Chap. XI.] 



VOCAL ORGANS OF BIRDS. 



39 1 



Some of the lower rings of the trachea unite to form 

 a tympanic chamber ; the tracheal orifices of the two 

 bronchi are separated by a membranous septum, and 

 on either side there is a tympaniform membrane 

 formed on the inner side of the uppermost bronchial 

 rings ; the air which passes through the bronchial clefts 

 sets in vibration the membranes which bound them, and 

 the character of the note produced is affected, on 

 purely physical principles, 

 by the position of the 

 bronchial half-rings, and 

 by the length of the 

 column of air in the 

 trachea. The position of 

 these half-rings is not 

 constant, owing to the 

 fact that they are moved 

 by proper muscles, which 

 act on their ends, and so 

 rotate them. 



In Steatoriiis (one of 

 the night-jars), the syrinx 

 is completely bronchial, 

 the fifteenth and sixteenth 

 bronchial rings being only 



half-rings, as are also some that follow them ; the 

 space thus formed is filled in with membrane, which 

 can be rendered tenser by the contraction of the 

 lateral muscle of the trachea, which is attached to the 

 middle of the fifteenth ring. In those American 

 crows in which the syrinx is completely tracheal, we 

 have an arrangement which is essentially similar. 

 Among the Katite birds the syrinx is best developed 

 in Rhea ; in the American vultures the voice organ is 

 found in its simplest condition. 



It is obvious that the length of the trachea must 

 have a very considerable influence 011 the character of 



Fig. 166. Larynx of Peregrine 

 Falcon. A, Front view; B, in 

 section. 



