456 C. W. PRENTISS 



11. Between stages of 15 and 25 cm., the inner cells of the 

 greater epithelial thickening change from a high pseudostratified 

 columnar type to that of a simple cubical epithelium. These 

 cells lose their attachments to the membrana and the space which 

 as high columnar cells they occupied, becomes the spiral sulcus. 

 The change is brought about by the degeneration of many of the 

 cells and the transformation of those remaining. 



12. In sections of the cochlea at full term the membrana was 

 found attached to the inner supporting cells of the spiral organ 

 and to the outer hair cells as well as to the labium vestibulare. 

 This attachment is regarded as normal because it was indicated 

 by dissection of fresh cochleae; because in development it is so 

 attached; because the attached membrane when shrinking under 

 the action of reagents exerts such a pull upon the spiral organ as 

 to distort it. Lastly, because physiologically and anatomically 

 it is the condition which we should expect to find if the membrana 

 is functional in transmitting sound waves to the auditor^^ hairs. 



13. Although usually described as lying above, the normal posi- 

 tion of the membrana may be directly beneath the spiral organ. 

 As it is slightly heavier than the endolymph if unattached it 

 would float free especially when actively moved or jarred. This 

 would interfere seriously with the function of the organ. 



14. Reasons why the membrana detaches itself from the spiral 

 organ more readily than from the labium vestibulare are as 

 follows : (a) The outer portion of the membrana being chambered 

 shrinks much more than the inner zone which is a solid cuticular 

 plate; (b) Shrinkage of the outer zone affects not only the width 

 but the length of the membrana; (c) Being a spiral structure, the 

 decrease in length decreases the diameter of the turns thus draw- 

 ing the membrana inward. This would tend to separate it from 

 its outer attachment to the cells of the spiral organ. 



15. The arguments against regarding the basilar membrane 

 as a medium for transmitting sound waves to the hair cells, do 

 not hold for the membrana tectoria. 



16. The membrana tectoria is a delicate chambered cuticular 

 structure, co-extensive with the spiral organ. It is attached by 

 its inner zone to the labium vestibulare by its outer zone between 



