PHYLUM CHORDATA 



581 



Insectivores and Rodents, and in Notoryctes among the Marsupials, 

 the eyes are imperfectly de- 

 veloped and functionless. 



The ear of a Mammal is 

 more highly developed than 

 that of other Vertebrates, 

 both in respect of the 

 greater complexity of the 

 essential part — the mem- 

 branous labyrinth — and in 

 the greater development of 

 the accessory parts. A large 

 external auditory •pinna, 

 supported by cartilage, is 

 almost invariably present, 

 except in the Monotremata, 

 Cetacea, and Sirenia. This 

 is a widely open funnel, of a 

 variety of shapes in different 

 groups, having the function 

 of collecting the waves of 

 sound. By the action of a 

 system of muscles it is 

 usually capable of being turned about in different directions. 



Fig. 120ft. — Sagittal section througli the nasal and 

 buccal cavities of the human head. /, ■ //, ///, 

 the three olfactory ridges formed by the turbinals ; 

 ?jc, entrance to the mouth ; Ig. tongue ; os, open- 

 ing of Eustachian tube ; an' , frontal sinus ; s/i", 

 sphenoidal sinus ; v. i, atlas vertebra ; v. ii, axis 

 vertebra. (After Wiedersheim.) 



Fio. 1210,— Parts of the Human ear (diagrammatic). Cch. cochlea ; R Eustachian tube ; Kv. 

 outer opening of ear ; X. labyrinth ; M. tympanic membrane ; N. entrance of auditory nerve ; 

 Oi- 0^. (?g. the three auditory ossicles— stapes, incus, malleus. (After Headley ) 



Enclosed by its basal part is the opening of the external auditory 

 passage (Fig. 1210, Ex.). This, the length of which varies, leads 



