THE EAR. 519 



THE MINUTE STRUCTURE OF THE MEMBRANOUS COCHLEA. 



The position of the membranous cochlea between the 

 scala vestibuli and scala tympani has been referred to. Its 

 shape as a triangular tube running from the sacculus to 

 the top of the cochlea has been stated. Speaking approxi- 

 mately its boundaries are as follows: The upper side formed 

 by the membrane of Reissner separates it from the scala 

 vestibuli ; the lower side is formed by the basilar membrane 

 and a part of the lamina spiralis, these two separating it 

 from the scala tympani. Its outer wall is formed by the 

 wall of the bony cochlea itself. 



1. Basilar Membrane. The essential structure in this 

 membranous cochlea is the basilar membrane. This, al- 

 though called a membrane, really consists of a number of 

 strings of varying length stretched from the edge of the 

 lamina spiralis to the outer wall. These strings are shortest 

 at the base of the cochlea and become gradually longer 

 towards the top. If this basilar membrane were removed 

 from the cochlea and straightened out it would have the 

 appearance given in Figure 161. Such a structure suggests 

 at once various kinds of musical instruments built on this 

 plan. The soundingrboard of a piano is fashioned after 



Fig. 161. DIAGRAM SHOWING THE GRADUAL LENGTHENING OF THE CORDS IN THE BASI- 

 LAR MEMBRANE, , d, d, d' , AND THE CORRESPONDING INCREASE IN THE WIDTH OF 

 THE ORGANS OF CORTI, a, ft, a, 6'. 



such a plan, the shorter strings of the board representing 

 the treble notes, the longer ones the bass notes. This 



