760 



SPECIAL SENSES. 



where the nerve enters, is a transverse fold, projecting into the canal and 

 occupying about one-third of its circumference, called the septum trans- 

 versum. 



The nerves terminate in essentially the same way in the sacs of the ves- 

 tibule and the ampullas of the semicircular canals. At the points where the 

 nerves enter, in addition to the otoliths, are cylindrical cells of various forms, 

 which pass gradually into the general endothelium of the cavities. In addi- 

 tion to these cells, are fusiform, nucleated bodies, the free ends of which are 

 provided with hair-like processes, called fila acustica. These are about ^^ 

 of an inch (31 /*) in length and are distributed in quite a regular manner 

 around the otoliths. The nerves form an anastomosing plexus beneath the 

 endothelium, and they probably terminate in the fusiform bodies just de- 

 scribed as presenting the fila acustica at their free extremities. In the sacs 

 of the vestibule and in the semicircular canals, nerves exist only in the macula 

 acustica and the ampullae. 



The cochlear division of the auditory nerve breaks up into a number of 

 small branches, which pass through foramina at the base of the cochlear, in 

 what is called the tractus spiralis foraniinulentus. These follow the axis of 



the cochlea and pass in 

 their course toward the 

 apex, between the plates of 

 the bony spiral lamina. 

 Between these plates of 

 bone, the dark - bordered 

 nerve-fibres pass each one 

 through a bipolar cell, these 

 cells together forming a 

 spiral ganglion, known as 

 the ganglion of Corti. Be- 

 yond this ganglion the 

 nerves form an anastomos- 

 ing plexus and finally enter 



F:G. 272. Distribution of the cochlear nerve in the spiral lamina ^VIQ ni-iarlvila+aT-al nanal nr 



of the cochlea. The cochlea is from the right side and is seen lut ( l udu lti1 ' 



from its antero-inferior part (Sappey). t h e cana j Q f Corti. As they 



1, trunk of the cochlear nerve : 2, 2. 2. membranous zone of the 



spiral lamina : 3, 3. 3. terminal expansion of the cochlear pass into this Canal they 

 nerve, exposed in its whole extent by the removal of the su- 

 perior plate of the lamina spiralis : 4. orifice of communica- Suddenly become pale and 

 tion of the scala tympani with the scala vestibuli. , . , m , 



exceedingly fine. They 



probably are connected finally with the organ of Corti, although their exact 

 mode of termination has not yet been determined. The course of the nerve- 

 fibres to their distribution in the cochlea is shown in Fig. 272. 



Organ of Corti. In the quadrilateral canal, bathed in the endolymph, 

 throughout its entire, spiral course, is an arrangement of pillars, or rods, which 

 are regular, like the strings of a harp in miniature. These are the pillars of 

 Corti. These pillars are external and internal, with their bases attached to the 

 basilar membrane and their summits articulated above, so as to form a regu- 

 lar, spiral arcade, enclosing a triangular space which is bounded below by the 



