SIR CHARLES BELL ON THE ORGANS OF THE HUMAN VOICE. 303 
trachea; the larynx being, in a manner, divided in its office. At the upper 
opening there is the structure, and action, and sensibility, constituting it a 
guard against foreign matter ; but the proper organ of sound is formed on the 
lower extremity of the trachea and in the chest. Hence, in birds, there is this 
remarkable difference, that the sound must ascend along the trachea. Directed 
by this consideration, it is not without interest that we notice the absence of 
the thyroid gland in them ; that the trachea itself is a firm tube with cartilages 
of entire circles ; and that there is nothing to suffocate the rising vibrations. 
In no animal is the thyroid gland of the same relative magnitude as in man. 
But it is easy to prove that the trachea has no influence upon the voice. 
Both in the open pipe or flute, and the pipe stopped at the bottom, as the 
syrinx, the length determines the note, — lengthening the tube depresses the 
note, and shortening it makes the sound more acute. A similar effect should 
result from the elongation and shortening of the trachea, if the changes of 
the voice depended upon it : but, on the contrary, the trachea is lengthened 
during the high note, while it is shortened as the voice descends, and the notes 
become graver*. I have no ear to determine what harmonic sounds attend 
the human voice ; but supposing that sounds proceed from the trachea, which is 
shortening, at the same time that they proceed from the upper part of the tube, 
which is lengthening, it is clear to demonstration that the two portions of the 
tube can never consent or keep any proportion in their vibrations. 
For these reasons I apprehend that in the structure and condition of the 
trachea, the design manifestly is to suffocate the vibrations of sound, and so to 
impede the motions originating in the larynx from being propagated down- 
wards. 
Pursuing our inquiry into the organs of the voice independently of articula- 
tion, and looking more particularly to the Larynx, we shall find that the com- 
mon opinion is confirmed by experiment and every analogy, that the glottis is 
the primary seat of sound — the source of the vibrations communicated to the 
air as it is breathed. But to consider the motions of the glottis, and even the 
modulations of the air in the larynx, as the sole source of sound, would be 
* Fabuicius ab Aquapendente, seeing the contraction and elongation of the trachea during the 
changes of the voice, presumed that these motions must be the cause of them. Dodakt showed the 
incorrectness of this. 
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