880 



ORGANS OF SPECIAL SEXSE 



compressed, and measures about one-twentieth of an inch (l-o nnn.) in diameter. 

 One extremity of each canal is suddenly dilated to nearly double its size to form 

 the ampulla. As two of the canals join to open by a common oritice, collectively 

 they present only five o])enings. 



The superior semicircular canal, the highest of the canals, is almost vertical, 

 and is directed transversely. A smooth eminence on the anterior surface of the 

 petrous l)one closely corresponds to the summit of its arch. Its ampullary end is 

 at the outer extremity, and presents a separate opening at the upper part of the 

 vestibule; the opposite end, which is not dilated, joins to form a common orifice 

 with .the similar extremity of the posterior canal. 



Fig. 501. — Interior of the Osseous Labyrinth of the Left Side. 

 (Modified from Soemmerring. Enlarged.) 



Superior Bemiciroular oanal 



Fovea hemielliptiea 

 Fovea hemispherica 



Lamina spiralis 



Seala tympani of 

 the cochlea 



Posterior semicircular canal 



External semicircular oanal 

 Orifice common to the superior 



and posterior semicircular 



canals 

 Orifloe of the vestibular 

 aqueduct 



Orifice of the cochlear aqueduct 



The posterior semicircular canal is the longest. Its dilated end is placed at 

 the lower and back part of the vestibule; its non-dilated extremity joins the 

 superior canal. It is placed nearly vertically, and is directed backwards. 



The external semicircular canal curves horizontally outwards, and presents 

 two openings, of which the anterior is ampullated, in the upper and back part of 

 the vestibular cavity. 



The Cochlea has a rough resemblance to a snail's shell, and coils itself almost 

 horizontally immediately in front of the vestibule. Its base corresponds to the 

 lower and front part of the fundus of the internal auditory meatus (fig. 498a), 



Fig. 502. — Interior of the Osseous Cochle.\. (Enlarged.) 



Scala vestibuli 



Scala tympani 



i— i^>_i_ Lamina spiralis 



and is there marked by the series of minute holes which form the tractus spiralis 

 foraminulentus, whilst its apex is directed forwards and outwards, towards the 

 u}»i)er and fore part of the inner wall of the tympanum. 



The cochlea is about a quarter of an inch (6 mm.) in length, and its basal meas- 

 urement is nearly the same. It is composed of a central pillar, tbi' modiolus, or 

 columella, around which curls for two and a half turns a spiral canal, which 

 contains a delicate spiral shelf or plate, the lamina spiralis. This ])artially divides 

 its cavity into two portions, as it follows its windings from base to ajx'x. Near the 

 apex it ends in a hook-like process, the hamulus, which bounds a small opening, 



