An Essay on Fat and Muscle. 
251 
appears in respiration, anil what is the origin of the carbonic acid 
jras ? T'he blood consists of the same elements as the food which 
the animal consumes, containing, as we have seen (4), a very 
large proportion of carbon and hydrogen; and as carbonic acid gas 
consists of oxvgen and carbon, it is evidently the result of the 
combination of the oxygen with the carbon of the food. Accord- 
ing to Boussingault, " a horse consumes in this manner in twenty- 
four hours 97^ oz. of carbon, and a milch cow 69y7r oz. ; and the 
former requires to convert the carbon into carbonic acid 13 lbs. 
3^oz. of oxygen, and the milch cow 11 lbs. lOf oz. in the same 
time." ' 
8. In whatever way carbon may combine with oxygen, the act 
of combination cannot take place without the disengagement of 
heat. There exists in the living body no other known source of 
heat but the chemical action between the elements of the food and 
the oxygen of atmosphere, and it signifies nothing what inter- 
mediate changes (6) the food undergoes in becoming assimilated 
to organised tissues, or in its passage thi'ough the liver in the for- 
mation of bile; the last change is uniformly the combustion of the 
carbon and hydrogen, and the production of carbonic acid, watery 
vapour, and animal heat. No part of the oxygen taken into the 
system is given out again in any other form than that of a com- 
pound of carbon and hydrogen ; and as these substances are 
supplied in the food, it is clear that the amount of nourishment 
required for any animal will be proportionate to the quantity of 
oxygen taken into the system. 
9. There are two causes which chiefly contribute to increase 
the consumption of oxygen gas; these are temperature and 
KXKRCisE. We will proceed to consider the effect of these in the 
living organism by a few examples from the farmer s everv-day 
practice. In the winter, the air is more condensed than in the 
summer, consequently the same volume of air in the winter con- 
tains a larger per centage of oxygen than in the warm weather, 
when it is more rarified. It is for this reason that a larger supply 
of food is consumed by persons living in cold countries than by 
those who are inhabitants of hot climates. We thus perceive an 
explanation of the apparently anomalous habits of different nations. 
The maccaroni of the Italian and the train-oil of the Greenlander 
are not adventitious freaks of taste, but necessary articles fitted to 
administer to their comforts in the different climates in which 
they have been born. 
Example No. 1. — A flock of Leicester sheep on tolerably good 
keep will increase in weight throughout the year about 52 lbs. of 
mutton for each sheep, but this accumulation takes place chiefly 
during the spring and summer months, for during the cold weather 
it requires all the farmer's supplies of food to keep them at the 
