PLATE XLIV. 



FIRST VIEW OF THE PTERYGOID REGION WITH THE 

 MASSETER MUSCLE REMOVED. 



In this dissection the zy#omat,ic arch was sawn through in front of the external 

 lateral ligament of the jaw and just behind the attachment of the zygomaticus major to 

 the malar bone, and both arch and masseter muscle turned down. The sterno-mastoid 

 and splenius capitis were cut close to their insertion into the mastoid process, and part 

 of the skull on either side of the superior curved line sawn away to expose the lateral 

 sinus. 



A probe has been introduced into the front of the sinus to show how it dips down- 

 wards, grooving the mastoid process in the fossa sigmoidea. The external auditory 

 meatus is the remnant of the visceral cleft between the mandibular and hyoid arches. 

 The lateral sinus varies considerably in its course, but, roughly speaking, it runs down on 

 the mastoid, opposite the line of reflection of the skin, on to the pinna posteriorly. The ac- 

 companying diagram shows the usual course of the lateral sinus (A) upon the exterior of the 



skull, and the position of a trephine hole (B) to expose it, the point of a J-inch trephine 

 being placed 1 inch behind the meatus, and on a level with its upper border, t.e. 

 J inch above the centre of the meatus. Its course, however, varies extremely in its 

 position and extent, but generally it lies i inch behind the external auditory meatus ; 

 hence in tapping the antrum mastoidea place the drill (c), which should be | inch 

 diameter, as near as possible to the posterior edge of the meatus on a level with its 

 upper limit ; do not penetrate more than f inch, or the risk will be run of injuring the 

 external semicircular canal ; the cells are usually opened at a depth of % inch. If the drill 

 is placed more posteriorly, risk is run of perforating the lateral sinus. The line marked 

 (D) in diagram is Reed's base line, which is drawn from the lower margin of orbit 

 through centre of meatus, and usually touches the external occipital protuberance or 

 (alls a little below it. 



