CHAP, iv.] HEARING. 211 



may however occasionally be found in the ampullae, or even in the 

 perilymph chambers of the cochlea. 



The Cochlea. 



830. As we have seen, the canalis cochlearis is a long spiral 

 tube triangular in section, the apex of the triangle being attached 

 to the edge of the spiral lamina, and the base to the opposite wall 

 of the bony canal. In a dried specimen the bony spiral lamina 

 ends in a thin edge, but in the fresh state the edge is thickened 

 by connective tissue into a projection of peculiar form called 

 the limbus (Fig. 177 lb.), which presents two edges, placed one 

 above the other and seen in vertical section as two lips separated 

 by a groove. The upper lip, which when looked at lengthways 

 is seen to end in a number of projections or teeth, " auditory 

 teeth," is called the vestibular lip, labium vestibulare (Fig. 177 

 Lv., 179 Lv.), the lower lip is called the tympanic lip, labium 

 tympanicum (177 Lt., 179 l.t.\ and the groove between them is 

 called the spiral groove, sulcus spiralis. The vestibular lip and 

 upper portion of the limbus is composed of a somewhat peculiar 

 connective tissue, consisting of a homogeneous matrix in which 

 are imbedded corpuscles ; this is covered, except over the auditory 

 teeth themselves, by a thin layer of flat epithelial cells, and deeper 

 down passes into the bony tissue of the lamina. The vestibular 

 lip serves for the attachment of the structure known as the 

 tectorial membrane, membrana tectoria (Figs. 177, 179 m.t.). The 

 tympanic lip, jutting farther outwards than does the upper lip, is 

 the more direct continuation of the bony spiral lamina, and ends 

 in an even edge composed of connective tissue which serves for 

 the attachment of the basilar membrane (Figs. 177, 179 m.b). 



The membrane of Eeissner, stretching across from the limbus 

 of the spiral lamina to the opposite wall and so forming the 

 vestibular wall of the canalis cochlearis (Fig. 177 m.E.), is a thin 

 membrane, the basis of which is a sheet of homogeneous or 

 obscurely fibrillated connective tissue, continuous on the inner, 

 median side with the connective tissue of the limbus and on the 

 opposite side with the periosteum of the bony shell of the cochlea. 

 On the side looking towards the scala vestibuli this basis is 

 covered with a single layer of lymphatic epithelioid plates; on 

 the opposite side, in the cavity of the canalis cochlearis, it is 

 covered with a single layer of flat polygonal cells, similar to 

 those lining the non-auditory part of the vestibule, and like them 

 presenting minor differences between themselves, some cells being 

 more granular than others. 



The periosteum which lines the whole of the bony canal of the 

 cochlea, and which over the limbus may be supposed to be repre- 

 sented by the peculiar connective tissue spoken of above, is on 



