THE SENSE OF HEARING. 377 



thin, free edge of the bony lamina spiralis is, in the recent state, thickened bv 

 a development of connective tissue forming a promontory known as the limbics. 

 The free edge of the limbics is in turn shaped in such a way as to make a short, 

 sharp projection in the plane of the upper surface of the lamina and a longer 

 projection in the plane of its lower surface, leaving the free margin between 

 them hollowed out. The upper projection, which is known as the vestibular 

 lip, labium vestibvlare i serves for the attachment of the tectorial membrane, 

 membrana tedoria, presently to be described. The lower projection is called 

 the tympanic lip (labium tympanicum) ; to it is attached the inner margin of 

 the basilar membrane, on whose inner half is seated the very complex struct- 

 ure known as the organ of Corti. 



The basilar membrane is a thin sheet of fibrillated connective tissue stretched 

 tightly between the tympanic lip of the limbns on the inside and the spiral 

 ligament (see p. 379) on the outside. The more median part of the membrane, 

 which supports the organ of Corti, is thin and rigid and is fibrillated in a 

 radial direction. The outer part, which is first thicker and then thinner again 

 near its point of attachment, is distinctly composed of radial fibres cemented 

 together ; the isolated fibres are characterized by being stiff and brittle. 



The organ of Corti (Figs. 193, 194) has as its supporting basis a series of 

 peculiarly modified epithelial cells, known as the rods of Cord (Fig. 195, B, r.'), 

 which are disposed along the edge of the spiral lamina in two rows, an inner 

 and an outer. The inner rods have their feet on the basilar membrane near its 

 median attachment; they lean outward and upward, and at their upper extrem- 

 ity join or articulate with the heads of the outer rods, whose feet are fastened to 

 the basilar membrane more externally. The two rows of rods are thus joined 

 together like the rafters of a house, and enclose beneath them a canal known 

 as the tunnel of the organ of Corti. The inner rods are more numerous than 

 the outer, so that the latter are fastened rather between than to the ends of the 

 former. Leaning against the inner or median side of the inner row of rods 

 is a single row of hair-cells (Fig. 19 1), much like those described as seated on 

 the maculae and crista? of the labyriuth, to which hair-cells filaments of the 

 auditory nerve are distributed. Closely applied to the .-ingle row of hair- 

 cells, on the inner side, are several rows of columnar cells gradually decreas- 

 ing in size toward the median line, and beneath the whole is a group of nuclei. 

 External to the outer row of rods, and separated from it by a space, are four 

 parallel rows of hair-cells known as the cells of Corti; their bodies do not 

 reach downward as far as the basilar membrane, and jusi below each row is a 

 bundle of nerve-fibres which have traversed the tunnel of Corti and then have 

 changed their direction from a radial t<» a longitudinal or spiral one. These 

 fibres, and others having a more direci course, one by one end in clusters 

 encircling the individual hair-cells. 



Four rows of peculiarly-modified columnar cells, the cells of Deiters, are 

 inserted closely between the cells of Corti, the OUtermosI row being external 

 to the fourth row of Corti. These cells rest below on the basilar membrane. 

 Still external to these groups of cells is a series of rows of tall columnar cells 



