ORGAN OF HEARING. 443 



part of the bony vestibule. It is oval in shape, 

 smaller than the utriculus, its longest diameter 

 measuring about 3 mm. From its lowest part a short 

 canal, the duct of Hensen, opens into the ductus 

 cochlearis. Anteriorly there is an oval, whitish 

 thickening, the macula acustica sacculi, innervated by 

 the neurons of the auditory nerve. The histology 

 of the sacculus is like that of the utriculus. 



(2) Semicircular Canals. There are three osseous 

 canals situated behind and above the vestibule. 

 The superior canal is vertical, the external is hori- 

 zontal, and the posterior is vertical. They open into 

 the vestibule by five apertures, since the inner ex- 

 tremity of the superior and the upper extremity of 

 the posterior join to form a common duct, the 

 canalis communis. Each canal presents an en- 

 largement or osseous ampulla near its origin with 

 the vestibule. The membranous semicircular canals 

 partly fill the bony canals, to which they conform. 

 The peripheral border of each canal is fixed to the 

 periosteum of the bony canals while the opposite part 

 is free. Each canal is dilated in the bony ampulla 

 to form a membranous ampulla. These canals com- 

 municate with the utriculus and possess a fibrous 

 wall clothed with simple pavement epithelium, ex- 

 cepting in the ampullae, where it is columnar. In the 

 latter slender sustentacular cells intervene between 

 shorter neuro-epithelial cells, or hair cells, similar to 

 those of the maculae. The neurons of the auditory 

 nerve arborize around the bases of the hair cells. 



(3) The Bony and Membranous Cochlea. The bony 

 cochlea assumes the form of a short cone and con- 



