ORGAN OF CORTI. 121 



other hand, somewhat resembles the outline of a swan's head ; the rounded part, 

 which represents the back of the head, fitting into the concave surface on the head 

 of the corresponding inner rod or rods, while the part which represents the bill 

 projects outwards and is connected with the reticular lamina, aiding to form the 

 first series of rings for the transmission of the auditory hairlets. Both inner and 

 outer rods are more slender about the middle of their length and expand again 

 below, so as to rest upon the basilar membrane by a somewhat widened foot. They 

 are distinctly striated throughout their length (fig. 138). 



In the head of the outer rod is an oval part free from fibres, and staining with 

 carmine more deeply than the remainder of the rod (pseudo-nucleus). A similar, 

 but smaller clear body, staining deeply with carmine, is seen in the head of the inner 

 rod, and the substance of the rod in its neighbourhood has a somewhat granular 

 appearance (fig. 1 38). 



The inner rods are more numerous than the outer ; l they are also more closely 

 set and touch one another along their whole length, whereas the outer rods are only 

 in contact laterally by their heads ; finally the outer rods are in all parts longer 

 than the inner, and in the upper turns of the cochlea considerably so. 



How the two sets of rods are jointed together is not very clear. It is certain that 

 the individual rods have little, if any, independent movement ; they are securely 

 fixed below to the basilar membrane, and the heads of adjacent rods are in close 

 contact. 



Basilar cells. In connection with both inner and outer rods, there is seen a 

 protoplasmic cell occupying the angle which the rod makes with the plane of the 

 basilar membrane (figs. 136, 137). Sometimes these cells extend along the membrane 

 until they come into contact, and they may, especially in young subjects, be seen to 

 rise up and partly envelope each rod. They are usually regarded as the cells by 

 and from which the rods have been formed. 



Hair-cells and cells of Deiters. The inner hair-cells, some 3,500 in number 

 in all, are closely applied against two or three of the corresponding rods, the cells 

 being considerably larger in diameter than the rods. Seen from above they are 

 oval and marked by a curved line, which is the line along which the hairlets are 

 attached (fig. 140, i.h.) They are very like somewhat short, columnar epithelium- 

 cells, and are prolonged below into a process (which may be branched) by which 

 they are connected with the upper surface of the spiral lamina. Beneath them, and 

 extending also under the gradually decreasing columnar epithelium of the spiral 

 groove, is a layer of protoplasmic cells with large round nuclei, amongst which fine 

 nerve-fibres appear to run in a spiral direction. Around the top of each inner hair- 

 cell is a sort of ring of cuticular substance, which is connected with slight projections 

 on the flattened heads of the inner rods, and perhaps represents the reticular lamina 

 in this place. 



The outer hair-cells are peculiar in shape. They are cylindrical at the upper 

 end, where they fit into the rings of the reticular lamina and bear the hairlets, but 

 lower down they are flattened from within out, so that, in profile, they look narrow, 

 but broader when seen on the flat (fig. 138). These cells end below with a rounded 

 extremity (fig. 136), extending about as far as to the narrowing part of Deiters' cells. 

 The hairlets, as with the inner cells, are about 20 in number on each cell, and spring 

 also from a curved line on the upper surface of the cell. Beneath the hair-cells and 

 resting by a broad base upon the basilar membrane, certain other cells are found 

 which are known as the cells of Deiters (fig. 136). These, which are of a cylindrico- 



1 According to Retzius there are altogether in the human cochlea about 5,600 of the inner rods 

 and nearly 4,000 of the outer ones. 



