1342 GENERAL STRUCTURE OF COCHLEA. [BOOK in. 



These two spaces moreover are each entire, not being broken up 

 or subdivided in any way by bridles or bands of connective tissue. 

 Such an arrangement obtains all the way along the successive 

 whorls except at the extreme top and extreme bottom. The 

 projecting sheet of bone, as it is traced from the bottom to the 

 top, describes a spiral, and hence is called the lamina spiralis. 

 The canalis cochlearis also describes a spiral, winds in fact like a 

 turret staircase, as do as well the two perilymph spaces, and the 

 latter are hence called scalae. The one lying above the canalis 

 cochlea, when followed to the bottom of the cochlea is found to be 

 continuous with, to open freely into, the perilymph space of the 

 vestibule ; hence it is called scala vestibidi (Sc. V.). The one lying 

 below the canalis cochlearis ends blindly at the bottom of the 

 cochlea, but in the bony wall of its blind end is an orifice, which 

 we have already spoken of as the fenestra rotunda, the membrane 

 covering which shuts off the scala in question from the cavity of the 

 tympanum ; hence this scala is called the scala tympani (Sc. T.). 

 The canalis cochlearis thus lying between these two scalse is 

 sometimes called the scala media; but this name is undesirable 

 since the canalis cochlearis being a part of the membranous 

 labyrinth, a derivative of the otic vesicle, differs essentially in 

 nature from the two scalse, which are merely lymphatic, perilymph 

 spaces. 



The whole tube of the cochlea diminishes in size from the 

 bottom of the lowermost whorl to the top of the highest ; but 

 the diminution affects the two scalse alone, and the scala tympani 

 more rapidly than the scala vestibuli ; the canalis cochlearis so far 

 from growing less, increases, except at the very top, from below 

 upwards and especially, as we shall see in the dimensions of one 

 of its sides. At the top the lamina spiralis comes to an end, 

 finishing off in the form of a hook, hamulus, and the canalis 

 cochlearis suddenly diminishing ends blindly. Beyond the tip of 

 the canalis cochlearis, the scala vestibuli which formed its roof, 

 becomes, by a round orifice, helicotrema, continuous with the end 

 of the scala tympani which formed its floor ; here, and here alone 

 does the one scala open into the other ; and by this connection only 

 has the fluid in the scala tympani access to the scala vestibuli and 

 so to the perilymph space surrounding the vestibular portion of 

 the labyrinth. The pulse which each thrust of the stapes at the 

 fenestra ovalis imparts to the perilymph of the vestibule at the 

 cisterna, passes into the scala vestibuli, and must either be trans- 

 mitted to the scala tympani across the canalis cochlearis or must 

 travel along the scala vestibuli to the apex of the cochlea, and 

 down the whole length of the scala tympani before it breaks on 

 the membrane of the fenestra rotunda. 



If then the bony tube of the cochlea were unrolled and made 

 straight it would appear as a tube diminishing to a pointed end ; 

 the lamina spiralis would appear as a longitudinal plate running 



