or IJEEA AA'D UEIJS'AET WATER. 
If, therefore, we consider the tissue-fluids as a part of the circulating medium, and 
the products of food as perpetually chculating amongst the tissues, we take away the 
distinctive pecuharities of these two \iews. 
All experiments have shown (and none more clearly than those of Bischoff and Voit) 
that there is the closest correspondence between the amount of nitrogen which has entered 
the blood from food and that excreted as urea in flesh-feeding animals, and that the 
former might in these animals really measure the latter. In man and other animals there 
is a small elimination of nitrogen by other sources, as certain organic compounds pass- 
ing ofi" by the skin and lungs, by the uiic acid, creatine, and hippuric acid, by which the 
urea in the urine does not fully represent the nitrogen contained in the food which has 
entered the blood ; but, on the whole, as it does not probably exceed 5 grs. of nitrogen in 
man, or its equivalent of about 11 grs. of urea, we need not consider it in the following 
observations. The prison experiments recorded in this paper, when compared with those 
on myself, explain the cause of some discrepancies in the results heretofore obtained. In 
prisoners the urea was lessened on Sundays, when, with no change of food, there was no 
exertion, and thereby there was relatively an increase of food ; whilst in myself there was 
great increase in the urea, with no exertion and a small increase of food. Two explana- 
tions offer themselves : — 1st. I am over-fed, so that the full bulk of my tissues is at all 
times maintained, and therefore I most readily show the effects of increase ; but they are 
rather under-fed, and their tissues have not therefore their full bulk, and hence they 
appropriate increased food, and do not show the full effects of increase. 2nd. In the 
absence of the powerful stimulus of riolent exertion, the assimilation of food was 
lessened in the prisoners, and a less proportion of the food was admitted into the blood. 
Urea is a measure of exertion only so far as it measures food, if the nitrogen of the food 
be sufficient to maintain the vital actions ; but if the tissues lose weight, the excess of 
nitrogen thus evolved will be added to the urea. If during muscular action there be 
waste of nitrogen in the muscle, there will be also appropriation of nitrogen by the 
same muscle, and the equilibrium will be maintained. If the appropriation proceed 
'pan jiCLSSU with the waste, there w ill be as much nitrogen consumed from the food as 
is excreted by the tissue, and the flnal quantity of urea will be always unchanged ; and 
if the waste should exceed the appropriation during exertion, and appropriation exceed 
waste in the intervals of rest and at night in the same proportion, the balance will be 
maintained, and the amount of urea eliminated in the whole tw^enty-four hours will be 
unchanged. Hence, with sufficient food, there cannot be any increased elimination of 
nitrogen from exertion, unless the tissue weight be lessened, and the urea will, de facto, 
represent the nitrogen taken into the blood with the food. On the other hand, if the 
bulk of the muscles increase with sufficient food, there will be a loss in the excretion of 
nitrogen, because there has been an appropriation of it, “ 
Hence the erident relation of urea must be with food, although it may be derived 
partly from tissue and partly from food ; and the precise correspondence in quantity will 
be regulated by the relation of appropriation to excretion of nitrogen in the tissues. As 
the urea is derived from various sources, it is impossible that it can measure either tissue- 
waste or food separately. 
MDCCCLXI. 5 T 
