THE INTERNAL EAR. 485 



divides the bony cochlear canal into two parts, the division being 

 completed by a fibrous tissue membrane, the lamina spiralis mem- 

 branacca, which extends from the free edge of the osseous spiral 

 lamina to a thickened periosteal ridge, the ligamentum spirale, lining 

 the outer wall of the bony cochlear canal. The canal above the 

 lamina spiralis (bony and membranous) is known as the scala 

 vestibuli, that below as the scala tympani. Both are perilymphatic 

 canals, and communicate in the region of the last half-turn of the 

 cochlea, by means of a narrow canal, the helicotrema, partly sur- 

 rounded by the termination of the bony spiral lamina, the hamulus. 

 The scala vestibuli is in free communication with the perilymphatic 

 space of the vestibule ; while the scala tympani communicates with 

 perivascular spaces surrounding the veins of the cochlear aqueduct, 

 which latter empty into the jugular veins. The scala tympani ter- 

 minates at the secondary tympanic membrane, closing the fenestra 

 rotunda. 



The cochlear duct, which, as will be remembered, communicates 

 with the sacculus by means of the canalis reuniens, is a long tube 

 closed at both ends,' the one end representing the vestibular sac, or 

 ccecum vestibulare, and the other the cupolar extremity, or cacum 

 cupolare, also known as the lagena. The cochlear duct forms about 

 two and three-fourths spiral turns, its length being about 3.5 mm. 

 Its diameter gradually increases from its lower to its upper or distal 

 extremity. The cochlear duct lies above the lamina spiralis, and, 

 in a section of the cochlea parallel to the long axis of the modiolus, 

 it is of nearly triangular shape, with the somewhat rounded apex 

 of the triangle attached to the osseous lamina spiralis. In the 

 cochlear duct we may distinguish the following parts : (i) the outer 

 wall, which is intimately connected with the periosteum of the bony 

 cochlear canal ; (2) the tympanal wall, resting on the membranous 

 basilar membrane, with its highly differentiated neuro-epithelium, 

 the spiral organ of Corti ; and (3) the vestibular wall, bordering on 

 the scala vestibuli, the intervening structures forming a very delicate 

 membrane the vestibular or Rcissnci- ' s membrane. 



From the account given thus far, it may be seen that within the 

 bony cochlear canal there are found three membranous canals, 

 running parallel with one another and with the osseous lamina spi- 

 ralis about which they are grouped. Two of these membranous 

 canals, the scala vestibuli and the scala tympani, are perilymphatic 

 spaces, and are consequently lined by endothelial cells ; between 

 them is found the cochlear duct, from its position known also as 

 the scala media, lined by epithelial cells. These three membranous 

 canals retain their relative position in their spiral course about the 

 modiolus, and, in a section through the cochlea parallel to the bony 

 axis of the modiolus, would be met with at each turn, and at each 

 turn present essentially the same relative position and structure. 

 In figure 372, which is from a longitudinal section of the cochlea 



