388 
Co.>iijwsif.iou and Nutritive Value of Strait: 
c-onccnied, but eadi according to circumstantcs can lend itsolf'to 
the work which is the more peculiar province of the other. The 
proportion of carbon in fatty matter amounts to rather more than 
80 per cent., and is much larger than in gum, sugar, or starch. 
Oil and fat, for tliis reason, are not only better producers of fat than 
starchy and sugary compounds, but are likewise more powerful 
agents for the support of respiration and the maintenance of animal 
heat — the heat generated in the body being proportionate to the 
amount of carbon consumed in a given time during respiration. 
Gum, sugar, mucilage, starch, and a few similar compounds 
may be represented as consisting of carbon and water only, and 
on account of the simplicity of their composition they are well 
adapted to support respiration. The quantity of carbon con- 
sumed by the respiration of animals varies at different times and 
in different species, according to the rapidity of their breathing 
and their mode of living. Under all circumstances, however, it 
is considerable, especially in the case of ruminating animals. 
Thus cows consume four'-ninths of the carbon contained in their 
ordinary daily food by respiration, and throw it off in their exha- 
lations in the form of carbonic-acid gas. Hence the absolute 
necessity of supplying large-sized animals with abundance of 
carbonaceous food. 
As straw contains no starch and but a small proportion of 
gum, mucilage, and sugar, and thus is deficient in the better 
kinds of respiratory constituents, it cannot rank high as a heat- 
producing material, unless it can be shown that cellular and 
woody fibre can be assimilated and used for the same purpose 
for which starchy compounds are usually employed in the animal 
economy. 
The question then arises — and it is an important one — is 
cellular or woody fibre digestible or not ? and upon a correct 
and trustworthy answer to this question mainly depends the 
decision whether or not straw is really as nutritious as some 
maintain. To arrive at as trustworthy a reply to this question as 
can be given in our present state of knowledge, we have to 
inquire, in the first place, what is understood by woody fibre ? 
If any vegetable substance— straw, for instance — is treated suc- 
cessively with cold and boiling water, next with alcohol and 
ether, then with a dilute solution of caustic potash, and finally 
with dilute sulphuric acid, an insoluble residue is obtained, 
differing in quality and texture according to the original material 
used in the experiment. This insoluble residue is called by 
chemists indiscriminately cellular or woody fibre. It is in 
reality generally a mixture of cellulose, the substance of which 
the walls of the cells of plants consist, and of woody substances 
which are deposited around the original cell-walls. These 
