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elements of tlie various tissues, are thrown off as waste, and as 
such give rise to what are commonly known as the excretions 
of the body. It is obviously to this last class that we must 
look for the measure of the wear and tear of the body and 
of the evolution of force of which that wear and tear is the 
exponent. 
Now, of all the different substances which are thus thrown 
off from the body as the result of the decay which is con- 
tinually going on within it, there is one, urea, which is pre- 
eminently important, not from its mere predominance in bulk 
over all the others, but because it is the one which gives us the 
most accurate gauge of the amount of waste of which it is the 
product. 
If we were to be told that the quantity of urea which is 
daily manufactured and eliminated from the body of a healthy 
man, weighing about 150 lb., varies from 400 to 630 grains, 
it is probable that many of us would not be much the wiser for 
the information. We must, therefore, see if we can learn what 
this represents in another way. 
The daily work which is performed by the body of an ordi- 
nary human being may be classed under four heads. (1) There 
is the vital work, or that which is required to keep the 
machinery of life going and in proper order; e. g., to make the 
heart beat, the stomach digest, the liver secrete bile, and so 
on ; just as a certain portion of the power of a steam-engine is 
expended in merely moving the machinery which it sets in action. 
(2) Then there is what may be called the calorific work , or that 
which is required to maintain the temperature of the body, and 
which will obviously be much greater in winter than in 
summer, and in cold climates than in warm ones. Although 
this is intimately connected with the preceding variety of 
work, still it is for many purposes sufficiently distinct and 
important to justify our considering it under a separate head. 
(3) Next we have the mechanical ivorh which is involved in the 
physical exercise we take, such as walking, talking, eating, &c. 
(4) And, lastly, there is the mental work , which we each of us 
perform in the acts of thinking, seeing, hearing, and in the 
exercise of our nervous functions generally. One of the great 
problems which physiology has of late been endeavouring to 
solve is, how much of the total daily work of the body is 
absorbed by each of these four departments of bodily activity 
separately; or, to put the question in another point of view, 
how much of the total daily waste of the body is due to them 
severally ? The recent researches* of a distinguished medical 
divine — for, by a strange coincidence, though a clergyman by 
profession he is also a physician by education (the Eev. Pro- 
* Dublin Quarterly Journal of Medicine , 1859, 1860. 
