778 MR. \i. C. MLI.ir. (»\ TlIK AXATOMV [iMnV'il. 



tlie proiuiiu'iit eA^e of tlie hasiocc-ipital. Tlu' jiuteriorpyition of 

 the iinuT side of tliis depression niid the front of the recess are 

 bounded hy the pterygoid and alisphenoid liones, which are fused 

 together ; the hitter also form tlie roof of the anterior half of the 

 depression. Thus the anterior portion of tlie cavity is bounded 

 on three sides by the pteiygoiil and externally by the squamosal, 

 and is known as the pterygoi<l fossa. 



Ill this recess the united tympanic and periotic* bones lie. 

 The latter consists of three parts : an anterior or pro-otic, a central 

 or lalivrinthic, and a posterior or opisthotic. The antei'ior oi- 

 pro-otic bone resembles a three-sided pyramid with its apex pointing 

 forwards and projecting into a cavity in the squamosal bonealiove 

 the roof of the pterygoid fos.sa. 



On the inferior surface of the pro-otic, at its posterior eml, there 

 is a longitudinally flattened pedicle which unites with the anterior 

 extremity of the tympanic bone. Just behind this anteiior jiedicle 

 of the tympanic— -that is, between the pro otic portion and the 

 central or labyrinthic — there is a groove running in a transverse 

 direction. This furrow forms the roof of the auditory meatus. 

 The central or labyrinthic portion is the smallest of the three 

 divisions of the periotic, and it contains the vestibule, the cochlea, 

 and the diminutive semicircular canals. The inferior surface of 

 this l)one is dome-shaped and forms the roof of the tympanic cavity. 

 The fenestra ovalis is situated in the depression which marks the 

 outer border of tliis bone on the under side. The fenestra rotunda 

 is placed some little distance behind the fenestra ovalis, and is 

 separated from the latter by a. i-idge Avliich forms the posterior 

 boundary of the tympanic cavity aliove. The fenestra I'otunda is 

 thvis on the posterior surface of the labyrinthic segment, and 

 thus outside the tynqianic cavity. The superior surface of the 

 labyrinthic bone is irregular in shape and is turned towards the 

 cranial cavity. Two foramina open on this surface — the aque- 

 ductus Fallopii anteriorly and the internal auditory meatus 

 posteiiorly. The aqueductus perforates the bone and opens at 

 the base of the lalnrinthic segment, and is continuous with 

 a groove in the opisthotic division. At the posterior end of the 

 lal)yiintliic the opisthotic segment extends outwards ; it is con- 

 stricted at its origin, but broadens and expands towards its 

 extremity between the s(iuam{)sal and exoccipital l)ones almost at 

 right angles to the pro-otic portion. Jn fact, the labyrinthic may 

 he said to lie at the angle formeil by the pro-otic and opisthotic. 

 The tympanic segment is united to the ends of these two bones 

 by pedicles, which are flattened in the direction of their length. 

 Thus the posterior pedicle is situated on the infeiior surface of 

 the opisthotic, near its junction with the labyrinthic, and is at, 

 right angles to the anterior pedicle. 



The tynq)anic bonet (I'l. LXXIV. fig. 3. d). which is attached 

 to tlie periotic as described above, is usually aliout five inches long 



• Carlo A Mncalister, Pliil. Trims. lrt(!7, |il. vi. %. 11. 

 t Carte & M:u-!ilistcr, l<h: vil. \<[. vi. fig. 10; pi. vii. fig. 6. 



