OF UEEA AND TJEINAET WATEE. 
833 
4. That, as elimination and formation of urea are different acts, but incapable of sepa- 
ration, the influence of agents exerted over the production of urea must be examined on 
many successive days. 
5. As urea is a mixed product from food and tissue, it cannot in any case (even in 
prolonged fasting) be taken as an absolute indication relative to the one or the other. 
6. As the relations of urea are chiefly ndth food, and, other things being equal, urea, 
with the nitrogen in the faeces, will measiu’e the amount of nitrogen contained in the 
food, its chief value in health and under ordinary conditions is in indicating the amount 
of food which has entered the blood as distinct from that which has passed through the 
bowel. In fasting, it will measure nearly the whole of the loss of nitrogen sustained by 
the tissues, minus an unlmown amount which the tissues have received from the albu- 
minous elements of the blood. 
7. The relation of urea to weight of body is indefinite and intermediate, since the 
relation is true only so far as weight of body may represent the consumption of nitro- 
genous foods. 
8. It is probable that the determination of the amount of urea eliminated under the 
ordinaiT conditions of life, is much less valuable than is at present supposed. 
9. The relation of period of the day to the elimination of urea is that of fluids to the 
solids in food, and the necessity for fluid in the body, and is one of elimination chiefly, 
whilst the relation of season of the year and its coexistent circumstances is with the 
solids in food and the production of m’ea. 
10. The appetite for food, and the amount of nitrogen contained in the food, chiefly 
affect the production of urea, whilst the statics of the body, and their relation to atmo- 
spheric pressure and to temperature, by regulating the quantity of fluid to be retained 
and emitted, affect the elimination of urea. Certain fluids, and perhaps solids, have the 
power of retarding the elimination of urea, by causing a retention or accumulation of 
fluids in the body. 
11. The alternations in the quantity of mine emitted on succeeding days are due 
chiefly to atmospheric influences, acting with, or in opposition to, the statics of the body. 
12. The excretion of urea is not affected by purging, except so far as the supply of 
food may be varied, and fluid be emitted by the kidneys in less quantity than is usual. 
13. The relation of urea to such disordered states of the system as are found in 
derangements of the stomach and in headache is probably less than that of fluid 
retained ; for relief is found rather with the emission of fluid by the bowel or by the 
kidneys, than with any remarkable and sudden increase in the elimination of urea. 
14. The production of carbonic acid is the best measure which we have of the activity 
of the rital functions attending muscular exertion. 
15. The variations in the amount of urine excreted at different seasons of the year 
are different from what (and not so great as) has been stated, when the amount of fluid 
drunk is not limited, and the inquiries extend over a lengthened period. 
16. It is now most desirable that inquiries should be directed to determine the rela- 
tive amount of nitrogen in the food which enters the blood to that which remains in 
