390 Mr. Brodie’s Experiments and Observations 
The facts now, as well as those formerly adduced, go far 
towards proving, that the temperature of warm-blooded ani- 
mals is considerably under the influence of the nervous sys- 
tem ; but what is the nature of the connection between them ? 
whether is the brain directly or indirectly necessary to the 
production of heat ? these are questions to which no answers 
can be given, except such as are purely hypothetical. At 
present we must be content with the knowledge of the insu- 
lated fact: future observations may, perhaps, enable us to 
refer it to some more general principle. 
We have evidence, that, when the brain ceases to exercise 
its functions, although those of the heart and lungs continue 
to be performed, the animal loses the power of generating 
heat. It would, however, be absurd to argue from this fact,, 
that the chemical changes of the blood in the lungs are in no 
way necessary to the production of heat, since we know of no 
instance in which it continues to take place after respiration 
has ceased. 
It must be owned, that this part of physiology still presents 
an ample field for investigation. 
Of opinions sanctioned by the names of Black, Laplace, 
Lavoisier, and Crawford, it is proper to speak with caution 
acid is evolved, the inspirations are much diminished in frequency, and the blood in 
the arteries of the general system assumes a darker hue; but its natural colour may 
be restored by artificially inflating the lungs, so as to furnish a greater supply of air 
to the blood circulating through them. 
We may suppose, that, on the division of these nerves, the sensibility of the lungs 
is either extremely impaired, or altogether destroyed, so ihat the animal does not 
feel the same desire to draw in fresh air : in consequence his inspirations become 
less frequent than natural, and hence arise the phenomena -produced by this opera- 
tion. 
