XIII 



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 

 almpst 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. 1209. Sagittal section through the nasal and 

 buccal cavities of the human head. 7, //, ///, 

 the three olfactory ridges formed by the turbinals ; 

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

 ing of Eustachian tube ; ,', frontal sinus ; sn", 

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

 vertebra. (After Wiedersheim.) 



Fio. 1210. Parts of the Human ear (diagrammatic). Cell, cochlea; E. Eustachian tube; Ex. 

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

 01- <?2- 03- 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 



