374 ELEMENTARY PHYSIOLOGY less. 



each other, except at the end of the case to which the 

 ruler does not reach. In a similar way, the cochlear tube, 

 containing endolymph, divides the cavity containing 

 perilymph, in which it lies, into two passages, called 

 scalas, which are seen in section (Fig. 120) to be placed 

 one above and the other below the triangular cavity of 

 the cochlear tube itself, and which communicate with 

 each other at the far end of the cochlear tube, but not 

 elsewhere. 



In one point, however, the comparison with the ruler 

 and its case is not exact. The cochlear tube is not nearly 

 so wide as the containing cavity ; and the sharp edge 

 opposite the convex adherent face would not be in direct 

 connexion with the bony walls, were it not for a bony ledge 

 which, projecting from the bony walls towards the thin 

 edge of the cochlear tube, is united to it by membrane 

 and thus forms a partition or septum, which separates 

 the two scalse in the region where the cochlear tube 

 itself would otherwise leave a communication between 

 them. 



As has been already stated the cochlear tube is not 

 straight or even simply curved, but is twisted up on itself 

 into a spiral of two and a half turns. In these twists it 

 is accompanied by the cavities above and below it, and 

 also by the septum spoken of above, which thus takes a 

 spiral course, and is spoken of as the lamina spiralis 

 (Figs. 120, L.S, 121, l.s). The whole arrangement some- 

 what resembles the shell of a snail ; hence the name. All 

 along the spiral the edge of the cochlear tube attached to 

 the lamina spiralis is directed inwards and the convex 

 face outwards ; so that when a section is made through 

 the axis of the spiral a succession of rounded spaces are 

 cut through, each space exhibiting, above and below, the 

 somewhat half-moon-shaped section of a scala, the two 

 scalse being separated on tlie outer side, by the cochlear 

 tube, and, on the inner, by the lamina spiralis (Fig. 120). 



The triangular cavity which, as we have seen, contains 



