EXCRETION OF SUPERFLUOUS AZOTIZED NUTRIMENT. 295 
separate or strain-off, as it were, the products of the decompo¬ 
sition of the tissues formed from it, when their term of life 
had expired (§ 161). But it is certain that Man (as well as 
other animals which have in some degree learned his habits) 
frequently consumes much more food than is necessary for 
the supply of his wants ; and a little consideration will show, 
that the surplus must pass-off by these excretions, without 
ever forming part of the living fabric. For new muscular 
tissue is not formed in proportion to the quantity of aliment 
supplied, but in proportion to the demand created by the 
exercise of it (§ 587); consequently, if more food be taken-in 
than is necessary to supply that demand, no use can be 
made of it. We never find that a Man becomes more fleshy 
by eating a great deal and taking little exercise ; indeed, the 
very contrary result happens, his flesh giving place to fat. 
But let him put his muscles to regular and vigorous exercise, 
and eat as much as his appetite demands, and they will then 
increase both in strength and bulk. 
348. Hence, if more azotized food be taken-in, than is 
required to supply the waste of the muscular and other azotized 
tissues, the surplus must be carried-off by the organs of 
excretion—chiefly, indeed almost entirely, by the Kidneys. 
By throwing upon them more than their proper duty, they 
become disordered and unable to perform their functions ; 
hence the materials which they ought to separate from the 
blood accumulate in it, and give rise to various diseases of a 
more or less serious character, which might have been almost 
certainly prevented by due regulation of the diet. The most 
common of these diseases among the higher classes are Gout 
and Gravel; both of these may be often traced to the same 
cause,—the accumulation in the blood of lithic acid, which 
results from the decomposition of the superfluous azotized 
food, and which the kidneys are not able to throw-off in the 
proper state, that is, dissolved in water. That these diseases 
are, comparatively speaking, rare among the lower classes, is 
at once accounted-for by the fact, that they do not take-in 
any superfluous azotized food;—all that they consume being 
appropriated to the maintenance of their tissues, and the 
kidneys having only to discharge their proper function of 
removing from the blood the products of the decomposition 
of these. 
