1876.] Organ of Corti in Mammals. 351 



varices that portion which covers the organ increases considerably in thick- 

 ness. 



From the foregoing observations it will be seen that the rods, membrana 

 reticularis, and trabecule are developed from the walls of the original 

 epithelial cells, whereas the hair- and various other cells are formed from 

 their contents ; and, lastly, that the membrana tectoria is a secretion from 

 the same original epithelium. 



The Function of the Organ of Corti. 



It is generally assumed that this part of the labyrinth is concerned in 

 appreciating the pitch and quality of sounds ; and there are two prevalent 

 theories to account for the manner in which this is accomplished. The 

 first, Helmholtz's, assumes that each of the rods of Corti, which are not of 

 uniform dimensions but graduated in length, as I clearly demonstrated 

 in 1871, vibrates in unison with a certain note, when produced in the 

 outer wall, and in vibrating affects certain nerve-filaments, which carry off 

 the impression to the brain. 



The second theory regards these rods as mere supports, and assumes the 

 hair-cells to be the only essential structures. 



The only objection of importance to Helmholtz's theory is the fact that 

 there are no rods in the cochlea of birds. 



As regards the second theory it is very incomplete, as it does not by 

 itself explain the appreciation of the variations in pitch and quality. To 

 make up for this deficiency, however, some further mechanical theories 

 have been suggested. 



Hensen was of opinion that the membrana basilaris, whose breadth is 

 also graduated, acts as the mechanical appreciator, whereas Hasse believed 

 that office to be performed by the membrana tectoria. Both of these ideas 

 are not only very unsatisfactory but obviously fallacious, when the posi- 

 tion and structure of these membranes are considered. 



For my own part I prefer adhering to Helmholtz's view, with the 

 modification that the rods are concerned only indirectly, and that their 

 vibrations affect the hair-cells connected with them, so as to cause an im- 

 pression to be transmitted through the nerve-filament to the brain. I 

 consider it highly probable that the graduation in the length of the bristles 

 alone may account for the appreciation of the variations to a considerable 

 extent ; but I cannot believe for one instant that the rods and rest of the 

 organ are so beautifully graduated for no special purpose. 



The membrana tectoria is undoubtedly analogous to the otolith mass 

 of the vestibule ; and I agree with Waldeyer in regarding it as a damper 

 to the vibrations. 



The method of preparation usually employed was as follows : — 



The cochleae were always quite fresh. 



They were hardened in a i-per-cent. solution of chromic acid in ordi- 

 nary methylated spirit ; ten days required. 



