THE EAR. 



771 



The membranous zone, having a constant width of 0-2 of a line, is 



Fig. 310. 



Fig. 309. subdivided into a zona denticulata 



and a zona pectinata, the former 

 of which constitutes about the 

 inner two-thirds, and the latter 

 the outer third of the breadth of 

 the membranous lamina, and are 

 both characterized by a great 

 complexity of structure, the im- 

 portance of which was first, in 

 more recent times, pointed out 

 by Corti (I c.) (vid. Figs. 309, 

 310). 



1. The zona denticulata (d-v) 

 may be again subdivided into two 

 portions, an internal, the habe- 

 nula interna s. sulcata (d-g), and 

 an external, the habenula externa 

 s. denticulata (h-t). The former 

 is developed at d, as an immediate 

 continuation of the periosteum of 

 the lamina spiralis ossea, and, in 

 fact, only from that portion of it 

 which looks towards the scala 

 vestibuli, increasing in width and 

 thickness from the commence- 

 ment to the termination of the 

 cochlear canal. In the first and 

 second turns of the cochlea, its 

 under surface supplies the place 

 of the periosteum upon the most 

 external portion of the osseous 



Fia. 309. Vertical section of the lamina spiralis, at 6 lines from its commencement, mag- 

 nified about 225 diameters (Cat or Dog) ; the epithelial layer investing its upper and lower 

 surfaces is removed : a, periosteum of the osseous zone ; 6, the two lamellae of the osseous 

 zone near its free border; c, cf, (?', termination of the auditory nerves; d-w, lamina spiralis 

 mcmbranacea d-w, zona denticulata; d-d f -f, habenula sulcata] d, place where the periosteum 

 is thickened ; e, granules in the groove of the habenula sulcata ; f-g, " teeth of the first series ;" 

 g-f-h, sulcus s. semicanalis spiralis ; A, its inferior wall: k, epithelial cells at the entrance of 

 the sulcus spiralis ; h-iv', habenula denticulata ; h-m, apparent teeth ; n-t, " teeth of the second 

 series;" n-p, their posterior joint ; o, nucleated enlargement upon them; p-q and q-r, articular 

 portions; r-t, anterior joint of the second row; s s s, three cylinder-cells, placed upon them; 

 l-v, membrane covering the habenula denticulata; u, one of the epithelial cells underneath; 

 7I/-M), zona pectinata ; :r, periosteum, attaching the lamina spiralis (muse, cochlearis, Todd and 

 Bowman) ; y, vas spirale internum ; z, its inner coat. After Corti. 



FIG. 310. Vestibular surface of the lamina spiralis membranacea, magnified 225 diameters. 

 The letters are in part the same as in Fig. 309 : a a, cylindrical elevations of the habenula 



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