8 ANATOMICAL AND PHYSIOLOGICAL STUDIES ON 



The next important matter is the determination of the plane 

 of the section. For the measurement of growth changes it was 

 necessary to obtain corresponding sections from the several 

 cochleas. In an organ like that of Corti, which changes in its 

 details from one end to the other, however, it is very difficult 

 to accomplish this, but I believe that I have overcome most of 

 the difficulties. 



After much testing, I found that a section parallel to the 

 under surface of os occipitale in the fronto-occipital direction 

 runs nearly exactly parallel to the axis of the modiolus of the 

 cochlea. In order to get the same direction from right to left, 

 I have taken as the standard the transverse plane of the under 

 surface of the os occipitale, controlling the direction of the section 

 with a magnifying glass. Thus nearly the same radial direction 

 and nearly corresponding places in the cochlea were obtained 

 in the several series of sections. This makes possible a trust- 

 worthy comparison of the measurements and drawings. 



The cross-section of the cochlea was gotten by making the 

 plane of the cut transverse to the axis of the modiolus. To get 

 the corresponding levels is difficult. At first I divided all the 

 serial sections by 23/2, which is the number of complete turns in 

 the cochlea of the albino rat. Next, from the number of the 

 slides representing each turn, I determined nearly the corres- 

 ponding level in the cochlea according to age. 



All the sections were 10V in thickness. The sections were 

 stained for the most part with haematoxylin and eosin, but 

 sometimes by Heidenhain's iron haematoxylin or the iron hae- 

 matoxylin and Van Gieson's stain. For the measurements, 

 however, only the sections stained with haematoxylin and eosin 

 were used. 



For the examination of the larger parts of the cochlea and 

 their relations, the sections were projected on a sheet of paper 

 by the Leitz-Edinger projection apparatus, at a magnification 

 of exactly a hundred diameters, and the outline of the image 

 accurately 1 raced. The remaining measurements of the ganglion 

 cells and the smaller portions of the cochlea were made directly 

 un< ler the microscope. The measurements made on the tympanic 



