302 SIR CHARLES BELL ON THE ORGANS OF THE HUMAN VOICE. 
circles, and which have no compressing muscles? Does it explain the pecu- 
liarity, that all the air-tubes of birds are dry ; that their lungs are motionless ; 
and that in the air respired by them there is no moisture ? 
These are the reasons why I must reject the opinion of Portal, that the 
transverse muscle of the trachea is to give force to the breath in speaking. 
The trachea, and all that portion of the windpipe which extends from the 
larynx to the lungs, may be considered as the porte-vent, or tube which conveys 
the air from the bellows to the reed of the organ-pipe ; and it has even less in- 
fluence on the quality of sound than the porte-vent. If this portion of the air- 
tube were to vibrate and give out sound, it would interfere with, and confuse 
those which proceed from the glottis. The imperfect circle formed by the car- 
tilages of the trachea, and their isolation from each other, are ill suited to con- 
vey sound. — But I am now to notice a more particular provision against the 
propagation of sound downwards by this passage. 
If on inspecting a musical instrument we should find a spongy body of the 
consistence of firm flesh in contact with a cord or tube, and an apparatus by 
which this body might be pressed against the vibrating part, we would not 
hesitate to conclude that it damped or limited the vibration. The thyroid 
gland is a vascular, but firm substance, which, like a cushion, lies across the 
upper part of the trachea*. Four flat muscles, like ribbons, arise from the 
sternum, first rib, and clavicle, and run up to the thyroid cartilage and os 
hyoides, over the surface of this glandular body. These muscles are capable 
of bracing it to the trachea. If it be admitted that the vibration of the tra- 
chea would only produce a continued drone, rising over the inflections of 
the voice and adding nothing to its distinctness, we may perceive in the ad- 
justment of the thyroid gland to the trachea the most suitable means of suffo- 
cating or stopping the vibrations from descending along the sides of the tube. 
Comparative anatomy is often a test of the correctness of our inferences 
drawn from the human body. I reflected that if I were right in my idea of 
this being one of the uses of the thyroid gland, there should be no such body, so 
placed, in birds : and that, following up the inquiry, if we were not likely to 
discover the function of that gland, we might nevertheless learn why it is so 
singularly placed. In birds the sounding apparatus is at the lower part of the 
* See Plate X. fig. 1. D.D. 
