Purchased Food, and of its Residue as Manure. 207 
food, without undergoing much change, are therefore readily 
assimilated by the animal organism, and, when given in excess, 
are stored up as animal fat. 
The proportion of carbon in fat amounts to about 80 per cent., 
and is much larger than in starch or sugar. In round numbers, 
one part by weight of fat or oil is as valuable a feeding-material 
as two-and-a-half parts of sugar or starch. Besides this, fat 
serves important functions in the processes of digestion and 
nutrition. It has been shown by actual experiments, that albumi- 
nous substances deprived of fat remain longer in the stomach, 
and require more time for their conversion into cells and muscular 
fibre, than when associated with fatty matters. There is good 
reason for believing that fat is largely concerned in the forma- 
tion of bile, and that the digestive power of the pancreatic fluid 
is due, in great measure, to its presence. 
Fat certainly possesses high digestive powers, and appears to 
assist the solution of food, and its absorption into the blood. 
Colourless blood-corpuscles receive, perhaps, the first impulse 
of their formation from the metamorphosis of fat, and thus it 
may be an important aid in the formation of blood. 
Fat thus takes an active part in the processes by which the 
nutritive constituents of food are converted into butcher's meat. 
Not only is it concerned in the formation of new tissue, but it 
also pervades, and finally disintegrates, the older structures, 
especially when their vitality is low. In this manner it helps in 
the solution of effete nitrogenous products, and their subsequent 
removal from the animal body. 
Starch, gum, mucilage, and sugar are appropriately called 
carbon-hydrates, for in them carbon is combined with the same 
relative proportions of oxygen and hydrogen in which the two 
latter elements form water. In starch, sugar, and analogous 
carbon-hydrates, the hydrogen is therefore fully oxidised, and 
the carbon only is capable of oxidation, and of generating animal 
heat by its oxidation or combustion. As already stated, the 
heat-producing power of fat or oil is about twice and a half 
as great as that of starch or sugar. 
The carbon-hydrates of food not merely generate animal heat, 
which is, in reality, the final result of their oxidation, but they 
likewise give rise to lactic and other organic acids, which per- 
form important functions in the digestion of food. The pre- 
sence of lactic acid in the stomach appears to be essential to 
the digestion of the albuminous compounds of food, and its 
occurrence in the juice of flesh probably assists the solution of 
effete tissues. 
When food rich in starch or sugar is given to animals in 
larger quantities than is required to support respiration, and 
