GENERAL OKGANOLOGT. 



121 



hairs, the olfactory tubiiles of the Crustacea and the olfactory 

 cones of insects. In a similar way certain nerve end organs in 

 the region of the mouth are considered as organs of taste, since the 

 taste organs of verteljrates, the so-called taste buds, are abundant 

 in the mouth cavity, especially on the tongue. 



Organs of Hearing and of Sight are called the higher sense- 

 organs, because they are of much greater importance for the 

 totality of our perceptions tlian the other organs, since they fur- 

 nish sensations which are quantitatively and cpialitatively much 

 more definite. Ears and eyes have therefore a complicated and 

 characteristic structure, wliich renders them easily recognizable 

 by the almost invariable presence of certain structures accessory to 

 their functions. 



History of the Auditory Organs. — The auditory organs of 

 vertebrates and of most of the other animal groups can be traced 

 back to a simple fundamental form, the auditorij vesicle (fig. 70). 



Fig. 79.— Auditory vesicle of a mollusc (Plrrntrarliea}. N. auditory llel■^■l■ : 1 Iz, audi- 

 tory cells with the central cell; C'z, ciliated cells ; Wz, ciliated cells ; 0(, otolith. 

 (After Glaus.) 



This has an epithelial wall, a fluid contents, the endolyinph, and 

 an auditory ossicle or otolith, formed from a single or from several 

 fused auditory concretions. In some instances the otoliths, to the 

 number of thousands, may remain separate. In a definite region 

 of the epithelial wall the cells are developed into the crista 

 acustirn, the auditory ridge; they are in connexion with the 

 auditory nerve and bear the auditory hairs projecting into the 

 endolymph. The otoliths themselves are concretions of carlxmate 

 or of phosphate of lime (exceptionally in Mysis of fluoride of 

 calcium). They usually float free in the centre of the vesicle, and 

 are often held in place by bundles of cilia which project from the 

 non-sensitive epithelial cells. 



