DEVELOPMENT OF THE EAR. 



769 



vesicle by bending in of its walls under it, and thus the elevation is converted into 

 a tube open at each end, which subsequently becomes elongated and presents an 

 ampullar dilatation. The cartilage which forms the osseous labyrinth is continuous 

 with that of the rest of the primordial cranium. The cartilaginous walls of the 

 cavity are united by connective tissue to the vesicle : this connective tissue, according 

 to Kolliker, becomes divided into three layers, of which the outer forms the lining 

 periosteum, the inner forms the external walls of the membranous labyrinth, while 

 the intervening layer swells up into gelatinous tissue, the meshes of which become 

 wider and wider, till at last the space is left which ultimately is found containing 

 perilymph. 



Fig. 525. 



Fig. 525. LABYRINTH OF THE HUMAN FCETUS OP FOUR 

 WEEKS, MAGNIFIED (from Kolliker). 



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

 r v, recessus vestibuli, giving rise later to the aqueduct ; 

 c s, commencement of the semicircular canals ; a, 

 upper dilatation, belonging perhaps to another semi- 

 circular canal ; c, cochlea. 



The cochlea appears at first as a prolongation 

 downwards from the auditory vesicle, but afterwards 

 become tilted forwards. This prolongation of the 

 auditory vesicle is the rudimentary canalis mem- 

 branacea. Close to it is placed the cochlear nerve, 



with a gangliform extremity. The canal becomes elongated in a spiral direction, and 

 the ganglion, which is elongated with it, becomes the ganglion spirale. Between the 

 canal and the cartilaginous wall which afterwards surrounds it a large amount of con- 

 nective tissue intervenes, and in this the cavities of the scala vestibuli and scala tym- 

 pani appear at a later period, precisely as does the space for the perilymph in the 



Fig. 526. 



Fig. 526. TRANSVERSE SECTION OF 

 THE COCHLEA IN A FCETAL CALF, 

 MAGNIFIED (from Kolliker). 



C, the wall of the cochlea, still carti- 

 laginous ; c c, canalis cochleae ; I s, 

 placed in the tissue occupying the 

 place of the scala vestibuli indicates 

 the lamina spiralis ; n, the central 

 cochlear nerve ; g, the place of the 

 spiral ganglion ; S, the body of the 

 sphenoid ; c k, chorda dorsalis. 



vestibule. The modiolus and spiral 

 lamina, according to Kolliker, are 

 ossified without intervention of car- 

 tilage. Within the canalis mem- 

 branacea Kolliker finds in the em- 

 bryo a continuous epithelial lining, 



thin on the membrane of Reissner and on the outer wall, but forming a thick eleva- 

 tion in the position of the rods of Corti, and a larger elevation more internally, filling 

 up the sulcus spiralis. On the surface of this latter elevation he observes a transpa- 

 rent body, the membrane of Corti. 



With regard to the middle and external ear, it has been already explained at pages 

 65 and 66 that the external aperture, the tympanic cavity, and the Eustachian tube, 

 are formed in the posterior or upper part of the first branchial cleft, which remains 

 open except at the place where the passage is interrupted by the formation of the 

 membrana tympani ; and also that the incus and malleus are formed in the first 

 branchial lappet from the proximal part of Meckel's cartilage, and the stapes and 

 stapedius muscle and the styloid process in the second lappet. It is pointed out by 



3 E 



