208 
On the Theoretical and Practical Value of 
to generate animal heat, the excess of the carbon-hydrates supplied 
in the food is converted into fat, which is stored up in the body. 
It was denied at one time that animals possess the power of 
eliminating from starchy compounds and analogous substances 
the elements which are subsequently reconstructed into fat ; but 
Boussingault's, Liebig's, and Lawes and Gilbert's experiments 
have clearly proved that fat may be, and always is, derived from 
the carbon-hydrates of the food of fattening-stock ; and common 
experience fully confirms the results of these experiments, for. 
it is well known that the meal of cereal grains, and of other food 
rich in sugar, is highly esteemed as good fattening material. 
Cellulose, or woody fibre, according to its condition of digesti- 
bility, depending upon the more or less matured state of the 
vegetable containing it, displays similar, or the same, functions 
in the animal economy as starch and sugar. The tender cellular 
fibre of unripe straw, or of hay, is certainly assimilated to a very 
large extent by herbivorous animals, whilst the hard woody fibre 
of over-ripe grass or straw is digested less perfectly, and rejected 
in larger proportions in the dung. 
Oxen appear to be capable of digesting cellulose, and deriving 
nourishment from it in a larger measure than sheep ; but it 
appears doubtful whether pigs are able to digest cellulose or 
woody fibre at all. Thus a bulky food, containing much straw- 
chafF, may be given with more advantage to cattle than to sheep. 
3. The saline or mineral constituents of food are largely con- 
cerned in the metamorphosis of matter, for it is a special func- 
tion of these substances to give a soluble form to the plastic 
constituents of food and of the animal tissues. They are, in 
fact, the chief, if not the only, media for the transference of 
organic matter from place to place in the animal body, being, on 
the one hand, the conveyers of nutritive materials into the system, 
and, on the other, the carriers of effete substances out of it. 
The saline or mineral constituents of food thus play an im- 
portant part in the phenomena of digestion, assimilation, and 
secretion, being required for the formation of blood, the juice 
of flesh, and other animal secretions. 
A considerable proportion of the mineral constituents of food 
consists of earthy phosphates : they not only supply to the 
animal body the materials of which the greater part of bones 
consists, but they also enter into the composition of flesh. 
Nearly the whole of the mineral matters of food pass into the 
liquid and solid excrements, only a small proportion being re- 
tained in the system, except in the case of young growing anirnals, 
which, requiring much phosphate of lime for the growth of bone, 
extract the earthy phosphates from food to a greater extent than 
full-grown fattening stock. 
