THE METABOLISM OF THE BODY. 459 
part they play when introduced into the blood, we soon find 
that our knowledge is very limited. 
Sulphur, and especially phosphorus, seem to have some im- 
portant use which quite eludes detection. It is important to 
remember that certain salts are combined with proteids in the 
body, possibly to a greater extent than we can learn from the 
mere analysis of dead tissues. 
Pathological —The withdrawal of any of the important salts 
of the body soon leads to disease, clear evidence in itself of their 
great importance. This is notably the case in scurvy, in which 
disease the blood seems to be so disordered and the nutrition 
of the vessel-walls so altered that the former (even some of the 
plood-cells) passes through the latter. 
Water.—The use of water certainly has a great influence 
over the metabolic processes of the body. The temporary ad- 
dition or withdrawal of even a few ounces of water from the 
regular supply of a dog in the course of a feeding experiment 
greatly modifies the results obtained for the time. . It is well 
known that increase of water in the diet leads to a correspond- 
ing increase in the amount of urea excreted. It is likely that. 
even yet we fail to appreciate the great part which water plays 
in the animal economy. 
THE ENERGY OF THE ANIMAL Bopy. 
As already explained, we distinguish between potential or 
latent and actual energy. All the energy of the body is to be 
traced to the influence of the tissues upon the food. Energy 
may be estimated as mechanical work or as heat, and the 
one may be converted into the other. All the processes of 
the organism involve chemical changes, and a large propor- 
tion of these are of the nature of oxidations; so that, speak- 
ing broadly, the oxidations of the animal body are the sources 
of its energy; and in estimating the quantity of energy, either 
as heat or work, that a given food-stuff will produce, one must 
consider whether the oxidative processes are complete or par- 
tial: thus, in the case of proteid food, if we suppose that the 
urea excreted represents the form in which the oxidative pro- 
cesses end or are arrested, we must, in estimating the actual 
energy of the proteid, subtract the amount of energy that 
would be produced were the urea itself completely oxidized 
(burned). 
If the amount of heat that a body will produce in its com- 
