596 
THE FOOD WE GIVE. 
place and form in their organism. When an animal subsists 
upon flesh, we find no difficulty in explaining its nutrition; 
for the flesh being of the same composition as its own body, 
the animal, in a chemical point of view, may be said to be 
eating itself; nor, with a knowledge of this identity of vege¬ 
table albumen with flesh, is there any difficulty in compre¬ 
hending the nutrition of vegetable feeders. 
Plants, then, in reality, form the^^<s^ of animals ; and the 
latter merely appropriate it a place in their organism. 
It follows, then, as a conclusion, that the analysis of any 
vegetable substance pretty accurately indicates its nutritious 
powers. It has been shown by many laborious chemical 
researches, to use the words of Dr. Playfair, that there are 
two kinds of food. The first, which contains nitrogen, is 
exactly of the same composition as the principal tissues of 
the human body, and is the only substance which can supply 
the waste of these tissues. The second kind of food is that 
destitute of nitrogen, such as starch, gum, and sugar, all of 
which are destined for the support of respiration and conse¬ 
quent heat of the animal. The latter kind of food, when in 
excess, is converted into fat, but never into muscle. The 
increase of flesh in an animal consists in two changes of the 
matter of the food, without any alteration in its composition. 
The albumen or nitrogenous constituent of the food is first 
converted into blood, without decomposition, and the blood 
is afterwards converted into flesh. In order to show that 
the transformation is actually effected without change, we* 
have only to refer to the following results of the analysis of 
vegetable albumen, of ox-blood, and of flesh : 
Vegetable 
Ox 
Albumen. 
Blood. 
Flesh. 
Carbon 
. 55-160 
54-35 
54-12 ' 
Hydrogen 
. 7-055 
7-50 
7-89 
Nitrogen 
. 15-996 
15-76 
15-67 
Oxygen 
. 21-18 
22-39 
22-32 
As muscle, then, is formed only by the albumen or gluten 
of the food, which albumen is in reality flesh itself, we can 
ascertain the comparative value of food, as far as the produc¬ 
tion of muscle is concerned, by estimating the exact quantity 
of the nitrogenous constituent of the food. The following 
table, continues Dr. Playfair, contains the approximate, 
though not perfectly accurate, information relative to the 
value of food for the support of respiration and production of 
fat: 
