SENSE OF HEARING. 



425 



between the true Reptiles and Fishes, there is a remarkable variation in this 

 respect, some having a tympanum, and some being completely destitute of 

 it. Wherever a tympanic cavity distinctly exists, there is an Eustachian tube 

 connecting it with the fauces. This cavity, in the true Reptiles, not only 

 possesses ihefenestra ovalis (or opening into the vestibule), but the fenestra 

 rotunda (or opening into the cochlea). The membrana tympani is usually 



[Fig. 182. 



A view of the axis of the Cochlea and the Lamina Spiralis, showing the arrangement of the three 

 Zones; the osseous zone and the membrane of the vestibule have been removed ; 1, the natural size of the 

 parts ; the other figure is greatly magnified ; 2, trunk of the auditory nerve ; 3, the distribution of its fila- 

 ments in the zona ossea ; 4, the nervous anastomosis of the zona vesicularis ; 5, the zona membranacea ; 

 6, the osseous tissue of the modiolus ; 7, the opening between the two scalae.] 



visible externally ; but it is sometimes covered by the skin. In Birds, the 

 structure of the ear is essentially the same as in the higher Reptiles. A dis- 

 tinct cochlea exists, though its form is not spiral but nearly straight : of its 

 character, however, there can be no doubt ; a division into two passages, by 

 a membranous partition on which the nerve is spread out, being evident. 

 Moreover the tympanum communicates with cavities in the cranial bones, 

 which are thus filled with air ; and these, by increasing the extent of surface, 

 produce a more powerful resonance. There is no external ear, except in a 

 few species of nocturnal Birds. In Mammalia, the organ of hearing is usually 



[Fig. 183. 





Cochlea of a new-born infant, opened on the side towards the apex of the petrous bone. It shows the 

 general arrangement of the two scalae, the lamina spiralis, and the distribution of the cochlear nerve. At 

 the apex is seen the modiolus expanding into the cupola, where the spiral canal terminates in a cul-de- 

 sac. The helicotrema is not visible in this view. From Arnold.] 



36* 



