Skull and Hyoid 217 



so-called " roots " : anterior (articular eminence), middle (post-glenoid tubercle) and 

 posterior or ascending (supramastoid crest) (Fig. 175). 



The outer end of the articular eminence is roughened by the attachment of the 

 external lateral ligament of the joint, and the temporal fascia is fastened to the upper 

 border of the free process and along the supramastoid crest. 



The prominent mastoid process projects downwards and forwards immediately 

 behind the meatus : between it and the tympanic plate is the point of emergence 

 of the posterior auricular branch of the vagus. The process is made by a mass con- 

 tinuous with the petrous, with a part of the squama covering its upper and front area 

 externally, so that a petro-squamous suture may be apparent on this surface of the 

 bone. Beside this it presents : (a) attachment of Retrahens aurem ; (b) some fibres 

 of origin of Occipito-frontalis ; (c) attachments of Sterno-mastoid, Splenius, and 

 Trachelo-mastoid ; (d) a mastoid foramen near its posterior border, opening internally 

 into the groove for the lateral sinus ; (e) digastric groove, etc., already considered. 

 The process is not present at birth, may owe its existence to the pull of the muscles 

 inserted into it, and is usually better developed in men than in women. 



The upper part of the region between the process and the external meatus exhibits 

 several vascular foramina and may present a small pos'-meatal spine projecting from 

 the squama : this depressed area (vascular spot) is of surgical importance, as it marks on 

 the surface the position of the mastoid antrum. 



On the front border the squama articulates with the great wing of the sphenoid 

 and with the parietal in the upper and back part. The meeting of these two surfaces 

 is marked on the articular bevelled area of the squama by a definite ridge (A in Fig. 175). 

 In front of this ridge the sphenoidal surface extends to the apex of the petrous, while 

 behind it the parietal extends to the prominent posterior angle on the mastoid margin. 

 The occipital articulates along the lower and inner margin from this angle to the apex 

 ' of the petrous. The malar joins the zygomatic process, and the lower jaw articulates 

 with the squama. The occipital articulation is largely through the medium of a 

 thin layer of fibro-cartilage : in this are sometimes found small ossifications, the 

 ossicles of Rio/an. 



The outer side of the petrous is covered by the other elements of the bone and cannot be 

 seen without removing these : this is impossible without the saw in the adult bone, though easily 

 effected in the young specimen. A semi-diagrammatic view of the outer side is given in Fig. 178. 

 The tympanic cavity is exposed by this removal of the squama and tympanic plate : the styloid 

 process is left and its upper end is seen embedded in the posterior wall of the tympanum (tympano- 

 hyal). The tympanic plate covered this in ; its area of articulation is shown. The rest of the 

 cut surface represents the extent to which the squama rested on the petrous. 



The cavity of the bony Eustachian tube is opened, leading to the tympanum, and behind 

 this again a narrower aditus opens into the mastoid antrum. Evidently these cavities are covered 

 in externally by the squama and tympanic plate ; the latter forms the outer wall of the tympanum. 

 The plate meets the jugular plate below and thus completes the tympanic floor : it also forms the 

 floor of the bony tube. The canal for the Tensor tympani is above the tube and also leads to the 

 tympanum. Mastoid air-cells are seen behind the antrum : these are very variable in extent 

 of development and may be found throughout the whole mastoid region, communicating with the 

 antrum. 



The inner ear is close to the inner wall of the tympanum, and the first turn of the cochlea 

 causes a bulging, the promontory, on the inner wall of the tympanum : immediately behind this 

 are the fenestra ovalis, for the footpiece of the stapes, and the fenestra rotunda, which is closed by 

 membrane : behind and below these is a depression, the sinus tympani. The facial nerve, having 

 run outwards from the inner meatus to the point X in the figure, turns back sharply here along 

 the upper part of the inner wall, in the bony canal known as the " aqujeductus Fallopii," to the lower 

 and inner side of the aditus ad antrum, where it turns down along the posterior wall to emerge 

 at the stylo-mastoid foramen. Outside the descending part of the canal is the " pyramid," a 



