AUDITORY TISSUES 221 



Meanwhile the sacculus had evaginated, from its inner epithelial 

 surface, a long tube which has curled into the snail-shell-shaped structure 

 mentioned above. This is the cochlea, whose rudiment is to be seen as a 

 small evagination from the inner surface of the sacculus in the lower 

 vertebrates, which has become elongated for some distance by continued 

 invagination in the birds, where it is straight or partly curved, and which 

 is thus curled up in the mammals on account of its great length and 

 development. 



This spiral, membranous tube does not retain a round shape, nor 

 does it fill the rounded bony cavity which is provided for it in the Guinea 

 pig. As is best seen in a transverse section of one of its coils, it is applied 

 by a third of its circumference to a narrow area of the outer wall of this 

 bony case, and is met by a bony and muscular septum or shelf that reaches 

 out from the inner wall to form a contact with a second third, which thus 

 lies at about right angles to the first mentioned. The last third of the 

 epithelium-bearing, membranous tube is stretched through the cavity 

 as a septum which divides the triangular interior of the membranous 

 tube from the large part of the bony tube which the membranous cochlea 

 occupies. 



In section, then, the membranous cochlea is, roughly, an equal-sided 

 triangle with one side applied to the wall of the bony tube, one side to the 

 septum that divides the bony tube into two parts, and the other stretched 

 across the upper division of the bony tube. On the first and third of these 

 the epithelium is of no further interest to us, and we shall study that part 

 which rests on the septum, for it is here that the auditory apparatus is 

 formed out of the layer of epithelial cells. The apparatus forms a band 

 which is cut several times in its spiral course by a median section through 

 the long axis of the cochlea. 



Figure 198 represents this basal section of the epithelium of the 

 membranous tube, as cut in the second spiral of a young Guinea pig's 

 cochlea. The apparatus is called the organ of Corti. 



Beginning from right to left, we find that the simple epithelium is 

 cuboidal where it first appears on the septum. These cells have been 

 called the cells of Claudius. They rise up against one of ^their neigh- 

 bors which is grown to five or six times their height and has acquired a 

 pointed end. This cell is known as H emeu's cell, and while but one of 

 them, representing a single row, is seen in the specimen from which 

 our drawing is taken, they form a double row of cells in the Guinea pig 

 and several rows in extent in the cat and some other mammals. 



The next six cells (representing six rows) are of two kinds placed 

 alternately. Three of them, including the first, are tall supporting cells 

 with narrow upper bodies that expand at the tip into plates. These 

 plates reach from one cell to the other and to the tip of the Hensen cells 



