280 THE ANATOMY OF THE HORSE. 



rotunda, by which, in the dried bone, it communicates with the 

 tympanum. At tlie apex of the cochlea it communicates with the scala 

 vestibuli by a small opening — the helicotrema. 



The Scala Vestihuli is separated from the preceding by the osseous 

 spiral lamina, and from the scala intermedia by Eeissner's membrane. 

 At the apex of the cochlea it communicates with the scala tympani by 

 the helicotrema, and at the base it opens freely into the osseous vestibule. 

 Like the vestibule, it therefore contains perilymph, and this passes also 

 by the helicotrema into the scala tympani. 



The Scala Intermedia is the smallest but the most important of the 

 three passages. It is the true membranous cochlea, and is also called 

 the ductus cochlearis. It is separated from the scala vestibuli by the 

 membrane of Keissner ; and from the scala tympani mainly by the 

 basilar membrane, but partly by the osseous spiral lamina near its 

 free edge. At the base of the cochlea it communicates by the 

 slender canalis reuniens with the sacculus, and it thus contains 

 endolymph. 



The terminal filaments of the cochlear division of the auditory nerve 

 are distributed in the substance of the basilar membrane ; and on 

 that surface of the membrane which is directed towards the scala 

 intermedia there occurs a peculiar arrangement of cells, termed the 

 organ of Corti. 



The Organ of Corti. — When the basilar membrane is examined in 

 transverse section, it is seen to support about the centre of the surface 

 directed towards the scala intermedia a double row of elongated rod- 

 like cells, termed Gorti's rods. The rods of the two rows, where they 

 rest on the basilar membrane, are separated by a slight interval ; but 

 they incline towards each other and meet at the opposite extremity, so 

 as to enclose a minute canal — the canal of Corti. On the outer side 

 of the external row of rods the basilar membrane supports four or five 

 rows of shorter cells, the free extremity of each of which bears a tuft of 

 stiff-hair-like processes. In the same way the membrane supports a 

 single row of hair-bearing cells on the inner side of the inner rods of 

 Corti. ■ On either side these hair-bearing cells are succeeded by cells 

 which become progressivelj' shorter and pass into the general columnar 

 cell lining of the scala intermedia. A delicate cellular membrane — the 

 membrana reticidaris — is spread over the outer hair-bearing cells. 

 Through apertures in this membrane the tufts of hair-like processes 

 project, in a manner comparable to tufts of grass springing through the 

 interstices of a wire net. Still another membrane — the membrana tectoria 

 — springs from the edge of the osseous spiral lamina between the lines of 

 origin of the basilar and Eeissner's membranes, and passes outwards 

 over the organ of Corti. 



The Auditory (8th) Cranial Nerve. This nerve enters the inter- 



