338 _ §$OURCES OF ANIMAL HEAT. 
supposed that Bees do not become torpid during the winter ; 
but this is now known to be a mistake. Bees, like other 
Insects, pass the winter in a state of hybernation ; but their 
torpidity is never so profound as to prevent their being aroused 
by moderate excitement. The temperature of a hive is usually 
from 5° to 20° above that of the atmosphere ; being kept at or 
above the freezing-point, when the air is far below it. Under 
such circumstances, their power of generating heat is most 
remarkable. In one instance, the temperature of a hive, of 
which the inmates were aroused by tapping on its outside, 
was raised to 102°; whilst a thermometer in a similar hive 
that had not been disturbed, was only 483°; and the tempe- 
rature of the air was 343°. 
412. The evolution of Heat in the Animal body may now 
be stated with tolerable certainty to depend for the most part 
on the union, by a process resembling ordinary combustion, 
of the carbon and hydrogen which it contains, with oxygen 
taken-in from the air in the process of Respiration. It has 
been elsewhere shown that, even in Plants, this union, when 
it takes place with sufficient rapidity, is accompanied by the 
disengagement of a considerable amount of heat (Veer. Pays., 
§ 381); and in all those Animals which can maintain an 
elevated temperature, we find a provision for this union, hoth 
in regard to the constant supply of carbon and hydrogen from 
the body, and to the introduction of oxygen from the air. 
The supply of carbon and hydrogen may be derived (as already 
shown, § 157), either directly from the food, a large proportion 
of which is thus consumed in many animals without ever 
forming part of the tissues of the body ; or it may be the 
result of the waste of the tissues, especially of the muscular, 
consequent upon their active employment (§$ 160), and con- 
verted into a substance peculiarly adapted for combustion by 
the agency of the liver (§ 366). Or, again, it may be derived 
from the store laid-up in the system in the form of fat ; which 
seems destined to afford the requisite supply, when other 
sources fail. Thus, when food is withheld, or when dis- 
ease prevents its reception, the fat in the body rapidly 
diminishes ; being burnt off, as it were, to keep up the 
temperature of the system. This is the case, too, during 
hybernation; the animals which undergo this change usually 
accumulating a considerable amount of fat in the autumn, and 
