180 
ANIMAL PATHOLOGY. 
been enabled satisfactorily to demonstrate it. Still, however, I 
believe it to exist; for I find the ultimate fibrils of these 
nerves, which I know to be those of organic motion and sensibi- 
lity, ramifying upon the bronchial membrane and the ultimate 
vesicles, as plainly and as palpably as on other tissues of the sen- 
sation and the power of motion of which I have no manner of 
doubt. I therefore adopt the language of Dr. Quain, for whom, 
as an anatomist and a man, I entertain the deepest respect ; “ as 
the mucous membrane of the trachea is prolonged into the vesi- 
cles, and forms their lining, it is a legitimate matter of inference, 
from the various facts disclosed by the general anatomy of 
tissues, that the fibrous lamellae and muscular structure extend 
to them also.” 
The Excitor of the Nerves of the Pulmonary Plexus. — Then, 
the nerves from these plexus, ramifying upon and governing the 
action of the muscles of the bronchi and the ultimate vesicles, what 
is their proper excitant ? What is the office of these vessels? 
To give passage to and to contain atmospheric air, in order that 
its oxygen may be employed in uniting with and carrying away 
the carbon of the blood, and also that a portion of that oxygen 
may enter into and mingle with the circulating fluid. And what 
is the proper action of the muscles? To close upon and expel 
the air from which this oxygen has been extracted, and again to 
expand for the admission of a fresh supply. Then what is evi- 
dently the stimulus which rouses these nerves to contract? The 
presence and the animal consciousness of the existence of an undue 
quantity of carbonic acid gas — the compound of the carbon of 
the blood and the oxygen of the air — in these passages. Car- 
bonic acid, existing in a certain proportion, is the stimulus or 
excitant of the nerves of the bronchi and the ultimate vesicles. 
The work of arterialization is suspended, and the worn-out and 
now deleterious gas is expelled, in order that more of the pabulum 
of life may be admitted. 
Experimental Proof of this. — Take a natural or deep inspira- 
tion. You have admitted a certain portion of atmospheric air 
to be acted upon by the blood ramifying over the membrane of 
the vesicles. You presently begin to feel an indescribable un- 
easiness within the chest. You may struggle with it for awhile ; 
but the seeming consciousness of approaching suffocation, com- 
pels you to make a deep expiration, and you are immediately 
relieved. You have got rid of the air charged with carbonic acid 
gas, and no longer able to support life ; s for, painfully irritating the 
bronchial membrane, the muscles have contracted upon it, and 
expelled it. Take two or three deep inspirations and expirations, 
by means of which you have completely expelled the vitiated 
