DEVELOPMENT OF THE EAR. 



91 



Other hollow projections appear near the dorsal end of the vesicle ; those form 

 the two superior semicircular canals ; the horizontal canal appears a little later. 



The mode of formation of the canals is somewhat peculiar. They first appear as flattened 

 semicircular hollow protrusions of the wall of the vesicle. Their sides then come together 

 and coalesce, except near the circumference of the semicircle, which now forms a tube con- 

 nected at both ends with the vesicle. Subsequently a separation or breach of continuity 

 occurs over the area of coalescence, so that the rest cf the tube is free. One of the ends 

 becomes dilated into an ampulla and connected with a branch of the auditory nerve. 



Whilst these processes are occurring at the dorsal and ventral ends of the now 

 elongated vesicle, a fold, or constriction, of the wall is beginning to make its 

 appearance about the middle, and thus the posterior part which is connected with the 

 semicircular canals becomes gradually separated (as the utricle) from the anterior 

 part, which forms the saccule, and is connected with the cochlea. This fold extends 

 into the beginning of the recess of the labyrinth, and separates it longitudinally fcr a 



Fig. 106. STAGES IN THE DEVELOPMENT OP THE MEMBRANOUS LABYRINTH. (W. His, jun.) 



A. Left labyrinth of a human embryo of about four weeks, viewed from the outer side, r, vestibu- 

 lar part ; c, cochlear part ; r.l, recessus labyrinthi (aquaeductus vestibuli). 



B. Left labyrinth with parts of the facial and auditory nerves of a human embryo of about 4^ 

 weeks, b.b, surface of the hind brain; u, utricular ; s, saccular part of labyrinth ; a.s.c. , p.s.c,, 

 e.s c., rudimentary folds representing the two vertical and the horizontal semicircular canals ; r.l, upper 

 part of recessus labyrinthi becoming enlarged into the endolymphatic saccule ; c.c, rudiment of cochlea ; 

 n.v, vestibular branch of auditory nerve ; g.v, vestibular ganglion (ganglion of Scarpa) ; g.c, cochlear 

 ganglion ; n.f, facial nerve, with geuiculate ganglion, g.g, 



C. Left labyrinth of a human embryo of about five weeks, viewed from without and below. Letter- 

 ing as before. The horizontal canal is still only a fold. The ampullae are beginning to be visible on the 

 two vertical canals. 



short distance into two tubes, one of which opens into the utricle, and the other 

 into the saccule, forming the only permanent means of communication between 

 their contents. Another fold, or constriction, appears presently, somewhat lower 

 down, and converts the connection between the saccule and the cochlea rudiment 

 into the narrow duct of Hensen (canalis re-uniens). 



In the meantime the cochlea-rudiment at the ventral end of the now labyrinthic 

 vesicle, becomes elongated into a tube, which, as it grows, becomes coiled upon 

 itself in such a manner as to produce the spiral structure of this part of the auditory 



