234 
Tlie Applications of Physiolocjy 
calculations to show that this fat is more than is necessary to 
account for the increase of tallow in an animal. He contends 
further, that the tallow of the food may be found completely in 
the milk of a cow fed upon it. In such a case he considers that 
none of it is lost. But even admitting this to be correct, M. 
Dumas must suppose that some digestive process converts the 
tallow into butter, for the composition of the fat found in vegetables 
is quite different from the butter of the milk. The conception 
of a digestive process is, therefore, quite as indispensable in his 
theory as in that of Liebig. I have shown, in a memoir read be- 
fore the Chemical Society, that the fat contained in food does not 
account for half the butter in the milk of a cow, even conceding 
that it is all transformed into butter. 
We know that fatty substances are of use in fattening animals. 
Mutton-suet is often given to fattening ducks. Linseed-oil is 
occasionally given as a substitute for linseed-cake. There cannot, 
then, be the smallest doubt that fatty matters are capable of produc- 
ing tallow in the body, but it is impossible to admit M. Dumas's 
opinion in all its generality. We know that sugar is a substance 
which occasions a great development of tallow ; but surely it 
cannot be affirmed that fat exists in it ready formed ? Rice given 
to fowls causes them quickly to be covered with fat, yet rice 
contains very little fat. 
We therefore do not see any reason to depart from the opinion 
f)f Liebig that fat is the product of a peculiar digestive process 
on the unazotized constituents of food. 
The form in which the food is given to cattle is far from being 
a matter of indifference. If the food be in a state in which it is 
either difficult to attain, or difficult to masticate when obtained, 
much of it will be lost in the production of force necessary to 
adapt it for the organs of digestion. The cutting of hay and 
straw to chaff is unwittingly done with a view to prevent any 
unnecessary expenditure of force. Less mastication is requisite, 
and consequently less of the tissues of the body are expended in 
grinding down the food. The use of saliva, according to Liebig, 
is to form a receptacle for air or oxygen, by which means it is 
mixed with the food and carried to the stomach. The use of 
mastication, then, is not only to comminute the food, but also 
to mix it with air or saliva. We find that a larger size of chaff is 
given to those animals which chew the cud than to those that do 
not. One great object of rumination is to obtain a repeated sup- 
ply of oxygen to the food. Hence, in our ordinary practice, we 
cut the hay- chaff 1 inch in length for oxen, ^ inch for sheep, 
and only \ inch for horses. The two first being ruminating ani- 
mals require it longer than the horse, which is not one. When 
we consider that fresh grass is much more easily masticated than 
