356 RESPIRATION. 



worthy of notice, that a dog which had been fed on mutton suet 

 neither exhaled nor absorbed nitrogen, and that only 69'4-g- of the 

 absorbed oxygen were employed in the formation of carbonic acid. 

 A considerable absorption of nitrogen was observed in hens which 

 had been fed on animal food after several days' starvation, but 

 when they had become habituated to this kind of food they began 

 again to develope nitrogen as in the normal condition : it was also 

 found by experiments on these birds that a far smaller quantity of 

 the absorbed oxygen was found in the carbonic acid when they had 

 been kept on animal food ; in two cases there were only 63 J of the 

 absorbed oxygen present in the carbonic acid which was exhaled. 

 On comparing the different experiments made on dogs and rab- 

 bits, we find that, when considered in reference to their dietetic 

 categories, they agree perfectly with the results yielded by Dulong's 

 observations on the respiration of animals. It is also found 

 that after an animal diet the interchange of gases in the lungs is 

 very similar to what we observe during fasting ; and this observa- 

 tion, which has also been made in reference to the urine and the 

 other excretions, seems to be explained by the fact that fasting 

 animals to a certain degree live upon their own flesh. 



Although our attention is at present most especially turned 

 to direct observations, and although we shall treat fully of the 

 influence of diet upon the molecular movements in the animal 

 body when we enter upon the subject of " Nutrition," the con- 

 sideration of the question, how far the nature of the food partaken 

 of influences the absorption of oxygen and the excretion of car- 

 bonic acid, can scarcely be deemed out of place in the present 

 part of our work. In considering this subject, we have to take 

 our stand upon a postulate, the inductive proof of which we defer 

 for the present; we assume that all the carbon and hydrogen of 

 the fats and carbo-hydrates derived from the food are entirely 

 oxidised in the living body into carbonic acid and water. It 

 must be obvious to all who are acquainted with the composition 

 of these substances, that very different quantities of oxygen are 

 required for their perfect oxidation. The mean composition of the 

 fats is about 78-13C, 11'64H, and 10*130). The oxidation of the 

 carbon (into carbonic acid) and of the hydrogen, which are con- 

 tained in 100 grammes of fat, would require (208*35 + 93*92 

 grammes) = 302*27 grammes of oxygen; but as the fat already 

 contains 10*13 per cent, of oxygen, it would only require to 

 absorb 292*14 grammes of oxygen to effect its entire combustion 

 into carbonic acid and water. When we compare the composition 



