388 PHILOSOPHY OF ZOOLOGY. 
But admitting the principles on which the opinions of 
Brack and Crawrorp rest, it will be seen, from the condi- 
tions of the problem which have been stated, that they of- 
fer no explanation of many of the changes which occur. 
Nay, it may be affirmed with confidence, that the support- 
ers of these opimions have not put themselves to any 
trouble in order to obtain a knowledge of these conditions, 
but have suffered themselves to be seduced by the expla- 
nation they seemed: to afford of some of the phenomena. 
In consequence of this neglect, a theory which was long the 
boast of the chemist, has been rejected as visionary ; and 
the disappointed physiologist is now left to re-examine the 
properties of that vital principle he had inconsiderately 
abandoned. 
In the present state of physiological science, no rational 
theory can be offered to account for the production and 
regulation of animal heat. Perhaps the changes which take 
place in the fluids of animals may occasion a disengage- 
ment of heat or cold, in consequence of the difference in 
their capacities. But a variety of other circumstances ap- 
pear to operate. Thecompression of the air in the lungs in 
the act of breathing, and the compression of the blood, by the 
muscular power of the heart and arteries, may likewise ex- 
ercise some influence. We are inclined to believe, however, 
that the principal source of animal heat may with propriety 
be referred to the electrical changes which accompany those 
endless combinations and decompositions which take place 
in the system through the whole of life, changes which are 
necessary to the wellbeing of the principle of life, and de- 
monstrated to be subject to its control. 
