. Comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates. AUDITORY ORGAN 301 Especially in Crocodiles and Birds, in which the two Eustachian canals open by a single median aperture into the pharynx,1 the tympanic cavity is complicated, and is continued into pneumatic cavities in the neighbouring bones. The osseo-cartilaginous columella is well developed in most of the Sauropsida, and is not distinct from the stapedial plate ; in Hatteria it is continuous distally with the hyoid (p. 115). In most Lizards the .tympanic membrane is on a level with the skin, but in certain forms (e.
. Comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates. AUDITORY ORGAN 301 Especially in Crocodiles and Birds, in which the two Eustachian canals open by a single median aperture into the pharynx,1 the tympanic cavity is complicated, and is continued into pneumatic cavities in the neighbouring bones. The osseo-cartilaginous columella is well developed in most of the Sauropsida, and is not distinct from the stapedial plate ; in Hatteria it is continuous distally with the hyoid (p. 115). In most Lizards the .tympanic membrane is on a level with the skin, but in certain forms ( Ascalabota, Lacerta, Monitor) an indication of the develop- ment of an external audi- tory passage is seen, the tympanic membrane being partially covered posteri- orly by a small fold of skin, usually enclosing the -s^i ma mu rec. 7RS OF Turdus musieus, from the inner side. (After G. Retzius.) Letters as in previous Figures. anterior border of the di- gastric muscle : in Croco- diles there is a definite in- tegumentary valve moved by muscles (abductor of the mandible, supplied by the facial nerve) and en- closing a dermal bone, and Fia 2_20.—RIGHT MEMBRANOUS LABYRINTH in certain Birds also ( Owls) there is a movable valve. The tympanic membrane in Birds is situated some distance from the surface in an external auditory passage, and is stretched on a ring formed by several bones of the skull. Mammals.—The auditory organ of Mammals reaches a much higher stage of development, but in Monotremes it shows many points of resemblance to that of Reptiles and Birds. The cochlea now reaches its highest development, and forms a long tube which becomes spirally coiled on itself (Figs. 221 and 222).'2 In this, as well as in the more highly-specialised histological structure of the cochlea, lies the characteristic peculi- arity of the auditory organ of Mammals. The auditory nerve forms the axis of the spiral. In consequence of this development of the cochlea, the papilla basilans a
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