082 HANDBOOK OF PHYSIOLOGY. 



into the interior of the cavity, forming the crista acustica. Here, too, 

 the epithelium is of a special kind. The nerve fibres spread out and 

 radiate on the inner surface of the membranous labyrinth : their exact 

 termination is uncertain. The distribution of the other division of the 

 auditory nerve, the cochlear, will be more clearly understood after the 

 description of the cochlea itself. 



Structure. The structure of the membranous labyrinth consists of 

 three coats, externally a layer of areolar tissue, next a hyaloid membrane, 

 elevated into minute papillae, and internally a layer of flattened epi- 

 thelium. At the position where the branches of the vestibular branch 

 of the auditory nerve join it, viz., at the saccule, utricule, and ampulla? 

 of the semicircular canals, there is a marked difference in the structure, 

 the external and middle layers are thicker and the epithelium becomes 

 columnar. The epithelium in which the fibres of the vestibular nerve 

 are said to terminate are of two kinds, called cylinder or hair cells , and 

 rod cells. The hair cells occupy only one-half of the thickness of the 

 membrane; from their inner end hair-like processes project into the 

 cavity of the labyrinth. Their outer end is rounded and contains a 

 large round nucleus. To these cells the primitive fibrillaa of the axis 

 cylinders pass up, some of them being distinctly varicose. The exact 

 relation of the nerve fibrillae to the hair-cells is unknown ; by some they 

 are believed actually to enter the cells, by others they are stated to form 

 a kind of nest of fibrillas into which the cells fit. The rod-cells are of 

 somewhat varying form. They are elongated cells extending from the 

 surface to the basement membrane, broad at the upper or surface end, 

 and containing oval nuclei toward their attached end, but not exactly at 

 the same level in all cases. These nuclei, therefore, form a distinct 

 broad nuclear layer on a vertical section of the membrane, as the cells 

 are numerous, much more so, indeed, than the other variety of cell. 

 The lower or attached part of the cell may be branched. 



The membranous part of the cochlea, with a muscular zone, forming 

 its outer margin, is attached to the outer Avail of the canal. Commenc- 

 ing at the base of the cochlea, between its vestibular and tympanic open- 

 ings, it forms a partition between these apertures; the two scalae are, 

 therefore, in correspondence with this arrangement, named scala vesti- 

 Imli and scala tyinpani (fig. 403). At the apex of the cochlea, the 

 lamina spiralis ends in a small hamulus, the inner and concave part of 

 which, being detached from the summit of the modiolus, leaves a small 

 aperture named helicotrema, by which the two scalae, separated in all the 

 rest of their length, communicate. 



Besides the scala vestibuli and scala tympani, there is a third space 

 between them, called scala media or canal membranaceus (CC, fig. 403). 

 In section it is triangular, its external wall being formed by the wall of 



