664 MANUAL OF HISTOLOGY. 



be observed is that bundles of fibres, made up of medullated tubes 

 0'0031 mm. in thickness, radiate from the modiolus into the lamina 

 spiralis ossea, forming a dense plexus in its complicated passages. At 

 one particular spot, namely, at the point of exit from the bony portion of 

 the spiral plate, there is imbedded (as was first observed by Corti} in the 

 course of the primitive fibres a ganglion cell (ganglion spirale of Corti}. 

 Preserving their plexiform arrangement, the nerve fibres still pursue a 

 course outwards, passing eventually through the apertures in the habe- 

 nula, after having dwindled down to non-medullated axis cylinders. 

 Arrived now in the canalis cochlearis they present themselves as extremely 

 delicate pale fibrillse. 



From this on two different sets of fibres may be distinguished, namely 

 (a}, one for the internal, and (b) one for the external hair cells. 



The axis cylinders of the internal set (0'0015-0'002 mm. thick) are 

 supposed to penetrate into the apices of the internal hair cells (fig. 608). 

 The external, which are much finer, to pass across the " tunnel " of the 

 organ of Corti, about midway between floor and roof (Gottstein, Wal- 

 deyer) to unite with the external hair cells (w). These views are, how- 

 ever, still but very feebly supported. 



While we leave the greater part of the development of the organs of 

 hearing to the special works on embryology, we may here mention a few 

 general facts on the subject in conclusion. 



The labyrinth makes its first appearance as a vesicular structure, known 

 as the labyrinthine or auditory vesicle, a multilaminar infolding of the 

 corneous layer (Remak), which subsequently receives from the middle 

 germinal plate a fibrous, and then over that a cartilaginous covering in 

 the form of a capsule. 



The semicircular canals and the canalis cochlearis are then formed 

 from the auditory vesicle as secondary outgrowths. 



The canalis cochlearis, at first only a slight eminence, grows out into a 

 curving horn, which acquires in its further development the spiral pas- 

 sage (Koelliker). The two scalar, that of the tympanum and that of the 

 vestibule, are tertiary formations produced by the liquefaction of the con- 

 nective-tissue adjoining the canalis cochlearis. 



