CRANIUM. 



735 



two roots, there is a groove in which play 

 the posterior fibres of the temporal muscle. 

 The fossa articularis, which is between the 

 roots, is bounded behind by the Glasserian 

 fissure before mentioned; it forms, with the 

 adjoining fossa parotidea, the glcnoid cavity. 

 The zygomatic process extends forwards about 

 an inch from its anterior root ; being, therefore, 

 convex externally and concave internally. Its 

 upper border gives attachment to the temporal 

 fascia; its inferior (which is about half the 

 length of the superior) to the masseter muscle. 

 Its external surface is covered by the integu- 

 ment, and its internal forms the outer boun- 

 dary of the temporal fossa, in which is situ- 

 ated the temporal muscle. The extremity of 

 the zygomatic process forms a point, on account 

 of the under margin being bevelled and den- 

 ticulated to articulate with the malar bone. 



The circumference of the squamous process 

 is sharp, in all that part which is above the 

 level of the zygomatic process, and denticu- 

 lated, at the expense of its outer table, in the 

 rest of its extent ; so that it rests on the sphe- 

 noid bone. 



The connexions of this bone and the me- 

 chanical effects which result from its position, 

 will be readily understood. Its petrous por- 

 tion being wedged between the basilar process 

 of the occipital bone, which serves it as a 

 fulcrum, and the ala major of the sphenoid, 

 which binds it against that fulcrum ; the in- 

 ferior part of its squamous process resting on, 

 and being sustained by the sphenoid bone, 

 while its mastoid process is braced in by the 

 posterior inferior angle of the parietal, and by 

 the occipital bone the fronting squamous 

 margin will effectually resist the lateral thrust 

 of the parietal ; the more so that a limited 

 yielding movement is allowed at the fulcrum. 

 The zygomatic process advancing forwards to 

 the malar bone, will, with its fellow of the 

 opposite side, give stability to the several 

 bones of the face ; and, in common with the 

 pterygoid processes of the sphenoid bone, 

 maintain the integrity of the various arches 

 which they form. It is also connected with 

 the lower jaw. 



This bone is developed from six points of 

 ossification : viz. one for each of the three 

 great divisions, and one each for the zygomatic 

 and styloid processes and the auditory canal. 

 At birth it consists of four pieces, the squa- 

 mous (), mastoid (c), petrous, and an in- 



Fig. 374, 



complete bony ring (d), to which the mem- 

 brane of the tympanum is attached. The 

 bony ring is the first to join, by its upper part, 

 the squamous ; after which it is consolidated 

 with the petrous, and then extends itself out- 

 wards and backwards to form the meatus 

 auditorius externus, and all the four pieces are 

 then united. In infancy the bone sustains 

 great changes ; the squamous process from 

 being straight becomes curved ; the zygomatic 

 process recedes from the squamous and in- 

 creases the space between them ; the mastoid 

 portion becomes more tumid, is developed 

 upwards and backwards, and sends forth the 

 nipple-like process which gives to it its name. 

 The eminentia articularis and fossa articularis 

 from an oblique assume a transverse direction, 

 and become, the one more concave, the other 

 more convex. The styloid process, though 

 ossified in its middle, is frequently, to an ad- 

 vanced age, connected with the bone by carti- 

 lage only. 



The parietal bone (on parietale; Germ, die 

 ScheitelbeineoderSeitenbeine) (jig- 372, 373 P) 

 constitutes with its fellow the greater portion 

 of the vault of the skull, and forms with it a 

 sort of bridge, the corners of which on each 

 side are fixed, the one on the great wing of 

 the sphenoid, the other on the mastoid process 

 of the temporal bone, the squamous process 

 of which braces in the intervening space. 



The external surface offers in its centre a 

 prominence which marks the spot at which 

 ossification commenced; and it marks also 

 the widest part of the skull. Below this is a 

 semicircular line (the linea temporalh)^ to 

 which are attached the temporal fascia and 

 muscle ; still more inferiorly is a plane surface 

 occupied by the temporal muscle; and be- 

 tween it and the lower border, is a lunated 

 articular portion with converging striae, to be 

 applied against the squamous portion of the 

 temporal bone. Near the posterior part of the 

 bone and a little removed from its upper border 

 is the foramen parietale, for the passage of a 

 vein to the longitudinal sinus. 



The inner surface exhibits the usual indi- 

 cations of the convolutions of the brain, and 

 also arborescent sulci, which mainly proceed 

 from the anterior inferior angle of the bone, 

 and are directed upwards and backwards to 

 the fossa parictalis, which answers to the pa- 

 rietal prominence on the outer surface ; these 

 sulci lodge the branches of the middle menin- 

 geal artery. Along the upper border is a de- 

 pression, which, with a similarly disposed edga 

 of the other bone, forms a groove for the 

 lodgement of the longitudinal sinus, and 

 hence is termed sulcus longitudinalis ; near to 

 it are sometimes seen small depressions (fossa 

 Pacchionii) for the granulations of the dura 

 mater, called glandulae Pacchionii externse. 



The borders are of various lengths; the 

 superior is the longest, the inferior is the 

 shortest, and the anterior is longer than the 

 posterior. The superior is united to the same 

 border of the opposite bone by the regular 

 interchange of serrations of the outer table ; 

 the anterior and posterior reverse the arrange.- 



