THE ORGAN OF COHTI. 659 



ment of the feet, contimionsly into that of the basilai* memorane, to 

 which they are thus intimately connected (fig. 470). Occupying most 

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

 staining with carmine more readily than the rest : this appears to 

 represent the nucleus of an epithelial cell from which the rod was 

 originally developed. A similar, but smaller nucleus is sometimes, 

 but not always, to be seen in the head of the inner rod, at a place cor- 

 responding with the i^art that represents the root of the coronoid process ; 

 and the substance of the rod presents in its neighbourhood a somewhat 

 granular protoplasmic appearance, as if there were here the remains of 

 an original epithelial cell (fig. 474), But there is another trace of 

 cell-formation always to be found in connection with both inner and 

 outer rods, in shape of a little mass of protoplasm, enclosing a 

 rounded nucleus, which occurs near the base of the rod and fills up 

 the angle which it makes with the plane of the basilar membrane 

 (fig. 471). Sometimes these masses of protoplasm extend along the 

 last-named membrane until they come into contact, and then the floor 

 of the arch of Corti is covered by them. 



As before mentioned, the inner rods are more numerous than the 

 outer,* they are moreover more closely set so as to touch one another 

 along their whole length, whereas the outer rods are only in contact 

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

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

 (Winiwarter, Urban Pritchard). How the two sets of rods are jointed 

 together at their heads is not very clear ; it is certain that but very 

 little movement can be permitted, if any ; for the basilar membrane 

 to which they are securely fixed below is stiff and seemingly inelastic, 

 and the heads of adjacent rods are in close juxtaposition laterally. 

 It is not improbable that the service which the rods and the other 

 cuticular structures here serve, is to keep the parts which more im- 

 mediately minister to the sense of hearing in a state of sufficient tension 

 to be readily acted on by vibrations. 



Hair-cells. — The inner hair-cells are closely applied against the inner 

 side (that turned towards the limbus) of two or three of the corresponding 

 rods, the cells being considerably larger in diameter than the rods. 

 They are very like somewhat short, columnar epithelium, and are 

 jirolonged below into a process, or it may be more than one, which, 

 according to Waldeyer, is directly connected to one of the nerve fi.bres 

 which turn up through the lamina spiralis just below these cells. 

 Under them, and extending also below the gradually decreasing columnar 

 epithelium, is an ill-characterised layer of protoplasmic cells with large 

 round nuclei, amongst which fine nerve-fibres are said to run in a radial 

 direction. Uniting the upper ends of the columnar cells internal to 

 the rods, is a considerable amount of intercellular cuticular tissue, and 

 this also forms around the top of each inner hair-cell a sort of ring, 

 which is connected with slight projections on the flattened heads of the 

 inner rods, and appears to represent the lamina reticularis here. 



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

 at the upper end, where they fit into the rings of the lamina reticu- 

 laris and bear the hair lets, but lower down are ribbon-shaped, being 

 flattened from Avithin out, so that, when seen in profile, they look 



* According to Waldeyer there are altogether in the human cochlea about (3000 of tho 

 inner rods and 4500 of the outer ones. 



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