146 



EAR 



upper and inner angle of the vesicle, and is known as the recess of the labyrinth 

 (fig. 186, r.L). Eventually it develops into a long epithelial tube, which passes 



p.s.c. 



p.s.c. 



FIG. 187. MODELS OP THE MEMBRANOUS LABYRINTH : 

 AND B OF AN EMBRYO OF 13 MM. 



sacc, saccule; cc, cochlea; r.L, recessus labyrinth!; a.s.c. 

 canal. The future lateral semicircular canal is represented by 

 sacc ; vr vestibular, cr cochlear division of auditory nerve. 



A OF A HUMAN EMBRYO OF 11 MM., 



(After Streeter.) 



superior, p.s.c. posterior semicircular 



the fold projecting outwards above 



p.s.c. 



through the petrous bone, with an expanded end lying within the skull under- 

 neath the dura mater. This tube and its expanded termination form respectively 

 the endolymph canal and endolymph saccule (fig. 189). 



In the meantime the audi- 

 tory vesicle becomes elongated 

 and begins to be irregular. 

 It shows a larger triangular 

 swelling in its dorsal part to 

 which the endolymph canal 

 is attached, and a smaller 

 flattened sac which is the 

 rudiment of the epithelial 

 canal of the cochlea. At the 

 junction between the two 

 moieties a bulging, described 

 by Denis and named atrium 

 by Streeter, 1 constitutes the 

 rudiment of the utricle and 

 saccule. The dorsal or vesti- 

 bular pouch soon shows a 

 vertical and a little later a 

 horizontal fold (fig. 186) ; the 

 former is the rudiment of the 

 two vertical (anterior and 

 posterior) semicircular canals 

 and their common opening 

 (crus), the latter the rudiment 

 of the horizontal (external) 

 canal. The folds which give rise to the canals are flattened semicircular 

 hollow protrusions from the wall of the vesicle. The layers of the folds next 



1 Amer. Journ. of Anat. vi. 1907. 



FIG. 188. MODEL OF THE MEMBRANOUS LABYRINTH OF A 

 HUMAN EMBRYO OF 20 MM. (After Streeter.) 



u, utricle; s, saccule; cc, cochlea; r.L recessus labyriiithi ; 

 a.s.c. superior, p.s.c. posterior semicircular canal ; vr ves- 

 tibular, cr cochlear division of auditory nerve. 



