General Principles of Physiology. 99 
become more or less so of all habitual acts of volition? We 
frequently hear such observations as the following :—ifI did so, 
I did it unconsciously. If we stop a person who is walking, he 
cannot tell which leg he last moved; or a person who is playing 
on an instrument, he cannot tell which fingers he last employed ; 
yet all such acts are strictly acts of volition. If we are re- 
minded of them, we can always interrupt, renew, retard, or ac- 
celerate, them at pleasure. We have no difficulty in perceiving 
and changing in any way we please the motions of respiration 
when we choose to attend to them; but as there is no other act 
of volition so habitual, there is none so apt to escape attention. 
The above explanation of the manner in which the removal of 
the brain puts a stop to respiration will be readily admitted, I 
think, when we consider to what part of the brain impressions 
from the lungs are conveyed. It is evidently to the part where 
the eighth pair of nerves, which supplies them, joins that part of 
the brain from which the spinal marrow originates. These 
nerves are no ways connected with the intercostal muscles, and 
diaphragm ; yet it appears from the experiments in which M. le 
Gallois removed the brain by slices, that respiration continued 
till he removed the origin of these nerves, and then instantly 
ceased. In these experiments the power of the muscles of respi- 
ration, and the nervous power which excites them, still remain, as 
may be easily ascertained by stimuli properly applied to the spinal 
marrow; hence M. le Gallois’s difficulty. It is the sensorial power 
alone, the sensation which induces us to inspire, that is lost. 
We cannot, perhaps, have a better instance of the distinct 
operation of the sensorial, nervous and muscular powers, than in 
the case before us, although they all here conduce to the same 
end. We may destroy any one of them, and leave the others 
unimpaired. The destruction of the sensation, in consequence of 
which we will to inspire, we have just seen, neither destroys the 
nervous nor muscular power employed in expanding the chest. 
By means applied to the muscles of respiration, we may destroy 
their contractility without destroying any part of the neryous 
power, or at all impairing the sensation just mentioned; and 
we may destroy the nervous power which excites these muscles 
Vor. XIV. H 
