828 
DE. SMITH ON THE ELIMINATION 
The true measure of exertion, and therefore, by inference, of tissue-waste also, is the 
evolution of carbonic acid, which, as I showed in my former paper, is increased imme- 
diately, and in a definite and almost proportionate degree by every degree of exertion, 
Avhilst no such relation can be traced between exertion and urea. 
Hence I venture to submit — 
1. That nitrogen is essential to vital transformation as well as to the constitution of 
certain tissues, and that all in excess of the latter quantity is after a short period cast 
out of the system, 
2. That Avhen the built of the tissue is maintained, urea represents nearly all the nitro- 
gen Avhich has entered the blood from the food ; when the bulk of the tissue is increased, 
the urea represents the nitrogen in the food minus the nitrogen gained by the tissues ; and 
when the bulk is lessened, the urea represents the nitrogen supplied by the food plus the 
nitrogen lost by the tissues ; and as in a Avell-balanced adult system the tissues maintain 
a standard bulk Avith some uniformity, the first is the representative condition of urea. 
3. That commonly the urea varies as the food. 
4. That Avhilst it is probable that all vital transformations take place in connexion 
Avith tissues, it does not follow that vital changes (including the production of urea) do 
necessarily imply interchange of tissue-elements (solids). 
5. The period and amount of elimination of urea is dependent upon the existence of 
Avater Avithin the body in excess of that necessary to maintain the due bulk of the tissues 
and the blood. The urea may therefore accumulate, or, on the other hand, it may be 
rapidly eliminated. The bulk of the body regulates the emission, but the supply of food 
and the activity of the vital actions regulate the production of urea. 
6. The effect of temperature and atmospheric pressure is probably that of food abso- 
lutely or the relation of food to the vital actions in the production of urea, whilst it is 
that of the statics of the body in relation to the elimination of urinary water and urea. 
7. That daily variations are not necessary evidences of variations in the formation of 
urea. They are lost on a long average by a compensating power, which controls the 
emission of fluids. 
Rhume. 
The folloAAing is a resume of the principal facts which are contained in the preceding 
communication : — 
1. The inquiiT was intended to show the changes normally proceeding in a healthy 
and abundantly fed system, under the ordinary and varying conditions under Avhich men 
live in the course of the year, and also the effect of treadwheel [and other] labour 
upon prisoners who are well fed, but have no surplus nutritive material in them. 
2. The inquiry upon myself was continued during [two years and two months, and 
embraced 635 days, and more than 1400 analyses for urea.] 
3. The average daily evolution of urea was 519 grs. [on the average of two years 
500 gi's.], and the proportion to each lb. of body-weight was 2-73 grs. The extremes 
were 298 and [876] grs. daily, and 1‘56 to [4'38] grs. per lb. of body-weight. 
The quantity of between 400 and 500 grs. daily was found in 46 per cent., and 
between 400 and 600 grs. daily in 85 per cent of the whole. 
