272 THE ANATOMY OF THE HORSE. 



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

 vestibuli by a small opening — the helicotrema. 



The Scala Vestibuli is separated from the preceding by the osseous 

 spiral lamina, and from the scala intermedia by Reissner'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 called also the 

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

 brane of Reissner ; 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 c/rgan 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 Corti'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 

 stifi', 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 progressively shorter and pass into the general columnar 

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

 membrana reticularis — is spread over the outer hair-bearihg cells. 

 Through apertures in this membrane, the tufts of hair-Uke processes 

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

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

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

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

 over the organ of Corti. 



The Auditory (Sth*) Cranial Nerve. This nerve enters the inter- 

 nal auditory meatus in company with the 7th, which passes into the 

 aqueduct of Fallopius. The 8th divides into two branches, one for the 

 cochlea, the other for the vestibule and semicircular canals. The 



