e inorganic elements are carried to the
earth in the urine.

These changes take place in the healthy animal body during every
moment of life; a waste and loss of substance proceeds continually;
and if this loss is to be restored, and the original weight and
substance repaired, an adequate supply of materials must be
furnished, from whence the blood and wasted tissues may be
regenerated. This supply is obtained from the food.

In an adult person in a normal or healthy condition, no sensible
increase or decrease of weight occurs from day to day. In youth the
weight of the body increases, whilst in old age it decreases. There
can be no doubt that in the adult, the food has exactly replaced the
loss of substance: it has supplied just so much carbon, hydrogen,
nitrogen, and other elements, as have passed through the skin,
lungs, and urinary organs. In youth the supply is greater than the
waste. Part of the elements of the food remain to augment the bulk
of the body. In old age the waste is greater than the supply, and
the body diminishes. It is unquestionable, that, with the exception
of a certain quantity of carbon and hydrogen, which are secreted
through the skin and lungs, we obtain, in the solid and fluid
excrements of man and animals, all the elements of their food.

We obtain daily, in the form of urea, all the nitrogen taken in the
food both of the young and the adult; and further, in the urine, the
whole amount of the alkalies, soluble phosphates and sulphates,
contained in all the various aliments. In the solid excrements are
found all those substances taken in the food which have undergone no
alteration in the digestive organs, all indigestible matters, such
as woody fibre, the green colouring matter of leaves ( chlorophyle),
wax, &c.

Physiology teaches us, that the process of nutrition in animals,
that is, their increase of bulk, or the restoration of wasted parts,
proceeds from the blood. The purpose of digestion and assimilation
is to convert the food into blood. In the stomach and intestines,
therefore, all those substances in the food capable of conversion
into blood are separated from its other constituents; in other
words, during the passage of the food through the intestinal canal
there is a constant absorption of its nitrogen, since only azotised
substances are capable of conversion into blood; and therefore the
solid excrements are destitute of that element, except only a small
portion, in the cons