THE INTERNAL EAR. 221 



hind the fovea hemispherica. It lodges the utricle, and presents a Macula 

 Cribosa for the entrance of branches of the vestibular nerve. 



What are the Semicircular Canals ? They are three C-shaped bony 

 tubes, each about -fa inch in diameter, situated above and behind the vestibule, 

 into which they open by 5 apertures. Each canal lies at a right angle with 

 the other two. The 



Superior and Posterior/ Canals, are each nearly an inch long, vertically 

 placed, joined together posteriorly where they open into the vestibule by 

 a common orifice. c 



External Canal, g is ^ inch long, horizontally placed, and directed out- 

 wards and backwards. 



Ampulla, is a flask-shaped dilatation of each canal at one extremity, 

 having a diameter of about fa inch. 



Describe the Cochlea. The Cochlea is a bony tube, about i^ inch long, 

 tapering from -fa to -fa inch in diameter, coiled around a central conical axis 

 for 2^ turns, and resembling a snail shell in appearance. It measures about 

 % inch in length and the same in breadth at the base. It is situated in front 

 of the vestibule, its apex pointing forwards and outwards, and is separated 



from the carotid canal in front by a thin 



FIG. IOQ. 

 wall. Its 



Modiolus, a is the central axis, conical 

 in shape, channeled by small canals 

 for the passage of arteries and nerves, 

 and by a central one, the Canalis 

 Centralis Modioli. Its apex is within 

 the last coil of the cochlea, and is ex- 

 panded into a funnel-shaped lamella, the Infundibulum. 



Lamina Spiralis, 6 is a thin, osseous plate, projecting from the modiolus 

 half way across the spiral canal, and winding around the modiolus ter- 

 minates near the apex in a hook-like or Hamular Process. Its deficiency 

 in the last half turn of the cochlea forms the Helicotrema, a space in 

 which the scalae vestibuli and tympani unite. (See below.) It consists 

 of two laminae, J , 2 , between which pass the filaments of the cochlear 

 nerve. 10 From its free edge the structures composing the membranous 

 cochlea pass to their attachment on the opposite wall of the cavity. (See 

 Fig. in.) 



Spiral Canal, is the space between the modiolus and the outer wall of the 

 cochlea. It makes 2^ turns, from left to right in the right ear, from 

 right to left in the left ear: and terminates in the Cupola, a cul-de-sac at 

 the apex of the cochlea. It is divided, by the osseous lamina spiralis and 



