DEVELOPMENT OF THE EAR. 



709 



Fig. 570. 



Fig. 570. — Transverse and slightly Oblique Section of the Head op a Fietal 

 iShkep, in the Eegion op the Hind Erain (from Foster and Balfour after 

 Boettclier). 



HD, inner surface of the tliickened walls of the hind brain ; RL, recessus vestilnili ; 

 VB, commencing vertical semicircular canal ; CC, canalis cochlew, Avith the cavity of the 

 primitive otic vesicle. On the left side parts only of these structures are seen ; GC, 

 cochlear ganglion of the right side ; on the left side, Gr', the ganglion, and N, the 

 auJitoiy nerve connected with the hind brain. 



the otic vesicle forms at first for a time a follicle open to the surface, 

 ami that it has therefore no original connection with the nervous 

 centre. Its position is at the side of the medulla oblongata, and in a 

 l)lace opposite to the interval between the first and second postoral 

 visceral arches. The outer opening of the follicle very soon contracts 



Fig. 571. 



Fig. 571. — Labyrinth of tee Human Fcetus of 

 FOUR aveeks, magnified (from Kolliker). 



A, from behind ; B, from before ; v, the vestibule ; 

 rv, recessus vestibuli, giving rise later to the aqueduct ; 

 r.s', commencement of the semicii'cular canals ; a, upper 

 dilatation, belonging perhaps to another semicircular 

 canal : c, cochlea. 



and becomes entirely closed. The follicle 



sinks down towards the basis of the cranium, 



and becomes imbedded in the formative 



mesoblastic tissue lying between the basi- 



occipital and alisphenoid matrices, undergoing chondrification and 



ossification at a very early period, as has been already described under 



the development of the head. 



There are therefore to be distinguished from an early period a part 

 corresponding to the internal membranous labyrinth proceeding from 



VOL. II. 3 D 



