494 THE ORGAN OF HEARING. 



Hence an inner attached and an outer free zone may be differentiated. 

 This membrane has no nuclei, and shows a fine radial striation. 

 Its free portion bridges over the sulcus spiralis internus and rests 

 upon the organ of Corti. Its outer margin extends as far as the 

 cells of Hensen. The development of this membrane is not 

 thoroughly understood, although it very probably represents a dis- 

 placed cuticular formation belonging to the cells of the limbus 

 spiralis. This acceptation has recently been confirmed (Exner). 



The auditory nerve gives off, soon after entering the internal 

 auditory meatus, vestibular branches to the maculae in the utriculus 

 and sacculus and to the cristae in the semicircular canals, and a 

 cochlear branch, which passes up through the modiolus in anasto- 

 mosing bony canals. From this centrally placed column of nerve- 

 fibers, a continuous sheet of nerve-fibers, arranged in the form of 

 anastomosing bundles, passes radially into the osseous spiral lamina 

 and thence to the organ of Corti. Near the base of the osseous 

 spiral lamina, along the entire length of this sheet of nerve-fibers, 

 there is situated in a special bony canal a ganglion, known as the 

 spiral ganglion of the cochlea. The ganglion cells of this ganglion 

 are bipolar, one of the processes of each cell, the dendrite, extending 

 outward through the osseous spiral lamina to the organ of Corti, 

 the other process, the neuraxis, passing through the bony canal in 

 the modiolus, through the internal auditory meatus, and thence to 

 the medulla. The dendritic processes of the nerve-cells of the 

 spiral ganglion form bundles of medullated nerve-fibers, which pass 

 outward within the osseous spiral lamina, forming, in the outer por- 

 tion of the latter, a closely meshed plexus, from which small bundles 

 of nerve-fibers proceed through the foramina nervosa of the labium 

 tympanicum to the organ of Corti ; immediately before passing 

 through these foramina, the medullated nerve-fibers lose their 

 medullary sheaths and neurilemma. 



These nonmedullated fibers, with or without further dividing, are 

 then arranged in small bundles, which, for a certain distance, 

 have a spiral course : that is to say, parallel to the tunnel of Corti. 

 One such spiral bundle is situated on the inner side of the inner 

 pillars, under the inner row of hair cells ; another, on the outer side 

 of the inner pillars, in the tunnel of Corti. Other fibers pass 

 through the tunnel of Corti, so-called tunnel-fibers, to reach the 

 outer side of the arches of Corti, where they are arranged in three or 

 four spiral bundles, at the outer side of the outer pillars and between 

 the rows of the cells of Deiters. From the nerve-fibers of these 

 spirally arranged bundles, terminal branches are given off, which 

 terminate, after further division, on the inner and outer hair cells 

 (Retzius, Geberg). 



Regarding the blood-vessels of the membranous labyrinth, it 

 should be mentioned that the internal auditory artery is a branch of 

 the basilar artery, and divides into the rami vestibulares and rami 

 cochleares. The branches of the former accompany those of the 

 auditory nerve as far as the utriculus and sacculus. At the maculae 



