1 520 HUMAN ANATOMY. 



upper end terminates in a triangular head, and whose lower extremity expands into the foot 

 resting upon the basilar membrane. The inner pillar is shorter, more nearly vertical and less 

 curved than the outer ; its head exhibits a single or double concave articular facet for the recep- 

 tion of the corresponding convex surface of the head of the outer rod. The cuticular substance 

 of both pillars adjoining the articular surfaces is distinguished by a circumscribed, seemingly 

 homogeneous oval area of different nature. The upper straight border of the head of each pil- 

 lar is prolonged outwardly into a thin process or head-plate, that of the inner lying uppermost 

 and covering over the head and inner part of the plate of the outer pillar. The head-plate of the 

 latter is longer and projects beyond the termination of the plate of the inner rod as the phalan- 

 geal process, which unites with the adjacent phalanges of the cells of Deiters to form the mem- 

 brana reticularis. The inner pillars of Corti are more numerous, but narrower than the outer 

 elements, from which arrangement it follows that the broader outer rods articulate with two and 

 sometimes three of the inner pillars, the number of the latter in man being estimated by Retzius 

 at 5600, as against 3850 of the outer rods. 



Immediately medial to the arch of Corti, resting upon the inner rods, a single row of spe- 

 cialized epithelial elements extends as the inner auditory or hair-cells. These elements, little 

 more than half the thickness of the epithelial layer in length, possess a columnar body contain- 

 ing an oval nucleus. The outer somewhat constricted end of each hair-cell is limited by a 



FIG. 1273. 



Membrana tectoria 



Nuel's space Inner hair-cells 



Outer hair-. 



cells 



Hensen's cells 



Oll-of Dritrr- 



Section showing details of Corti's organ from human cochlea ; owing to slight obliquity of section, width is some- 

 what exaggerated. X 375- Drawn from preparation made by Dr. Ralph Butler. 



sharply defined cuticular zone, from the free surface of which project, in man, some twenty-five 

 rods or hairs. The inner hair-cells are less numerous (according to Retzius about 3500), as well 

 as shorter and broader, than the corresponding outer elements. Their relation to the inner rods 

 of Corti is such, that to every three rods two hair-cells are applied. The inner sustentacular 

 cells extend throughout the thickness of the epithelial layer and exhibit a slightly imbricated 

 arrangement as they pass over the sides of Corti's organ to become continuous with the lower 

 cells of the sulcus spiralis. 



The cells covering tin- Kisilar membrane from the outer pillar to the basilar crest comprise 

 three groups: (a) those composing thr outer part of Corti's organ, including the outer hair- 

 //* and cells of Deiters ; (/>) the outer supporting cells, or cells of Hensen ; (c) and the low 

 cuboidal elements, the cells of Claudius, investing the outermost part of the basilar membrane. 



The outer auditory or hair-cells are about five times more numerous (approximately 18,000 

 according to Waldeyer) than tin- corresponding inner elements, and in man and apes are dis- 

 posed in thre, -or t'.uir rows. They alternate with tin- peculiar end-plates or "phalanges" of 

 1 tellers' cells, which separate the ends of the hair-cells and join to form a cuticular mesh-work, the 

 nifintimmi //<///</; /.v, through the openings of which the hair-cells reach the free surface. The 

 inner rciw< it tliesi- cells lies directly upon the outer rods of Corti, so placed that each cell, as a 

 rule, rests upon two rods. The cells of the second row, however, are so disposed that each cell 

 ingle rod, whilst the third layer repeats the arrangement of the first. In conse- 

 quencf of this grouping, these elements, in conjunction with tin; " phalanges," appear in surface 

 views like a checker-board mosaic, in which the oval free ends of the auditory cells are included 

 between the peruliar < nmpre->-,ed and indented octagonal areas of the end-plates of Deiters 1 cells 



