34 



PRACTICAL ORNITHOLOGY. 



For the present this must serve as an explanation of the 

 mechanism and functions of the respiratory and vocal organs 

 of birds. An opportunity may occur of resuming the subject. 

 But before concluding this lesson, I cannot with propriety 

 defer explaining a peculiarity in the windpipe of birds which 

 seems to have been overlooked. It has been remarked that 

 the rings in certain species have one side narrower than the 

 other, and this alternately, in the manner represented by the 

 accompanying figure ; but although this may be the case in a 

 few species, it is not so in those in which it has been pointed 

 out, for the disposition in question is merely apparent and not 

 real. The true state of such windpipes is this. Owing to the 

 frequent and extensive alternate contractions and elongations 

 of the neck, the trachea requires to have a structure allowing 

 it to undergo corresponding alterations, and this without any 

 great change in its diameter. Solid rings, connected by elas- 

 tic membranes, might, by the contraction of the latter, ap- 

 proximate so as greatly to diminish the length of the tube ; 

 but so great does this diminution occasionally require to be, 

 that to effect it in this manner, the rings would be too slender 

 or too distant to maintain the calibre of the tube in a perfectly 

 pervious state, and therefore a contrivance was necessary by 

 which strength and a great degree of contraction, with a uni- 

 form diameter in all cases, might be 

 combined ; and this has been effect- 

 ed in the following manner. Fig. 

 108 represents a portion of the wind- 

 pipe in a state of relaxation ; while 

 Fig. 109 represents a portion in a state 

 of contraction ; and Fig. 110 a por- 

 tion in an intermediate state. In 

 the first state, Fig. 108, the rings, 

 which are equal, or nearly so, stand 

 free, being separated by an interven- 

 ing space occupied by elastic inem- 

 brane. In the second case, i:* ig. lOy, 

 the rings appear as if incomplete, or the trachea seems to be 

 formed of alternate lateral half rings, presenting, at their meet- 



