THE INTERNAL EAR. 



^517 



rounded or somewhat swollen end containing a spherical nucleus. The central end, next to the 

 free surface, exhibits a differentiation into a cuticular zone, similar to that covering the inner 

 ends of the susten^cular elements. From the free border of each hair-cell, a stiff robust hair 

 (.020-.025 mm. long) projects into the endolymph. This conical process, however, is resolv- 

 able into a number of agglutinated finer hairs or rods. 



The free surface of the neuroepithelium v\ithin the saccule and the utricle is covered by a 

 remarkable structure, the so-called otolith membrane. This consists of a gelatinous membrane 

 in which are embedded numberless small crystalline bodies, the otoliths or ear-stones. Between 

 it and the cuticular zone is a space, about .020 mm. in width and filled with endolymph, through 

 which the hairs project to the otolith membrane. The otoliths (otoconia) are minute crystals, 

 usually hexagonal in form, with slightly rounded angles, and from .009-011 mm. in length. 

 They are composed of calcium carbonate with an organic basis. 



On reaching the macula the nerve-fibres form a subepithelial plexus, from which fine 

 bundles of fibres pass toward the free surface. The fibres usually lose their medullary substance 

 in passing through the basement membrane and enter the epithelium as naked axis-cylinders. 

 Passing between the sustentacular cells to about the middle of the epithelium, they break up 

 into fine fibrillie, which embrace the deeper ends of the hair-cells and give off* fine threads that 

 pass as free axis-cylinders between the cells to higher levels. 



The crista acustica and the planum semilunatum are covered with neuroepithelium similar 

 to that of the macula;. The hairs of the hair-cells, however, are longer and converge to and are 

 embedded within a peculiar dome-like structure, known as the cupola, which probably does not 

 exist during life, but is an artefact formed by coagulation of the fluid in which the ends of the 

 hairs are bathed. Otoliths probably do not exist in the cristas acusticae. 



The Cochlear Duct. — The membranous cochlea (ductus cochlearis) lies 

 within the bony cochlea, and like it includes from two and one-half to two and three- 

 quarter turns, named respectively the basal, middle and apical, the latter being 



Fig. T271. 



Organ of Corti 



Reissner s , ' /ir^ ^ 

 membrane^ 



r J /! 



Ligamen-____^ "T ,'^7" 

 turn -spirale / , ' ' 



Basilar membraiK. _- Z"'^^' , V>-i^>\~r-- 



Corti -. membrane 



Ganglion -.pirale 



Scala \eittbuH 



Ductus 

 -cochlearis 



jj^Scala 

 »' tynipani 



Ganglion spirale 



Cochlear ner\e in niteral 

 Modiolus ' auditory canal 



Section of human cochlea passing through axis of modiolus. >; 12. 



three-fourths of a turn at the ape.x; of the cochlea. The tapering tube of the bony 

 cochlea, winding spirally around the modiolus, is subdivided into three compart- 

 ments by the osseous spiral lamina and two membranes, namely, the membranous 

 spiral lamina and Reissner' s membrane. The membranous spiral lamina 

 (lamina basilaris) or basilar membrane extends from the free border of the lamina 

 spiralis ossea to the outer wall of the cochlea, where it is connected to an inward 

 bulging of the periosteum and subperiosteal tissue, called the spiral ligament. 

 The lower of the two tubes thus formed is the scala tympani and communicates, in 

 the macerated skull, with the tympanum through the round window. The upper 

 tube is subdivided into two compartments by an exceedingly delicate partition, 

 known as Reissner's membrane (membrana vestibularis) which extends from the 

 upper surface of the osseous lamina near its outer end, obliqutly upward and putward, 

 to the external wall of the cochlea. The compartment above this membrane is the 



