n sources of heat,
and also some mechanism by means of which both the production and loss
of heat can be regulated. This is conclusively shown by the fact that
both in summer and winter their mean temperature remains the same. But
it was not until the introduction of thermometers that any exact data
on the temperature of animals could be obtained. It was then found
that local differences were present, since heat production and heat
loss vary considerably in different parts of the body, although the
circulation of the blood tends to bring about a mean temperature of
the internal parts. Hence it is important to determine the temperature
of those parts which most nearly approaches to that of the internal
organs. Also for such results to be comparable they must be made in
the same situation. The rectum gives most accurately the temperature
of internal parts, or in women and some animals the vagina, uterus or
bladder.

[v.02 p.0049]

Occasionally that of the urine as it leaves the urethra may be of use.
More usually the temperature is taken in the mouth, axilla or groin.

_Warm and Cold Blooded Animals_.--By numerous observations upon men
and animals, John Hunter showed that the essential difference between
the so-called warm-blooded and cold-blooded animals lies in the
constancy of the temperature of the former, and the variability of
the temperature of the latter. Those animals high in the scale of
evolution, as birds and mammals, have a high temperature almost
constant and independent of that of the surrounding air, whereas
among the lower animals there is much variation of body temperature,
dependent entirely on their surroundings. There are, however, certain
mammals which are exceptions, being warm-blooded during the summer,
but cold-blooded during the winter when they hibernate; such are the
hedgehog, bat and dormouse. John Hunter suggested that two groups
should be known as "animals of permanent heat at all atmospheres" and
"animals of a heat variable with every atmosphere," but later
Bergmann suggested that they should be known as "homoiothermic" and
"poikilothermic" animals. But it must be remembered there is no hard
and fast line between the two groups. Also, from work recently done
by J.O. Wakelin Barratt, it has been shown that under certain
pathological conditions a warm-blooded (homoiothermic) animal
may become for a time cold-blooded (poikilothermic). He has shown
conclusively that this condition exists