1340 THE MEMBRANOUS LABYRINTH. [BOOK in. 



forked or horse-shoe shaped ridge placed athwart the long axis of 

 the ampulla and projecting into the interior ; it is called a crista 

 acustica (Figs. 175 cr.p., cr.s., cr.h., 176). Hence the vestibular nerve 

 ends exclusively in the macula acustica of the utricle, the macula 

 acustica of the saccule and the crista acustica of each of the three 

 ampullae. The superior branch before it ends in the utricle and 

 superior canal bears a ganglion of nerve cells, and the division 

 which the same branch gives off to the horizontal canal also bears 

 a group of nerve cells just before it joins its crista. The median 

 branch to the saccule and posterior canal likewise bears a ganglion 

 which is more or less continuous with the ganglion of the superior 

 branch. 



824. The cochlea may be considered as a prolongation of 

 the vestibule in the form of an elongated cone ; and indeed in some 

 of the lower animals, in birds for instance, it is a short blunt cone. 

 But it differs very widely from the rest of the labyrinth ; and its 

 special features may be broadly considered under three heads. 



In the first place, the elongated, almost tubular, bony cone is 

 not straight, but twisted closely on itself in two and a half whorls 

 (Fig. 163), and the whorls grow together so as to form a short 

 cone, the markings of the whorls being visible on the outside after 

 the fashion of a gasteropod shell ; hence the name. In the natural 

 position in the head the cochlea is nearly horizontal, with the 

 beginning of the first whorl at the base abutting on the median 

 wall of the tympanum and with the apex directed forwards, and to- 

 wards the median line ; but when we are dealing with it by itself 

 it will be convenient to consider it as if it were vertical in position 

 with the apex above and the base below. The axis, or ' modiolus ' 

 as it has been called, round which the whorls are coiled differs from 

 the walls of the whorls themselves in being formed of spongy, not 

 compact bone, and is traversed by canals for the passage of the 

 cochlear nerve, which entering it at the base from the meatus 

 interims, ascends along it to the apex giving off fibres as we shall 

 see all the way along. 



In the second place, in the vestibule and semicircular canals 

 the membranous labyrinth hangs, for the most part, loose within its 

 bony shell, supported by irregular bridles, or is so attached that in 

 any case there is no very definite arrangement of the perilymph 

 space in relation to the sac which it surrounds ; in the cochlea on 

 the contrary a very definite arrangement obtains. This is best seen 

 in a vertical section of the cochlea, in which the whorls are cut 

 transversely in succession. The whole lumen of the coiled bony 

 tube (Fig. 177) is seen to be roughly circular in section. Within 

 this lies the canalis cochlearis (C. Chi.}, the tubular continuation of 

 the membranous labyrinth. This however is not, as in the semi- 

 circular canals round or oval, but triangular in section. It is 

 moreover so placed that while the base of the triangle is firmly 

 attached to the outer wall of the bony tube, no perilymph space lying 



