THE METABOLISM OF THE BODY. 431 
tivity of the other. This is availed of in the treatment of dis- 
ease, 
Both the Malpighian capsules and the renal tubules have a 
true secretory function, though the larger part of the water of 
urine is secreted by the former. Blood-pressure is an important 
condition of secretion, though it is likely that this is so chiefly 
because it favors a rapid renewal of the blood circulating 
through the organ. Whether there are nerves that influence 
secretion directly, as in the case of the skin, is not determined. 
Suppression of the renal functions leads to symptoms in 
which the nervous system is recognized as suffering to the 
extent often of coma, ending in death. The urine of most other 
animals is more concentrated than that of man; this secretion 
in carnivora being acid, and in herbivora alkaline in reaction 
when passed a short time. 
Our information in regard to the kidneys has been derived 
experimentally chiefly from the study of the frog and a few of 
the domesticated mammals, especially the dog; and as regards 
some points of interest, so far as urine is concerned, from the 
bird (guano), and the horse, ox (aromatic compounds), etc. 
Man’s urine has been thoroughly studied; but the nature of 
the act of renal secretion is in his case a matter of inference 
from the facts of pathology, clinical medicine, therapeutics, etc. 
THE METABOLISM OF THE BODY. 
In the widest sense the term metabolism may be conven- 
iently applied to all the numerous changes of a chemical kind, 
resulting from the activity of the protoplasm of any tissue or 
organ. In amore restricted meaning it is confined to changes 
undergone.by the food from the time it enters till it leaves the 
body, in so far as these are not the result of obvious mechani- 
cal causes, The sense in which it is employed in the present 
chapter will be plain from the context, though usually we shall 
be concerned with those changes effected in the as yet compara- 
tively unprepared products of digestion, by which they are ele- 
vated to a higher rank and brought some steps nearer to the 
final goal toward which they have been tending from the first. 
As yet our attempts to trace out these steps have been little 
better than the fruitless efforts of a lost traveler to find a road, 
the general direction of which he knows, but the ways by which 
it is reached only the subject of plausible conjecture. But 
