HEARING. 



189 



is divided into three columns, one on one side of the partition of bony ledge and 

 the first membrane, one on the other side of the bony ledge and the second mem- 

 brane, and the third, enclosed between the membranes, roughly triangular in 

 shape, its third wall being made by the membrane of the outer wall itself. Of 

 the two outer columns one, the SCALA VESTIBULI, is closed, at the end which is 

 directed towards the middle ear, by the membrane of the fenestra ovalis and the 

 other, the SCALA TYMPANI, by the second membrane-covered opening in the parti- 

 tion, the fenestra rotunda. At the top of the spiral these two columns commun- 

 icate over the tip of the middle triangular one, which ends blindly a little short of 

 the upper end. Fig. 58 shows diagrammatically the relationship of the three. 

 Movements of the membrane of the fenestra ovalis cause vibrations of the same 



FIG. 57. One turn of the cochlear spiral, enlarged to show the organ of Corti. 



rate and shape in the double column of fluid contained in the two outer chambers, 

 play being allowed by the membranous end of the second column at the fenestra 

 rotunda, which moves out when the ovalis moves in and vice versa. Because the 

 partitions between them are only membranes, it is to be expected that the 

 oscillations of the two enclosing columns must affect the contents of the third. 

 It is among these structures that the endings of the auditory nerve are found and 

 we must look here for the essential organ of hearing. 



