680 DISSECTION OF THE EAR. 



Other views of the cavity may be obtained by sections of the temporal 

 bone in different directions, according to the knowledge and skill of the 

 dissector. 



The vestibular space (rig. 244) is ovoidal in form, and the extremities 

 are directed forwards and backwards. The larger end is turned back, and 

 the under part or floor is more narrowed than the upper part or roof. It 

 measures about ^th of an inch in length, but it is narrower from without 

 inwards. The following objects are to be noted on the boundaries of the 

 space. 



In front, close to the outer wall, is a large aperture (g) leading into the 

 cochlea ; and behind are five round openings of the three semicircular 

 canals (d, e,f). 



The outer wall corresponds with the tympanum, and in it is the aper- 

 ture of the fenestra ovalis. On the inner wall, nearer the front than the 

 back of the cavity, is a vertical ridge or crista (&). In front of the ridge 

 is situate a circular depression, fovea hemispherica (), which is pierced 

 by minute apertures for nerves and vessels, and corresponds with the 

 bottom of the meatus auditorius internus. Behind the crest of bone, 

 near the common opening of two of the semicircular canals, is the small 

 aperture of the aqueduct of the vestibule (c), which ends on the posterior 

 surface of the petrous portion of the temporal bone. 



The roof is occupied by a slight transversely oval depression, fovea 

 semi -elliptic a ; this is separated from the fovea hemispherica by a prolonga- 

 tion of the crista (b) on the inner wall before mentioned. 



The SEMICIRCULAR CANALS (fig. 245) are three osseous tubes, which 

 are situate behind the vestibule, and are named from their form. 



Dissection. These small canals will be easily brought into view by the 

 removal of the surrounding bone by means of a file or bone forceps. Two 

 may be seen opening near the aperture made in the vestibule, and may 

 be followed thence ; but the third is altogether towards the posterior aspect 

 of the petrous portion of the temporal bone. 



The canals are unequal in length, and each forms more than half an 

 ellipse. They communicate at each end with the vestibule, but the con- 

 tiguous ends of two are blended together so as to give only five openings 

 into that cavity. Each is marked by one dilated extremity which is 

 called the ampulla. When a tube is cut across it is not circular, but is 

 compressed laterally, and measures about ^th of an inch, though in the 

 ampulla the size is as large again. 



From a difference in the direction of the tubes, they have been named 

 superior vertical, posterior vertical, and horizontal. 



The superior vertical canal (a) crosses the upper border of the petrous 

 part of the temporal bone, and forms a projection on the surface. Its 

 outer end is marked by the ampulla, whilst the inner is joined with the 

 following. 



The posterior vertical tube (b) is directed backwards from its junction 

 with the preceding towards the posterior surface of the temporal bone; the 

 upper end is united with the superior vertical canal, and the lower end is 

 free and dilated. 



The horizontal canal (c) has separate apertures, and is the shortest of 

 the three. Deeper in position than the superior vertical, it lies in the sub- 

 stance of the bone nearly on a level with the fenestra ovalis; its dilated 

 end is at the outer side close above that aperture. 



