THE FUNCTIONS OF THE NERVOUS SYSTEM. 
275 
the exception of their instantaneous destruction as above mentioned, cease in 
death ; whether it be occasioned by injury of the sanguiferous or nervous 
system, or both. 
Such then appears to be the nature of respiration. The first act is the im- 
pression made on the sensorium, the sensation excited by the want of fresh air 
in the lungs. We are enabled to supply it, and thus remove the uneasiness, by 
exciting certain muscles subjected to the will. Through nerves which are 
fitted for this purpose, we apply a stimulus to certain muscles which perform 
the act required. Thus respiration is the combined act of the sensorial nervous 
and muscular powers. It is as effectually destroyed by a failure of the sensa- 
tion which makes us will to inspire, as by that of the nervous or muscular 
power by which the will effects its object. With this view of the subject be- 
fore us, and i can see no other which the facts admit of, it will be proper to 
examine the nature of respiration more in detail. 
I have already had occasion to observe, that the effort made in ordinary 
breathing is very slight. It is chiefly performed by the diaphragm, by the con- 
traction of which the cavity of the chest being slightly enlarged perpendicularly, 
the pressure of the atmosphere readily causes the air cells to be distended with 
air ; but if any obstacle occurs tending to prevent the passage of the air to the 
cells, a greater effort is required, and other muscles are called into action. It 
seems almost unnecessary to observe, that the sensation which induces us to 
make this greater effort, must, as the object is still the same, operate in the 
same way. The more powerful sensation indeed, and the trouble the effort 
gives us, by calling our attention to it, enables us at once to perceive that it 
is an effort of the same kind with any other voluntary effort by which we 
endeavour to relieve ourselves from a painful feeling, and, like any other power- 
ful voluntary effort long continued, produces the feelings of fatigue. Would 
any privation of air induce the struggle that we see in severe dyspnoea, if no 
sensation were excited by it ? This sensation is excited in the sensorium through 
the nerves of the lungs, and all that follows is evidently the result of it. 
The effort consists in two things, drawing the air into the chest with greater 
force, that is, expanding the chest more forcibly that the air may enter it with 
a greater degree of atmospheric pressure, and thus any obstacle to its entrance 
be overcome ; and doing all we can to enlarge the passage by which the air 
enters. 
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