108 COMPARATIVE ANATOMY. 



however, the auditory organ gradually sinks further and further 

 inwards from the surface. Thus a new method for conducting the 

 sound-waves is necessitated, and the following structures become 

 developed : a canal passing inwards from the surface, the external 

 auditory passage or meatus ; this opens into a spacious cham- 

 ber, the tympanic cavity, in which are situated the auditory 

 ossicles, and which is connected by the Eustachian tube with 

 the pharynx. The whole of this canal, which is divided into an 

 outer and an inner portion at the junction of the external auditory 

 passage and tympanic cavity by a vibratory membrane, the tym- 

 panic membrane, lies in the position of the first embryonic 

 visceral (hyomandibular) cleft, or, what comes to the same thing, 

 in the position of the spiracle present in many Fishes. From Rep- 

 tiles and Birds onwards the first indications of a pinna (that is, the 

 part of the external ear which projects from the head) are seen, 

 though it only reaches a full development in Mammals. 



The pinna arises from a series of swellings which surround the external 

 aperture of the hyomandibular cleft. These appear at an early stage in the 

 region of the mandible and hyoid, and soon fuse together to form a sort of 

 ring, from which are "formed later those characteristic protuberances of the 

 pinna which are known as tragus, antitragus, antihelix, &c. 



Fishes. Apart from Cyclostomes, the peculiarities of whose 

 auditory organ it is difficult to explain, that of all other Fishes 

 follows the general plan given above, and the same may be said for 

 all the higher Vertebrates. Everywhere we meet with a division 

 into a pars superior, represented by the utriculus and 

 semicircular canals, which remains essentially much in the 

 condition already described, and a pars inferior, constituted 

 by the sacculus and cochlea, which gradually becomes more 

 differentiated, and attains to a higher and higher degree of develop- 

 ment and functional perfection (Fig. 161). In Fishes the cochlea 

 consists simply of a small knob-like appendage ("lagena") of the 

 sacculus, which opens freely into the main cavity of the latter 

 by means of the sacculo-cochlear canal (Fig. 161, C}. The utriculus 

 and sacculus also communicate with one another by the sacculo- 

 utricular canal. 



Amphibia. Here all the parts remain much as described 

 above, with the exception of the cochlea, which, especially in the 

 Anura, points to a higher stage of development, in that it shows 

 an indication of a pars basilaris with another patch of nerve- 

 endings, the papilla acustica basilaris: it becomes further 

 constricted off from the lumen of the sacculus, with which it is 

 connected only by a very minute canal. 



A further advance in structure as compared with Fishes is the 

 appearance of a cartilaginous plate which fits into the feriestra 

 ovalis of the auditory capsule, and corresponds to the base of the 

 stirrup-bone (stapes) of the higher Vertebrata (Figs. 56 and 58, 



