THE SPIRAL LAMINA. 



369 



Fig. 186. 



Interior of the cochlea. 



Fig. 1ST. 



Section of the cochlea. 



its two scalar, might be regarded as a spiral process of the modiolus. 

 Through the modiolus and its spiral process, or lamina spiralis, the au- 

 ditory nerve gains access, through suitable channels, to the interior of 

 the scalas. 



Fig. 186, interior of the cochlea, rendered 

 visible by the removal of half of the outer wall: 



a, a, lamina spiralis, turning by its inner edge, 



b, around the axis of the cochlea ; <?, infundib- 

 ulum ; d, aperture of communication between 

 two scalaB; e, e, section of the outer wall; 

 fi f-> ft inferior or tympanic scala ; <?, g, g^ su- 

 perior or vestibular scala. 



Fig. 187, section of the cochlea in the direc- 

 tion of its axis : #, canals of the axis, or of 

 the columella, for the passage of the vascular 

 and nervous ramifications; , infundibulum ; 

 c, base of the modiolus, or columella ; d, d, d, 

 section of spiral lamina ; 6, e, 6, section of the 

 outer wall; f,f,fi inferior scala; <7, <?, superior 

 scala. 



It is necessary to understand the structure of the lamina spiralis more 

 particularly. As we have said, it divides the helical tube into The spiral 

 the two scake by extending transversely across it. Its bony por- lamina. 

 tion does not, however, extend more than about one third of the distance, 

 the rest of it being made up in part of a delicate membranous portion, 

 and completed by a muscular structure ; so that, if we consider the lam- 

 ina spiralis at any point, the region of it near the modiolus is bone, the 

 intermediate portion membranous, and the residual is muscular. Or, 

 considering the lamina spiralis in the aggregate, we might say that it 

 consists of a helix of bone, membrane, and muscle. To the muscle the 

 name of the cochlearis is given. Its obvious function is to tighten the 

 membranous region. Moreover, considered thus in the aggregate, the 

 lamina spiralis is a triangular plate wound round upon a central conical 

 core, and which, therefore, is broadest at the base of the cochlea, and 

 gradually tapers off toward the apex. It is to be understood that the 

 cochlea, like all other portions of the labyrinth, is filled with water. 



Upon the spiral lamina, issuing forth through its bony portion, are 

 placed the ultimate filaments of the auditory nerve. These, having cast 

 off their white substance r come into relation with elongated vesicles, 

 and are thus distributed upon the membranous portion, the membrane 

 being kept uniformly tense by the action of the cochlearis muscle. 



Fig. 188, p.. 370, section of the cochlea through its axis, magnified four 

 diameters, and showing the cochlear branch of the auditory nerve, ac- 



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