THE INTERNAL EAR. 



1519 



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

 teeth. The lower lip (labiuin tympanicum) 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 epithelium 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 neuroepithelium 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 toward 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 ligament and 

 constitute the so-called auditory strings. According to the estimate of Retzius, there are 24,000 



FIG. 1272. 



Bony capsule 

 of cochlea 



Stria vascularis 



Reissner's membrane 



Prominentia spiralis 

 Membrana tectoria 



Spiral ligament 



Crista basilaris 



Nerve- fibres 



< v- -i- Vas spiralis Bone 



membrane 



Cross-section of ductus cochleaiis from human cochlea. X 90. Drawn from preparation made by Dr. Ralph Butler. 



of these special fibres. Their length increases from the base toward the apex of the cochlea, in 

 agreement with the corresponding increase in breadth of the basilar membrane. The tympanic 

 lamella contains numbers of fusiform cells of immature character interspersed with fibres. In 

 this location the differentiation of the mesoblastic 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 

 which 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 of neuro-epithelium. 



The Organ of Corti. The organ of Corti (organon spirale) consists in a general way of a 

 series of epithelial arches formed by the interlocking of the upper ends of converging 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. 1273). 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 proto- 

 plasmic envelope, which presents a triangular thickening at the base directed toward the cavity of 

 the tunnel. Each pillar possesses a slender slightly sigmoid, longitudinally striated body, whose 



