THE COCHLEA AND THE ORGAN OF CORTI 653 



The Membranous Labyrinth. The membranous labyrinth cor- 

 responds generally with the form of the osseous labyrinth, so far as regards 

 the vestibule and semicircular canals, but is separated from the walls of these 

 parts by perilymph, except where the nerves enter into connection within it. 

 The labyrinth is a closed membrane containing endolymph. 



The Utriculus and the Sacculus. The vestibular portion of the inner ear 

 consists of membranous sacs, the upper, the utriculus, the lower called the 

 sacculus. The former is connected with the semicircular canals, the latter 

 with the cochlea by the cochlear canal. The utriculus and the sacculus have 

 on their floors each a special patch of sensory epithelium called the macula. 

 The fibers of the vestibular divisions of the auditory nerve end in the 

 maculae, figure 435. In the cavities of the sacculus and utriculus are small 

 masses of calcareous particles called otoliths. 



The Semicircular Canals. There are three semicircular canals for each 

 ear, one horizontal and two vertical ones placed almost at right angles to 

 each other. The three canals, therefore, occupy the three planes of space. 

 Each has a considerable enlargement or swelling, called an ampulla. The 

 epithelium of the ampulla is modified at the point of entrance of the nerve 

 into a thickened hillock called the crista acustica. This epithelium is com- 

 posed of rod cells or supporting cells which extend the full thickness of the 

 crista, and of hair cells, which occupy the 

 inner or free half of the crista. The hair 

 cells are the sensory cells. They have hair- 

 like processes which project from the free 

 ends of the cells out into the endolymph of 

 the cavity. Nerve fibrils run up into the 

 crista and apparently form terminal arboriza- F ic. 436 yiew of the Osseous 

 tions about the hair cells, or, according to Cochlea Divided through the 

 i , . ,, Middle, i. Central canal of the 



some observers, end in the cells. modiolus; 2, lamina spiralis ossea; 



The Cochlea and the Organ Of Corti. 3> scal a tympani; 4, scala vestibuli; 

 , i i i i , i ' i 5> porous substance of the modiolus 



The membranous cochlea is located in the ne ar one of the sections of the 

 spiral canal in the petrous bone, called the canalis spiralis modioli. x 5. 

 cochlear canal. It is attached to the wall 



of the cavity between the fenestra ovalis and the fenestra rotunda, and to 

 the outer wall of the canal and the free border of the lamina spiralis almost, 

 but not quite, to its summit. A small cavity is thus left around the 

 upper end of the cochlea connecting the scala vestibuli above with the scala 

 tympani below. A cross-section through the cochlear canal shows the 

 relations of the cochlear canal which was named scala media by the earlier 

 anatomists. The free portion of the membranous wall above is called the 

 membrane of Reisner, while that below is called the basilar membrane. The 

 basilar membrane supports the special sensory apparatus for the reception 

 of stimuli of sound waves. 



