Ixxxi 



can account for it, wlioreas the great capacity of this reservoir, 

 its free communication with the trachea, immediately above 

 the narrow bronchial aperture, its sudden distension, and as 

 sudden contraction, or the alternate partial action and relaxa- 

 tion of the distending and compressing agencies, together with 

 the free and elastic vibrating borders of the tracheal opening, 

 the resisting wall behind, and the long and softly resonant 

 tube, leading upwards, may, I think, satisfactorily account for 

 these peculiar vocal phenomena. What may have been the 

 design of imparting to this being this peculiar endowment, it 

 would be as vain to speculate upon as to attempt to account 

 for the infinite variety of voice that prevails throughout the 

 animal world; that it is voluntary I have no doubt, and may 

 be exercised either for sexual attraction or social union, or as 

 indicative of nervous emotion, the result of anger, terror, or 

 alarm. 



This remarkable air-bag is not only peculiar to this ani- 

 mal, but there is nothing exactly analogous to it in any other 

 member of the class Aves. In the trachea of several water 

 birds there have been long observed peculiar swellings, or 

 dilatations of the tube, in the structure of which the entire 

 of a certain number of rings are engaged, but there is no dis- 

 tinct sac' or bag. The laryngeal or tracheal sac in the cha- 

 meleon, bears, as Dr. Knox has remarked, some remote analogy 

 to it ; the laryngeal sacs in certain of the quadrumana, and 

 even the ventricles in the human larynx, and the sacculi la- 

 ryngis, which lead from each upwards and forwards, may be 

 regarded as rudimental conditions of the same structure. 



Explanation of the Plate. 



Pig, 1. — Superior glottis, tongue fimbriated. 



Fig. 2. — Front view of the tracheal sac; a few feathers re- 

 main. 



Fig. 3 Sac laid open, and drawn over to the right side: 



a, opening in the trachea : b^ sharp, thin extremities of the six 



