thp: cochlea 373 



It is divided roughly into two compartments, the utricle 

 and the saccule ; with tlie former is connected a system 

 of tubes known as the semicircular canals, with the 

 latter is connected a single tube which is coiled in the 

 shape of a helix. This tube is the canalis cochlearis, and 

 with the bony cavity in which it is enclosed, forms the 

 cochlea, the essential organ of hearing. 



(vi) The Cochlea. — Connected with the saccule by a 

 narrow canal is an extension of the original membranous 

 sac, in the form of a long tube closed at the end (Fig. 124, 

 Coch.). This cochlear tube is lined with epithelium, 

 contains endolymph, and is lodged in a bony cavity filled 

 with perilymph. The canalis cochlearis is much smaller in 

 section than the bony cavity in which it lies. The amount 

 of perilymph surrounding it is therefore considerable. In 

 the cochlea the contour of the cochlear tube is, along its 

 whole length, totally different from that of the containing 

 cavity ; for, in transverse section, while the contour of the 

 containing cavity is almost circular, that of the cochlear 

 tube itself is nearly triangular. The cochlear tube in fact is, 

 in shape, what is often called triangular (as when we speak 

 of a triangular file), but should be called trihedral ; that 

 is to say it has three sides or faces (and three edges) ; 

 one of the sides is however not flat but convex, i.e., bulges 

 somewhat outwards. 



The cochlear tube, or canalis cochlearis, closely adheres 

 to the bony wall, along the whole length of the tube, in 

 two regions, namely, over the whole of that face of the 

 trihedral tube which has just been described as being 

 convex, and at the edge opposite. Take a round ruler, 

 make a paper case which just fits it, and close the case at 

 one end. Then pare down the ruler on two sides until 

 it has two flat faces meeting at an edge, and slide it into 

 the case, so that it does not quite reach the closed end. 

 The ruler, if it were hollow, would represent the cochlear 

 tube ; and it will be observed that it divides the cavity of 

 the case into two passages, which are quite distinct from 



