186 
which we may add, in the 3d place, How is the 
body cooled, when placed in a temperature higher 
than that which is natural to it? 
Independent of direct experiment, there are 
many circumstances connected with animal tem- 
perature which would lead us to conclude that 
it is intimately connected with the function of 
respiration. In the first place, all animals that 
have a temperature much superior to that in 
which they are immersed have their lungs con- 
structed in the most perfect manner, and possess- 
ing the most elaborate organization ; while there 
is an obvious relation between the quantity of 
oxygen which they consume and the heat which 
they evolve. Thus, what are styled the warm 
blooded animals, have lungs of a large size, and 
so formed as to permit the blood and the air to 
exercise the most extensive influence over each 
| other. In amphibia, the pulmonary vessels of 
| the lungs are much more scanty: while the cir- 
culation is so arranged, that only a part of the 
blood passes through them during each circula- 
tion. The temperature of these animals is pro- 
portionably low ; and in fish, where there is only 
-asmall quantity of blood to receive the action of 
_ the air, and that in a less direct manner, the tem- 
perature is only a degree or two above that of 
the medium in which they live. In the second 
place, it is observed, that, in the same species of 
animals, or even in the same individual under 
different circumstances, whatever quickens the 
circulation raises the temperature; and that, 
when the respiration is impeded, either from dis- 
ease or from an original mal-conformation of the 
organs, the temperature is proportionably low- 
ered. Lastly, it may be urged in favour of the 
chemical theory of animal heat, that oxygen is 
actually united to carbon ; and that, according to 
the ordinary effect of this union, caloric must be 
liberated, so that it would be difficult to explain 
how it is disposed of, if it be not employed in 
raising the temperature of the body. There is 
also a farther circumstance to be held in view, 
that, if we reject the hypothesis of the lungs be- 
ing the source of animal heat, we have no other 
adequate cause for its production; for, although 
some writers have supposed that the stomach, 
and others that the nervous system is concerned 
in this function, yet these have been thrown out 
as mere conjectures, without being digested into 
any regular system, so as to point out, in either 
case, in what manner the effect follows the sup- 
posed cause. Upon the whole, therefore, we think 
ourselves warranted in concluding, in the pre- 
sent state of our knowledge upon the subject, 
that animal heat is derived, in the first instance, 
from the union of oxygen and carbon, which 
_ takes place in the lungs during the process of res- 
piration. 
If we have found it difficult to arrive at any 
certain conclusion on the first point that we pro- 
posed to discuss, we shall probably find it still 
more so with respect to the second—By what 
ANIMAL HEAT. 
means is the uniformity of the animal tempera- 
ture preserved? because any speculation which 
we may form upon this subject must, in a great 
measure, depend upon our ideas of the means by 
which animal heat is produced. With the gen- 
eral fact we are well acquinted, that, in warm- 
blooded animals, each species has a temperature 
that is natural to itself, from which it deviates 
very little while in its healthy state. 
perature of the internal parts of the human body 
is between 98° and 99°; and this temperature is 
The tem- 
preserved with as much regularity by the Green- 
lander as by the African. There are, no doubt, 
many circumstances in their modes of life by 
which the inhabitants of these different regions 
endeavour to counteract the extremes of heat and 
cold to which they are exposed ; but after making 
allowance for all these circumstances, some sys- 
tem of adjustment of the functions will be neces- 
sary in order to preserve that uniformity of tem- 
perature which is so essential to life. 
The experiments of Priestley and Crawford,which 
first threw some light upon this intricate subject, 
were repeated and considerably extended by Lavoi- 
sier. They led to the same conclusion, that the | 
union of oxygen and carbon in the lungs is influ- 
enced by the temperature of the inspired air; the 
lower the temperature the more tendency there is | 
to their union, there is a greater consumption of 
oxygen, and a more rapid generation of carbonic 
acid. Hence, according to our usual notions 
upon this subject, there must be a greater evolu- 
tion of animal heat; and this will naturally have 
the effect of counteracting the lower temperature 
in which the body, in this case, is conceived to 
be immersed. According to Crawford’s theory, 
the greater quantity of carbon is removed from 
the blood, the more perfectly is it converted from 
the venous to the arterial state, the more is its | 
capacity for heat increased, and the more will it 
require to supply this increased capacity, which 
will be afterwards liberated during the course of 
the circulation to maintain the due temperature 
of the body. Although this explanation proceeds 
in part upon the principles of Crawford’s theory, 
and may be so far considered as of doubtful au- 
thority, it depends, to a certain extent, upon the 
direct results of experiments that were performed 
without any view to this hypothesis, and which 
appear to be entitled to our confidence ; while, it 
must be admitted, that the admirable manner in 
which they explain the phenomena, affords at 
least some presumption of their truth. But al- 
though the experiments that have been per- 
formed are favourable to the hypothesis—that 
the formation of carbonic acid in the lungs is so 
regulated by the temperature of the air as to 
produce heat according to the demand for it in 
the system—still they are not sufficiently numer- 
ous or decisive to amount to a demonstration of 
its truth. Should it be confirmed by subsequent 
facts and experiments, it must be admitted to be 
one of the most beautiful examples of the adapta- 
a 
ee eres 
| 
