 gm. Of this only 246 gm. could have served for the respira- 

 tion, for 95 gm. are eliminated in the form of urea and 109 gm. in the 

 form of hippuric acid. . . . The experiment of Boussingault which shows 

 that a horse expires 2450 gm. of carbon in a day cannot be very far 

 from the truth." 



The nitrogen-containing substances of the fodder of the horse do not con- 

 tain more than one-fifth of the carbon necessary for the maintenance of the 



