THE SKULL THE CRANIUM 197 



posterior part of the auditory rim, and behind it is the rough edge left 

 by breaking off the posterior attachment of the ectotympanic part of 

 the bulla. It is bounded posteriorly and on the inside by the anterior 

 surface of a small, thin, and sharp plate or shelf, which runs upward 

 and forward and slopes inward and forward so as to end above in a 

 pointed process which joins the petrous just above the fenestra ovalis, 

 and may be named, for convenience, the bridge. This plate ends, 

 below, where the free upper edge of the ectotympanic ends, and where 

 the posterior root of the septum of the bulla begins. The posterior 

 inner surface of this plate is concave transversely, and, with its con- 

 tinuation, namely, the deep distinct groove which curves downward, 

 backward, and outward behind it, is converted by the bulla into a 

 canal, the lower portion of the aquseductus Fallopii. 1 This canal 

 separates the squamous from the mastoid. 



The internal surface of the squamous has the general outline of 

 the external surface. It possesses a boldly arched upper border, 

 which curves downward before and behind to meet the almost straight, 

 longitudinal, lower border. The surface is concave, the deepest part 

 of the concavity lying in front of and below the centre of the bone, 

 not much above and behind the lower anterior angle. This point is 

 the centre of an almost smooth circular area, which enters into the 

 formation of the inner surface of the cranium, contributing the lower 

 part of the side of the middle cerebral fossa. It is often marked 

 by grooves for the meningeal artery, whereof the most constant 

 appears to be a vertical groove near the posterior part. The part 

 of the surface lying below and in front of this area is a narrow 

 rough strip, slightly bevelled along the front and lower borders, which 

 joins the outer side of the alisphenoid and the alisphenoidal root of 

 the tentorium. A wider, thinner, bevelled band lies at the upper 

 and back part of the inner surface and overlaps the parietal and 

 joins the outer edge of the parietal root of the tentorium. The 

 upper part of the base of the petrous portion of the bone is firmly 

 coalesced with the posterior part of the lower border and the posterior 

 lower angle of the inner surface of the squamous, at which point the 

 squamous also unites with the mastoid. When the petrous is broken 

 away, the whole inner surface of the mastoid is exposed, as well as 

 the inner and the posterior part of the lower surface of the squamous. 



1 Fallopius (A.D. 1523-1562). 



