300 SIR CHARLES BELL ON THE ORGANS OF THE HUMAN VOICE. 
with parts which serve a double or even a treble function. But we ought 
not to be surprised at the intricacy of structure in the human organs of voice, 
when we find them capable of imitating every sound of bird or beast, excel- 
ling all instruments of music in clearness and expression, and capable of 
making those infinite changes on articulate sounds, which form the languages 
of the different nations of the earth. 
Although there be one subject, — Articulate language, — on which I shall prin- 
cipally comment, as being that in which the treatises on the voice are altogether 
defective ; yet, as there are lesser points in which I think authors are in fault, 
I shall take the subjects consecutively or systematically. I do this in the hope 
of affording, at the same time, a sounder foundation in anatomy, to those mem- 
bers of the Society who are more capable of pursuing this part of philosophy 
in all its curious and elegant subdivisions. 
It will be convenient to divide the inquiry into three heads : — the Trachea, 
the Larynx, and the Pharynx. 
Under the head of 7 'rachea, and through the whole investigation, it is neces- 
sary to keep the different functions of the part in mind ; or we shall be ap- 
propriating to the voice, structures which have reference to other functions. 
We read that the trachea is formed of imperfect hoops of cartilages, joined 
by membranes, and that it is flat on the back part, for these reasons : that it 
may be a rigid and free tube for respiring the air — that it may accommodate 
itself to the motions of the head and neck — and that it may yield, in the act 
of swallowing, to the distended oesophagus, and permit the morsel to descend. 
This is perfectly correct ; but there is a grand omission. Whilst all admit that 
a copious secretion is poured into this passage, it is not shown how the mucus 
is thrown off. 
There is a fine and very regular layer of muscular fibres on the back part 
of the trachea, exterior to the mucous coat, and which runs from the extremi- 
ties of the cartilages of one side to those of the other *. This transverse 
muscle is beautifully distinct in the horse. When a portion of the trachea is 
taken out, and everything is dissected off but this muscle, the cartilages are 
preserved in their natural state ; but the moment that the muscular fibres 
* See Plate X. fig. 3. A. 
