844 



SPECIAL SENSES. 



tube, following the turns of the bony cochlea, beginning below, at the first turn, by a 

 blind extremity, and terminating in a blind extremity at the summit of the cochlea. If 

 we make a section of the cochlea in a direction vertical to its coils, it will be seen that 

 this canal is divided, partly by bone and partly by membrane, into an inferior portion, a 

 superior portion, and a triangular canal, lying between the two, which is external. The 

 bony septum is in the form of a spiral plate, extending from the central column (the 

 modiolus) into the cavity of the cochlea, about half-way to its external wall, and termi- 

 nating above in a hook-shaped extremity, called the hamulus. The free edge of this 

 bony lamina is thin and dense. Near the central column, it divides into two plates, 

 with an intermediate spongy structure in which are lodged vessels and nerves. The 

 surface of the bony lamina looking toward the base of the cochlea is marked by numer- 

 ous regular, transverse ridges, or striae. 



FIG. 265. Otolithsfrom various animals. (Rudinger.) 



1, from tfce goat ; 2, from the herring ; 3, from the devil-fish ; 4, from the mackerel; 5, from the flying-fish ; 6, from 

 the pike ; 7, from the carp ; 8, from the ray ; 9, from the shark ; 10, from the grouse. 



Attached to the free margin of the bony lamina, is a membrane (the membrana basi- 

 laris) which extends to the outer wall of the cochlea. In this way, the canal of the cochlea 

 is divided into two portions, one above and the other below the septum. The portion 

 below begins at 'the fenestra rotunda and is called the scala tympani. The portion 

 above, exclusive of the triangular canal of the cochlea, communicates with the vestibule 

 and is called the scala vestibuli. 



Above the membrana basilaris, is a membrane (the limbus laminae spiralis) the external 

 continuation of which is called the membrana tectoria, or the membrane of Corti. Be- 

 tween the membrana tectoria and the membrana basilaris, is the organ of Corti. The 

 membrane of Eeissner extends from the inner portion of the limbus upward and outward 

 to the outer wall of the cochlea. This divides the portion of the cochlea situated above 

 the scala tympani into two portions ; an internal portion, the scala vestibuli, and an 

 external, triangular canal, called the canalis cochleas, or the membranous cochlea. 



In the anatomical description of the contents of the bony cochlea, the membranous 

 parts may be designated as follows : 



I. The portion below the bony and membranous septum, called the scala tympani. 

 This is formed by the periosteum lining the corresponding portion of the cochlea and the 

 under surface of the bony lamina, and the membrana basilaris. 



