THE ORGAN OF HEARING. 



disappear along its attachment to the bony semicircular canal 

 (Riidinger, 72, 88). 



The epithelium lining the membranous semicircular canals is 

 simple squamous in character and very evenly distributed over the 

 entire inner surface, including the papillae previously mentioned. 



On the concave side of each semicircu- 

 lar canal the epithelial cells are some- 

 what narrower and higher. This inner 

 and higher epithelium (raphe), extending 

 along the concave side into the ampullae, 

 marks the region at which the semicir- 

 cular canals were constricted off from 

 the pocket-like anlagen. The epithe- 

 lium of the ampullae (Fig. 371), with 

 the exception of that in the region of the 

 raphe, is of the squamous type. At the 

 cristae of the ampullae, however, there is 

 found a neuro-epithelium similar to that 

 of the maculae. The cells adjoining both 

 ends of the cristae are high columnar, 

 and to these the squamous epithelium 

 is joined. The columnar cells just men- 

 tioned form the so-called semilunar fold. 

 Otoliths are also present upon the neu- 

 ro-epithelium of the cristae. Here the 

 structure corresponding to the otolithic 

 membrane of the utriculus and sacculus 

 is called the cupida. In preserved spec- 

 imens it presents the appearance of a 

 coagulum, showing a faint striation ; in 



the fresh condition, it has never been recognized as a distinct struc- 

 ture, at least in the lower classes of vertebrates. 



k d 



Fig. 371. Part of a verti- 

 cal section through the anterior 

 ampulla, showing the membran- 

 ous wall, a portion of the "crista 

 acustica," and the "planum 

 semilunatum" (after Retzius) : 

 a, Semilunar fold ; b, crista acus- 

 tica ; c, nerve-fibers ; d, blood- 

 vessels. 



3. THE COCHLEA. 



The cochlea consists of an osseous portion, the bony cochlea, 

 a membranous portion, the cochlear duct, and two perilymphatic 

 canals. The bony cochlea consists of a central bony axis of conical 

 shape, the modiolus, around which is wound a spiral bony canal, 

 having in man a little over two and one-half turns, the modiolus 

 forming the inner wall of this canal. The summit of the cochlea, 

 which has the shape of a blunt cone, is formed by the blind end of 

 this bony canal, and is known as the cupola. The modiolus further 

 gives support to a spiral plate of bone, the lamina spiralis ossea, 

 which extends from the lower part of the modiolus, and, forming 

 two and one-half spiral turns, reaches its top, where it ends in a 

 hook-like process, the hamulus. This bony spiral lamina partly 



