THE INTERNAL EAR. 489 



membrane the surface toward the cochlear duct is known as the 

 cochlear surface, that toward the scala tympani as the tympanic 

 surface. Two layers are differentiated in the basilar membrane, 

 the lamina basilaris propria and the tympanic investing layer. The 

 lamina propria consists, in turn, of (i) radially arranged basilar 

 fibers, or acoustic strings ; (2) two thin strata of a homogeneous 

 substance, one above and the other below the layer of basilar fibers, 

 the upper of which is the thicker and nucleated ; and (3) a fine cuti- 

 cula, of epithelial origin, lying on the cochlear side. The tympanic 

 investing layer is highly developed in youth, but later becomes 

 thinner, and may then be differentiated into a connective-tissue 

 layer, regarded as a periosteal continuation of the tympanic por- 

 tion of the osseous lamina spiralis, and an endothelial cell layer 

 belonging to the lining of the perilymphatic space or the scala 

 tympani. In the vicinity of the labium tympanicum is a blood- 

 vessel situated within the tympanic investing layer of the basilar 

 membrane the vas spirale. 



Reissner's membrane consists of an exceedingly thin connective- 

 tissue lamella, lined on the side of the cochlear duct by a layer of 

 flattened epithelial cells and on the vestibular side by a layer of 

 endothelial cells. The epithelium lining the cochlear duct is occa- 

 sionally raised into small villus-like projections. 



The Organ of Corti. In the region of the labium tympan- 

 icum of the limbus spiralis and in the greater portion of the 

 adjoining basilar membrane, the epithelium of the cochlear duct is 

 peculiarly modified, forming here a neuro-epithelium, which receives 

 the terminal ramifications of the cochlear nerve and is known as the 

 spiral organ of Corti. 



Passing from the labium tympanicum to the ligamentum spirale, 

 the following three regions may be recognized in the organ of 

 Corti : An inner region, composed of the inner sustentacular cells 

 and the inner auditory cells ; a middle region, consisting of the 

 arches of Corti ; and an outer region, in which are found the outer 

 auditory cells and the outer sustentacular cells or Deiters's cells. 

 Two cuticular membranes are in close relationship to the organ of 

 Corti : namely, the lamina reticidaris and the membrana tectoria, or 

 membrane of Corti. 



In figure 374, a sketch of the organ of Corti and adjacent 

 structures, it may be observed that the epithelium lining the sulcus 

 spiralis internus (at the right of the figure) is of the pavement 

 variety, and that the epithelium becomes gradually thicker until the 

 organ of Corti is reached, where it becomes suddenly elevated in 

 the form of a wall. In this, two varieties of cells are distinguished 

 sustentacular cells and inner auditory cells. The sustentacular 

 cells, which follow the flattened cells, become gradually higher 

 from within outward and occupy three or four rows. Next come 

 the inner auditory cells, cylindric elements, somewhat rounded and 



