G62 THE EAK. 



to be described in connection with the organ of Corti. It arises, as 

 before stated, on the limbus, not for from the hne of origin of Eeissner's 

 membrane, but the extent to which it covers the limbus, varies in 

 different animals. It overlies the projecting teeth at the edge of the 

 limbus, and also the epithelium between : all this part of the mem- 

 brane is thin and delicate, imperceptibly shading off towards the inner 

 edge of attachment. As the membrane projects over the crest of the 

 limbus, it swells out below into a pad-like projection which, as before 

 stated, covers in and partly fills up the spiral groove, and rests below 

 upon the rods of Corti and contiguous structures. Towards its external 

 edge the membrane again thins out, and over-lies the outer hair-cell 

 region as a delicate film presenting a somewhat reticular appearance, 

 as if impressed by or moulded on the subjacent structures. The 

 thickened part of the membrane is distinctly fibrous in appearance (the 

 fibrillation extending from within, out), and after immersion in weak 

 solutions of chromic acid, or bichromate of potash, it appears to possess 

 considerable toughness and elasticity, Waldeyer states, however, that 

 in the perfectly fresh condition it is soft and pulpy, and he ccnsiders 

 that it may serve as a damper to prevent any too violent vibrations of 

 the fluid which is in immediate contact with the hair-cells. It must 

 be remembered that from its position the hairlets borne by the latter 

 must necessarily be in contact with the under surface of this membrane. 

 About its origin nothing certain is known, but it appears to be formed 

 as a cuticular deposit or secretion from the epithelial cells, upon which, 

 even at a comparatively early stage of development, it may be seer, ta lie. 



Vessels and nerves of the cochlea. — The cochlear hranclics. of the 

 internal auditory arterij, twelve or fourteen in number, arising at the 

 bottom of the internal auditory meatus, traverse the many small canals 

 in the modiolus and bony lamina spiralis, and form in the latter a 

 capillary plexus that joins at intervals the vas spirale, pre'viously 

 mentioned. From this plexus offsets are distributed in the form of a 

 fine network on the periosteum, but the vessels do not anastomose 

 across the membrana basilaris. 



The veins of the cochlea issue from the grooves of the cochknr axis 

 and, joining the veins of the vestibule and semicircular canals at the 

 base of the modiolus, pour their contents into the superior petrosal 

 sinus. 



The cochlear Iranch of the auilitorij nerve is shorter, flatter, and 

 broader than any of the other nerves of the internal ear, and perforates 

 the bone by groups of minute foramina at the bottom of the internal 

 meatus, below the opening of the Fallopian aqueduct. These groups are 

 arranged in a shallow spiral furrow (tractus spiralis foraminulectus) in 

 the centre of the base of the cochlea ; and they lead into small bony 

 canals, which follow first the direction of the axis of the cochlea, 

 through the modiolus, and then radiate outwards, between the plates of 

 the bony lamina spiralis. In the centre of the spiral tract is a larger 

 foramen which leads to the canalis centralis modioli, Tbroagh 

 this_ foramen and canal the filaments for the last half turn of the 

 lamina spiralis are conducted ; whilst the first two turns are supplied 

 by filaments which occupy the smaller foramina and bent carals. In 

 the bone, the nerves have dark outlines, and near the root of tl p spiral 

 lamina they pass outwards through a spirally wound gangiicuic C;)rd, 

 the so-called gamjlion sjjirale, situated m the s])ecial bony canal, canalis 



