PRODUCTION OF UREA. 127 



The amount of carbonic acid thrown off by a living animal 

 in a given time is a just criterion of the proportion of carbon 

 which has united with oxygen in that time, and therefore in- 

 dicates the quantity of heat produced by their combination 

 within the body. But the knowledge of the amount of car- 

 bonic acid formed within the living body for example, in a 

 day -does not indicate how much organic tissue has been dis- 

 integrated in the process, or to what extent the decomposition 

 of non-nitrogenised proximate principles has contributed to 

 the effect. Even the loss of weight does not solve this diffi- 

 culty ; for though an animal deprived of food burns its own 

 substance in the maintenance of its temperature, yet what 

 burns first under such circumstances is the fat which has 

 accumulated in the interstices of the solids, and then, and not 

 till then, the organic tissue shares the same fate. Moreover, 

 it appears probable, from various circumstances, that even 

 when an animal is deprived of food, disintegration of the 

 organism does not take place to any greater extent than cor- 

 responds to the amount of muscular exertion that continues 

 to take place that is, that the decomposition of the animal 

 solids hardly occurs unless when their vitality is first lost by 

 the exertion of living action. Whence it should follow that 

 the more a starving animal can be kept in motion, the longer 

 it will be able to support life by feeding on itself. 



When disintegration of the organism of the kind composing 

 the muscular tissue occurs, nitrogenised compounds, such as 

 urea, must be produced within the living system. Whence it 

 appears that an estimate of the nitrogenised products thrown 

 off by the urine in a given time should be an index of the 

 amount of organic tissue disintegrated within that time. Thus, 

 when an animal is very much exercised, a much larger pro- 

 portion of urea, the most abundant of the nitrogenised prin- 

 ciples thrown off by the kidney, is found in the urine. It 



