PHYSIOLOGICAL ANATOMY OF THE INTERNAL EAR. 



847 



spiralis foraminulentus. 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. Beyond 

 this ganglion, the nerves form an 

 anastomosing plexus and finally 

 enter the quadrilateral canal, or 

 the canal of Corti. As they pass 

 into this canal, they suddenly be- 

 come pale and exceedingly fine, 

 and probably they are connected 

 finally with the organ of Corti, 

 although their exact mode of ter- 

 mination has not yet been deter- 

 mined. The course of the nerve- 

 fibres to their distribution in the 

 cochlea is shown in Fig. 267. 



FIG. 267. Distribution of the cochlear nerve in the spiral lamina 

 of the cochlea (the cochlea is from the right side and is seen 

 from its antero-inferior part). (Sappey.) 



1, trunk of the cochlear nerve ; 2, 2, 2, membranous zone of the spiral 

 lamina ; 8, 3, 3, terminal expansion of the cochlear nerve exposed in 

 its whole extent by the removal of the superior plate of the lamina 

 spiralis ; 4, orifice of communication of the scala tympani with the 

 scala vestibuli. 



Organ of Corti. Qi all the 

 parts contained within the bony 

 labyrinth, the organ of Corti pos- 

 sesses the greatest physiological interest ; for it is this organ which is supposed to receive 

 the sonorous vibrations and communicate them directly to the terminal filaments of the 

 auditory nerves. Although this view has not received the support of actual demonstra- 

 tion, it affords an explanation, more or less plausible, of the mechanism of audition, car- 

 ried to the point of the actual reception of impressions by the nerves. In view of this, 

 it is important to have a clear comprehension of the arrangement of those parts which 

 are supposed to receive the sonorous vibrations ; and we shall, for the sake of simplicity, 

 eliminate from our description certain accessory structures, the functions of which are 

 obscure. 



In the quadrilateral canal, bathed in the endolymph, throughout its entire spiral 

 course, is an arrangement of pillars, or rods, regular, like the strings of a harp in minia- 

 ture, which are supposed to repeat the varied vibrations of sound. These are the pillars 

 of Corti. 



The structures contained in the quadrilateral canal are so delicate that their investi- 

 gation presents great difficulty ; but the arrangement of the pillars, or rods of Corti is 

 pretty well understood. These pillars are external and internal, with their bases attached 

 to the basilar membrane, and their summits articulated above, so as to form a regular, 

 spiral arcade, enclosing a triangular space, which is bounded below by the basilar mem- 

 brane. The number of the elements of the organ of Corti is estimated at about 3,500, for 

 the outer, and 5,200, for the inner rods, the proportion of inner rods to the outer being 

 about three to two. The relations of these structures to the membranous labyrinth are 

 seen in Fig. 266. The external pillar is longer, more delicate and rounded, and is also 

 attached to the basilar membrane. The form of the pillars is more exactly shown in 

 Figs. 268 and 269, the latter figure, however, exhibiting other structures which enter 

 into the constitution of the organ of Corti. It will be remarked that a small nucleated 

 body is attached to the base of each pillar. At the summit, where the internal and the 

 external pillars are joined together, is a delicate prolongation, directed outward, which 

 is attached to the covering of the quadrilateral canal. 



The above description comprises about all that is definitely known of the arrangement 



