116 AVES. 



against its fellow of the opposite side, and is attached higher up to 

 the posterior extremity of the first bronchial arch, which it serves to 

 elevate. The fifth pair of muscles is situated between the posterior 

 long, and t the anterior transverser, elevator, and is called the oblique 

 elevator (m. obliquus posterior s. rotator posterior] ; it arises exter- 

 nally from the superior border of the tympanum by a short and thick 

 muscular belly, passes obliquely backward, and is inserted into the 

 posterior extremities and lower border of the second bony arch, which 

 it rotates and draws in the direction outward. 



The voice of Birds appears like that of the human subject to be 

 produced by the combined vibrations of the laryngeal membranes 

 and the tongue. The flute-like tones of the Singing-birds are doubt- 

 less accomplished by a vibration of the whole column of air while 

 passing through the trachea. Where all the membranes are absen* 

 and the bronchial rings complete, the voice is also deficient, as in 

 the Stork. 



In addition to the already recorded peculiarities in the anatomy 

 of the inferior larynx, there occur in many Birds expansions of the 

 tympanum or apparatus of resonance which strengthen the tone of 

 their voice. To these belong the large bladder-like dilatations and 

 expansions of the tympanum met with hitherto only in the Natatores, 

 as the Drakes and Ganders. These expansions, which have been 

 called the labyrinth, are peculiar from their occurring only in the 

 male and never in the female sex, and in being asymmetrical or of 

 unequal dimensions upon the two sides, the left being always con- 

 siderably larger than the right ; as a rule, this structure is found upon 

 the left side, it being very rare for the right tympanum to be the 

 largest. 



In the Ducks the labyrinth consists for the most part of round 

 bony ampullae about the size of a pea in the lesser species, as Anas 

 crecca, still smaller in A. clypeata, but much more developed in the 

 larger species. More rarely there occurs a double irregular labyrinth 

 larger upon the right side than the left, as in Anas tadorna. These 

 ampullae are wanting in the Dipper (Hydrobates), and both sexes 

 are alike in this respect. Other Ducks, as Anas marila, fuligula, 

 glacialis, leucophthalmos, have a perforate labyrinth : the enlarge- 

 ment is here more angular, and partly provided with membranous 

 lenestrse. This also is generally the case in the male Mergansers, 

 where the labyrinth in M. merganser attains its highest grade of 

 development. The larynx here forms a thick-walled arid hard bony 

 expansion nearly one and a half inches in length, having an uneven 



