XTII 



PHYLUM CHORDATA 



563 



sn 



Mammals do the nasal chambers lose their sensory functions the 

 olfactory nerves being vestigial or absent. The organs of taste 

 are taste-buds in the mucous membrane covering certain of the 

 papillae on the surface of the tongue and in that of the soft palate. 



In essential structure the eye of the Mammal resembles that of 

 the Vertebrates in general (see p. 106). The sclerotic is composed 

 of condensed fibrous tissue. The pecten of the eye of Birds and 

 Reptiles is absent. In most Mammals there are three movable 

 eyelids, two, upper and lower, opaque and usually covered with 

 hair, and one anterior, translucent, and hairless the nictitating 

 membrane. The secretions of a lacrymal, a Harderian, and a series 

 of Meibomian glands moisten and lubricate the outer surface of 

 the eye-ball and its lids. In Moles, and certain other burrowing 

 Insectivores and Rodents, and 

 in Notoryctes among the Mar- 

 supials, the eyes are imper- 

 fectly developed and function- 

 less. 



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 membranous laby- 

 rinthand in the greater 

 development of the accessory 

 parts. A large external audi- 

 tory pinna, supported by car- 

 tilage, is almost invariably 

 present, except in the Mono- 



fr-QTviafci Pafanoa anrl Sirpnii FlQ. 1221. Sagittal section through the nasal and 



Uldtd,, ^ uacea, a buccal cavities of the human head. /, //, III, 



ThlS is a widelv Open fun- the three olfactory ridges formed by the tur- 



,. JL ' t binals ; be, entrance to the mouth ; Iff. tongue ; 



liel, OI a Variety OI Shapes os , opening of Eustachian tube ; an', frontal 



A'^tf^, ^4- lir,,-. sinus ; sn", sphenoidal sinus ; v. i, atlas vertebra ; 



in dltlerent groups, having ., 'axis vertebra, (After Wicdersheim.) 



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. 

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

 passage (Fig. 1222, Ex.). This, the length of which varies, 

 leads inwards to the tympanic membrane (M.), which separates it 

 from the cavity of the middle ear or tympanic cavity. The wall of 

 the external auditory passage is sometimes entirely membranous or 

 cartilaginous, sometimes in part supported by a tubular part of 

 the tympanic bone ; in Echidna it is strengthened by a series of 

 incomplete rings of cartilage. The tympanic cavity, enclosed by 

 the periotic and tympanic bones, communicates with the upper or 

 respiratory division of the pharynx by a longer or shorter tubular 



