1050 



ORGANS OF THE SENSES AND THE COMMON INTEGUMENT 



upper and lateral angle of the vestibule, just above the fenestra vestibuli, where 

 it opens close to the ampullated end of the superior canal ; its opposite end opens 

 at the upper and back part of the vestibule. The lateral canal of one ear is very 

 nearly in the same plane as that of the other; while the superior canal of one ear 

 is nearly parallel to the posterior canal of the other. 



Helicotrema 



Lamina spiralis 

 / ' ossea 



Tympanic cavity 



Cochlea 

 Scala vestibuli- 

 Scalatympani-- 



-Vestibular fenestra 

 .Fissura vestibuli 



. Recessus 

 sphcericus 

 \~Fossa cochlearis 



Lai. semicircular 

 canal 



Vestibule 



Post, semicircular 

 canal 



Aquasductus 

 vestibuli 



Reces suscepticus 



FIG. 923. The cochlea and vestibule, viewed from above. All the hard parts which form the roof of the internal 



ear have been removed with the saw. 



The Cochlea (Figs. 922, 923) . The cochlea bears some resemblance to a common 

 snail-shell ; it forms the anterior part of the labyrinth, is conical in form, and placed 

 almost horizontally in front of the vestibule; its apex (cupula) is directed forward 

 and lateralward, with a slight inclination downward, toward the upper and front 

 part of the labyrinthic wall of the tympanic cavity; its base corresponds with the 

 bottom of the internal acoustic meatus, and is perforated by numerous apertures 

 for the passage of the cochlear division of the acoustic nerve. It measures about 

 5 mm. from base to apex, and its breadth across the base is about 9 mm. It con- 

 sists of a conical shaped central axis, the modiolus; of a canal, the inner wall of which 

 is formed by the central axis, wound spirally around it for two turns and three- 

 quarters, from the base to the apex; and of a delicate lamina, the osseous spiral 

 lamina, which projects from the modiolus, and, following the windings of the canal, 

 partially subdivides it into two. In the recent state a membrane, the basilar 

 membrane, stretches from the free border of this lamina to the outer wall of the bony 

 cochlea and completely separates the canal into two passages, which, however, 

 communicate with each other at the apex of the modiolus by a small opening 

 named the helicotrema. 



The modiolus is the conical central axis or pillar of the cochlea. Its base is broad, 

 and appears at the bottom of the internal acoustic meatus, where it corresponds 

 with the area cochlea?; it is perforated by numerous orifices, which transmit fila- 

 ments of the cochlear division of the acoustic nerve; the nerves for the first turn 

 and a half pass through the foramina of the tractus spiralis foraminosus; those 

 for the apical turn, through the foramen centrale. The canals of the tractus 

 spiralis foraminosus pass up through the modiolus and successively bend outward 



