310 SIR CHARLES BELL ON THE ORGANS OF THE HUMAN VOICE. 
thorax. However powerful the muscles of expiration may be in compressing 
the chest, their influence is very small on the column of air in the windpipe ; 
the pressure there being no more than on any part of the walls of the chest, 
which is of the same diameter as the base of the tube. The closing of the glottis 
by this small muscle, throws all those of the chest and abdomen, which are 
otherwise muscles of respiration, free to act as muscles of the trunk and arms. 
But if any defect of the windpipe, or of the muscle which closes it, permit 
the air to escape, the muscles of the chest and abdomen sink with the falling 
of the chest ; they become muscles of expiration, and lose their power as mus- 
cles of volition, consequently all powerful efforts cease in the instant. When 
an unhappy suicide thinks to perpetrate self-destruction by dividing his 
windpipe, his sensations of sudden and total failure of strength announce 
the accomplishment of the act ; but he is deceived. In the moment of lunatic 
excitement, his energies are wound up, and his breath is drawn and confined ; 
but now the trachea being divided, in the instant he is seized with feebleness ; 
for the compressed air is let loose, the chest subsides, and the whole muscles 
of the trunk and arms are lost to the actions of volition. He feels as if struck 
with the sudden influence of death ; his actual death depends on other cir- 
cumstances. 
Thus we perceive that the muscle of the glottis, not weighing a thousandth 
part of the muscles of the trunk of the body, controuls them all ; changing 
them from muscles of respiration to muscles of volition ; and this it is enabled 
to do on the principle of the hydraulic press. 
We are by these instances prepared to understand the great importance in 
the animal economy, of power being employed on the lesser cavity in prefer- 
ence to the larger*; and how much will be saved if the appulse necessary in 
articulation be given by the pharynx instead of by the greater cavity of the 
thorax. 
* The principle is as important in its application to pathology as to the natural functions. It ex- 
plains the weak pulse which attends the dilated heart ; how the contractions of the uterus become 
more powerful in the progress of labour ; and why the distended bladder acts with diminished power 
in the expulsion of the urine through the urethra. On the same grounds we understand how a slight 
spasm in the canal of the urethra will resist the most powerful contractions of an enlarged and thick- 
ened bladder, aided by the pressure of the abdominal muscles. 
