AUDITORY ORGAN 



229 



raa 



Fig. 183. — Right Membkanods Labykinth 

 OF Turdu.1 musicus, from the inner side. 

 (After G. Retzius. ) Letters as before. 



In many Reptiles the free end of the ductus endolymphaticiis is 

 situated close under the roof of the skull beneath the parieto- 

 occipital suture, and in the Ascalabota the duct even leaves the 

 •cranial capsule, passes back between the muscles of the neck, and 

 in the region of the pectoral arch becomes swollen to form a large 

 folded sac, from which finger-shaped processes extend to the 

 ventral surface of the vertebral colunm and to the sub-mucous 

 tissue of the pharynx. These processes may also branch out in a 

 labyrinthic manner into the 

 •orbit, and they are always 

 filled with a white semi- 

 solid mass of calcareous 

 substance, as in Anura : 

 calcareous matter is present 

 in the ductus endolym- 

 phaticus of all Vertebrates, 

 at any rate in the embryo. 

 In Birds, the duct does not 

 pass out of the cranial 

 ■cavity. 



A tympanic membrane 

 is present in Birds and all 

 Reptiles except Hatteria, 

 Snakes, and Amphisbte- 

 mians ; and in the two last- 

 mentioned groups the tympanic cavity and Eustachian tube are also 

 wanting. In Crocodiles the tympanic cavity is very complicated, 

 and in them as well as in Birds, the two Eustachian canals open by 

 a single median aperture into the pharynx. The osseo-cartilaginous 

 ■columella is well developed in the Sauropsida, and is not distinct 

 from the stapedial plate ; in Hatteria it is continuous distally with 

 the hyoid (p. 92). 



In certain Lizards (e.g., Ascalabota, Monitor), an indication of 

 the development of an external auditory passage is seen, the 

 tympanic membrane being partially covered posteriorly by a small 

 fold of skin, usually enclosing the anterior border of the digastric 

 muscle ; and in Crocodiles there is a definite integumentary valve 

 moved by muscles. In certain Birds also {e.g., Owls), there is 

 a moveable valve. 



Mammals. — The auditory organ of Mammals reaches a much 

 ■higher stage of development (Fig. 184), but in Monotremes it 

 shows many points of resemblance to that of Reptiles and Birds. 



The cochlea now reaches its highest development, and grows 

 into a long tube which becomes spirally coiled on itself^ In this 



^ In Jlan it forms nearly three coils, and in other Mammals from one and a 

 half (Cetacea) np to as many as four or more. Tlius in the Rabbit there are two 

 and a half, in the Ox three and a half, in the Pig almost four, and in the Cat 

 three coils. The cochlea, as well as the sacciihis and all parts of the pars superior 

 of the membranous labyrinth, vary considerably both in form and arrangement in 

 the various types. 



