ORGAN OF HEARING IN MAMMALIA. 2111 



panded independently of any other movement of the face. The 

 1 depressor alye nasi/ ib./, arises from the outer border of the 

 sockets of the canine and contiguous incisor : the fibres ascend 

 to the alae, many of them arching over the outer and back pro- 

 minence of the nostril. The ' depressor septi,' ib. k, is detached 

 from the upper part of the 'orbicularis oris,' fig. 29, oo, the 

 fibres converging from each side toward the nasal septum. The 

 small triangular patch of pale fibres, fig. 161, g, is the ' com- 

 pressor narium minor : ' the larger quadrilateral muscle, h, is the 

 ' levator alas proprius.' In races, like the Mincopies of the 

 Andaman Islands l who scent the ripeness of indigenous fruits, 

 moving the thick ala? of their squab nose, as they explore their 

 dark forests for this purpose, the nasal muscles may be expected 

 to be well and instructively developed. 



§ 216. Organ of Hearing. — The advance in this sense-organ in 

 the mammalian class is seen in the extension of the cochlea, 

 fig. 162,/, into coils suggesting the name ; in the greater propor- 

 tion of the perilymph ; in the ossification of the cartilages between 

 the stapes and tympanum forming the ' mal- 

 leus,' and commonly also the ' incus ; ' and in 

 the presence, save in most swimmers and bur- 

 rowers, of an external ear or conch, served by 

 muscles for various movements to catch the 

 sound. Besides the cochlea, the labyrinth, 

 fig. 162, includes, as in other Vertebrates, the 0sseou 

 semicircular canals, c, d, e, and the interme- gj skle - Hl1,11;u1 > 11U 

 diate space or 'vestibule,' a, by which they 

 now communicate with the cochlea. The semicircular canals 

 form a smaller proportion of the labyrinth in Mammals than in 

 lower Vertebrates ; they retain, however, their posterior position 

 to the vestibule and cochlea. 



The larger opening in the bony wall of the labyrinth is called, 

 from its shape in Man, the ' foramen ovale,' or, from its situation 

 in the labyrinth, 'fenestra vestibuli,' fig. 162, a: it is closed by 

 the base of the stapes. A smaller 'round aperture,' ib. b, is 

 called ' fenestra cochlea3,' because it forms the terminal orifice by 

 which one of the turns of that part, ' scala tympani,' would open 

 into the tympanum, were it not naturally closed by membrane. 



A depression in the petrosal or bony case of the labyrinth 

 receives the facial and acoustic nerves, and terminates in a cul-de- 

 sac, one division of which gives passage to the facial, fig. 165, k; 

 the others receive divisions of the acoustic nerve, and transmit 



1 XXXVIl". 



