CHAP, iv.] HEARING. 981 



skin covering the head ; for a time the epithelium of the vesicle 

 is continuous with the epidermis of the skin, and wholly uncon- 

 nected with the developing brain ; later on the epithelial invo- 

 lution separates from the skin, becomes a closed independent 

 vesicle, and makes connections with the brain through the audi- 

 tory nerve growing out to meet it. The otic vesicle therefore is 

 not like the optic vesicle a part of the brain, and we find accord- 

 ingly the structure of the auditory epithelium much more simple 

 than that of the retina ; it corresponds to a part only of the 

 retina, to the more external layers of the retina, not to all of 

 them. 



We have seen that the optic fibres are connected with a part 

 only of the optic vesicle, with the anterior wall only of the re- 

 tinal cup and not with the whole of this ; the part of the anterior 

 wall which forms the pars ciliaris retinae and the whole of the 

 posterior wall make no connections with the optic fibres and 

 remain in the form of a relatively simple epithelium. The con- 

 nection of the auditory nerve with the walls of the otic vesicle 

 is still more partial ; the nerve fibres become connected with the 

 epithelium in a few limited areas. It is only in these areas that 

 the epithelium lining the otic vesicle becomes differentiated 

 into the special auditory epithelium; elsewhere it possesses 

 relatively simple characters. 



The cavity of the optic vesicle is, as we have seen, soon 

 obliterated by the coming together of the anterior and posterior 

 walls. The cavity of the otic vesicle is permanent, growing 

 with the growth of the organ and becoming filled with a peculiar 

 fluid secreted by the walls, called endolymph. The vesicle as it 

 grows soon loses its early simple, more or less spherical form 

 and assumes a most complicated shape, becoming divided into 

 the parts known as the utricle with the semicircular canals, the 

 saccule, and the canalis cochlearis ; of these we shall speak in 

 detail later on. 



611. While the vesicle is assuming this complicated shape, 

 the mesoblastic tissue investing it undergoes a differentiation. 

 The tissue immediately in contact with the epithelium becomes 

 connective tissue serving as a dermis to support the epithelium, 

 so that the vesicle becomes a (complicated) sac with membranous 

 walls lined with epithelium specially modified into auditory 

 epithelium at particular places, at which places and at which 

 places alone, the auditory nerve makes connections with the 

 walls. 



The outer portion of the mesoblastic tissue is converted into 

 bone of a somewhat dense character, and thus furnishes a bony 

 shell or envelope enclosing and to a large extent following the 

 contour of the complicated membranous sac. Between the 

 outer bony envelope and the inner membranous sac is developed 

 a large irregular lymphatic space which (Fig. 166) follows to 



