376 NORMAL HISTOLOGY. 



The tympanic wall or floor of the cochlear duct (Fig. 423) comprises the basilat 

 membrane, extending from the basilar crest to the outer end of the bony spiral lamina, 

 and the limbus lamincc spiralis, which includes this wall from the attachment of Reiss- 

 ner's membrane to the end of the bony lamina. The limbus or crista spiralis is a 

 thick mass of connective tissue upon the upper surface of the outer end of the osseous 

 lamina spiralis. Its outer extremity is deeply grooved to form a -gutter, the sulcus 

 spiralis internus, the projections of the limbus above and below the sulcus forming 

 respectively its superior (vestibular) and inferior (tympanic) labia. The upper sur- 

 face of the limbus is marked by clefts and furrows which are most conspicuous near 

 the outer margin of the upper lip, where the irregular projections between the furrows 

 form the so-called auditory teeth, because of their fancied resemblance to incisor teeth. 

 The lower lip is continuous externally with the basilar membrane and is perforated 

 near its outer end by some 4000 apertures (foramina nervosa) transmitting minute 

 branches of the cochlear nerve. The epithelium covering the elevated portions of the 

 limbus, including the auditory teeth, is of the flat polyhedral variety, the intervening 

 furrows and clefts being lined by columnar cells. The epithelium of the sulcus spiralis 

 consists of a single layer of low cuboidal or flattened cells, continuous with the epithe- 

 lium of the auditory teeth above and with the highly specialized elements of Corti's 

 organ below. 



The basilar membrane consists of a median (inner) and a lateral (outer) part. 

 The former, known as the zona arcuata, is thin and supports the modified neuroepi- 

 thelium constituting the organ of Corti ; the outer part, named the zona pectinata, is the 

 thicker division and lies external to the foot-plates of the outer rods of Corti. The 

 basilar membrane is made up of three distinct layers the epithelium, the substantia 

 propria, and the tympanic lamella. The substantia propria is formed of an almost 

 homogeneous connective tissue with a few nuclei and fine fibres, which radiate towards 

 the outer edge of the spiral lamina. The fibres of the zona arcuata are very fine and 

 interwoven, appearing to be an extension of those of the lower lip of the limbus, whilst 

 straight and more distinct fibres stretch from the outer rods of Corti to the spiral liga- 

 ment and constitute the so-called auditory strings. According to the estimate of 

 Retzius, there are 24,000 of these special fibres. Their length increases from the base 

 towards the apex of the cochlea, in agreement with the corresponding increase in 

 breadth of the basilar membrane. The tympanic lamella contains numbers of fusi- 

 form cells of immature character interspersed with fibres. In this location the differ- 

 entiation of the mesodermic cells lining the tympanic canal has never advanced to the 

 production of typical endothelial plates, the free surface of the lamella being invested 

 by the short fusiform cells alone. The inner zone of this layer contains capillaries 

 w hich empty into one, or sometimes two, veins, frequently seen under the tunnel of 

 Corti and known as the vas spirale. The epithelium covering the inner zone of the 

 basilar membrane forms the organ of Corti, the highest example of specialization as 

 neuroepithelium. 



The Organ of Corti. The organ of Corti, or organon spirale, consists in a general 

 way of a series of epithelial arches formed by the interlocking of the upper ends of con- 

 verging and greatly modified epithelial cells, the pillars or rods of Corti, upon the 

 inner and outer sides of which rest groups of neuroepithelial elements the auditory 

 and the sustentacular cells. The triangular space included between the converging 

 pillars of Corti above and the basilar membrane below constitutes the tunnel of Corti, 

 which is, therefore, only an intercellular space of unusual size. It contains probably 

 a soft semifluid intercellular substance serving to support the nerve-fibrils traversing 

 the space (Fig 424) . The pillars or rods of Corti, examined in detail, prove to be 

 composed of two parts, the denser substance of the pillar proper, and a thin, imperfect 

 protoplasmic envelope, which presents a triangular thickening at the base directed tow- 

 ards the cavity of the tunnel. Each pillar possesses a slender slightly sigmoid, longi- 

 tudinally striated body, whose upper end terminates in a triangular head, and whose 

 lower extremity expands into Ihe/oot resting upon the basilar membrane. The inner 

 pillar is shorter, more perpendicular and less curved than the outer ; its head exhibits 

 a single or double concave articular facet for the reception of the corresponding con- 

 vex surface of the head of the outer rod. The cuticular substance of both pillars ad- 

 joining the articular surfaces is distinguished by a circumscribed, seemingly homoge- 



