THE INTERNAL EAR. 



377 



neous oval area of different nature. The upper straight border of the head of both 

 pillars 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 phalangeal process, which unites with the adjacent phalanges 

 of the cells of Deiters to form the membrana 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 specialized 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 containing an oval nucleus. The outer somewhat constricted end of 

 each hair-cell is limited by a 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 



Nuel's space 



Inner hair-cells 



Membrana tectoria 



IJL^^f 77 ^ 



Outer hair cells 

 Hensen's cells 



FIG. 424. Section showing details of Corti's organ from human cochlea ; section is slightly oblique, 

 hence width is somewhat exaggerated. X 375. (Preparation by Dr. Ralph Butler.) 



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 ar- 

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

 lower cells of the sulcus spiralis. 



The cells covering the basilar membrane from the outer pillar to the basilar crest 

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

 the outer hair-cells and cells of Deiters ; (b) the outer supporting cells, or cells of 

 Hensen ; (c} and the low cuboidal elements, the cells of Claudius, investing the outer- 

 most part of the basilar membrane. 



The outer auditory or hair-cells are about five times more numerous (approxi- 

 mately 18,000 according to Waldeyer) than the corresponding inner elements, and in 

 man and apes are disposed in three or four rows. They alternate with the peculiar 

 end-plates or " phalanges " of Deiters' cells, which separate the ends of the hair-cells 

 and join to form a cuticular meshwork, the membrana reticularis, through the open- 

 ings of which the hair-cells reach the free surface. The inner row of these 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 lies 

 opposite a single rod, whilst the third layer repeats the arrangement of the first. In 

 consequence of this grouping, these elements, in conjunction with the phalanges, 

 appear in surface views like a checker-board mosaic, in which the oval free ends of 

 the auditory cells are included between the peculiar compressed and indented octag- 



