SIR CHARLES BELL ON THE ORGANS OF THE HUMAN VOICE. 301 
are cut across, the cartilages fly open. This muscle, then, is opposed to 
the elasticity of the cartilages of the trachea. By its action it diminishes the 
calibre of the tube, and by its relaxation the canal widens without the opera- 
tion of an opponent muscle. 
The whole extent of the air-passages opens or expands during inspiration ; 
and then the trachea is also more free ; but in expiration, and especially in for- 
cible expectoration and coughing, the trachea is diminished in width. The 
effect of this simple expedient is to free the passage of the accumulated secre- 
tion ; which, without this, would be drawn in and gravitate towards the lungs. 
When the air is inspired, the trachea is wide, and the mucus is not urged 
downwards ; when the air is expelled, the transverse muscle is in action, the 
calibre of the tube is diminished, the mucus occupies a larger proportion of the 
canal, the air is sent forth with a greater impetus than that with which it was 
inhaled, and the consequence is a gradual tendency of the sputa towards the 
top of the trachea. In the larynx, the same principle holds ; for as the open- 
ing of the glottis enlarges in inspiration, and is straitened in expiration, the 
sensible glottis, by inducing coughing, gets rid of its incumbrance. Without 
this change of the calibre of the trachea, the secretions could not reach the 
upper end of the passage, but would fall back upon the lungs. 
Experiments have been formerly made*, which, although no such view as I 
now present was in contemplation, prove how the action of the transverse 
muscle tends to expel foreign bodies. The trachea of a large dog being 
opened, it was attempted to thrust different substances into it during inspira- 
tion ; but these were always sent out with impetus, and could not be retained. 
Why the dog could not be thus suffocated is apparent ; the tube is furnished 
with this most salutary provision to secure the ready expulsion of all bodies 
accidentally inhaled ; the air passes inwards, by the side of the foreign body ; 
but in its passage outwards, the circumstances are changed by the diminished 
calibre of the canal, and the body, like a pellet filling up a tube, must be ex- 
pelled by the breath. 
Looking on the form and muscular structure of the trachea in man, as pro- 
viding for expectoration of the secretions poured into the tube, what shall we 
think of the tracheae of birds, which are formed by cartilages of complete 
* By M. Favieb. 
2 R 
MDCCCXXXII. 
