416 



Mr. H. J. Watt, 



9. It may be of interest to state the average index build of the mammalian 

 cochlea ; in relation to the diameter of the lowest whorl of the cochlea the 



proportions are : — 



Second whorl 0-61 



Diameter of tube in front of round window - 44 



Major axis oval window 0'32 



Length of basilar membrane 5 - 65 



Number of whorls (absolute) 2"5 + 



In Table III the correlations between the different cochlear indexes are 

 given. The following conclusions may be drawn : — 



Table III. — Cochlear Indexes. (Index = any dimension divided by the 



diameter of the lowest whorl.) 





Basilar membrane. 



Tube of the 

 cochlea. 



Major axis 

 of oval 

 window. 



Second 

 whorl. 



Number of whorls 



•730 ± -040 

 (- -042 ± -110)* 



•494 ± -077 

 - -069 ± -102 



•325 ± -101 



•060 ± -093 



•500 ± -070 

 •362 ± -082 



•051 ± "094 

 •094 ± -093 



■000 ± -093 



Second whorl 



Major axis of oval window ... 

 Diameter of tube of cochlea . . . 



* Between the number of whorls and the index of the basilar membrane contained in the first 

 two whorls. 



« 



10. There is a high correlation between the number of whorls and the 

 index of the basilar membrane, but not between the former and any other 

 cochlear index. This confirms the conclusion of (5) that variation in the 

 number of whorls from the typical number two and a half occurs without 

 any alteration in the basal plan of the cochlea ; extra turns are added on at 

 the top of the cochlea by simple continuation of the whorl, or if the turns 

 are fewer than usual, the whorl simply stops short at the required point. 

 The cochlea does not grow by accretions at the base as a shell does, but by 

 accretions at the apex ; if it grew in the former way, all the dimensions of 

 the cochlea would be correlated highly and positively with the number of its 

 whorls, which is not the case ; there is not even any correlation between the 

 number of turns and the index of the length of the basilar membrane 

 contained in the first two whorls (vide note to Table III). 



11. There is also a positive correlation — practically certain, if not quite 

 decided — between the index of the basilar membrane and the index of the 

 second whorl. This evidently means that the basilar membrane may be 

 increased by an increase in the diameter of the second whorl as well as by 



