166 OSTEOLOGY. 



The sutures around the summit of the great wing of the sphenoid are arranged 

 like the letter H placed obliquely, the cross.-piece of the H corresponding to the 

 spheno-parietal suture. When this is short, and becomes a mere point of contact, 

 the arrangement then resembles the letter X. This region is named the pterion. 



Curving over the lateral region of the calvaria in a longitudinal direction is 

 the temporal line. This is often double. The lower line marks the limit of the 

 attachment of the temporal muscle, whilst the upper ridge defines the attachment 

 of the temporal fascia. Commencing in front at the zygomatic process of the 

 frontal, the line sweeps upwards and backwards across the inferior part of that 

 bone, and then crossing the coronal suture at a point called the stephanion 

 it passes on to the parietal, over which it curves in the direction of its mastoid 

 angle. Here it is continued on to the temporal bone, where it sweeps forwards 

 to form the supra -mastoid crest, which serves to separate the squamous from 

 the mastoid portion of the temporal bone laterally. Carried forwards, this ridge 

 is seen to become continuous with the upper border of the zygomatic arch above 

 the external acoustic meatus. In front, the temporal ridge separates the temporal 

 fossa from the region of the forehead ; above and behind, it bounds the temporal 

 fossa which lies within its concavity, and serves to separate that hollow from 

 the surface of the calvaria which is overlain by the scalp. Above the level of 

 the temporal lines the surfaces of the frontal and parietal bones are smooth, the 

 latter exhibiting an elevation of varying prominence and position, but usually 

 situated about the centre of the bone, called the tuber parietale (parietal tuberosity). 

 A slight hollowing of the surface of the parietal behind and parallel to the coronal 

 suture is not uncommon, and is referred to as the post-coronal depression. 



As seen in profile, the part of the calvaria behind and below the lambdoid suture 

 is formed by the squamous part of the occipital bone. In line with the zygomatic 

 arch this outline is interrupted by the external occipital protuberance or inion. 

 The projection of this point is variable ; but its position can usually be easily 

 determined in the living. Passing forwards from it, and blending anteriorly with 

 the posterior border of the mastoid process of the temporal bone, is a rough crest, 

 the linea nuchae superior (superior nuchal or curved line), a little above which 

 there is often a much fainter line, the linea niichse suprema (highest curved line) ; 

 this affords attachment to the galea aponeurotica. These two lines serve to 

 separate the part of the cranium above, which is covered by scalp, from that 

 below, which serves for the attachment of the fleshy muscles of the back of the 

 neck, the latter surface (planum nuchale) being rough and irregular as con- 

 trasted with the smooth superior part (planum occipitale). The fulness of these 

 two parts of the occipital bone varies much. There is frequently a pronounced 

 bulging of the planum occipitale, and the position of the lambda can often be 

 easily determined in the living ; similarly the planum nuchale may be either com- 

 paratively flat or else full and rounded. These differences are of course associated 

 with corresponding differences in the development of the cerebral and cerebellar 

 lobes, which are lodged in relation to the cerebral aspect of these parts of the bone. 

 The further description of the planum nuchale is best deferred till the external 

 aspect of the base of the skull is studied. 



Fossa Temporalis. Within the limits of the temporal lines the side of the 

 cranium slopes forwards, medially, and down wards, thus leaving a considerable interval 

 between its lower part and the zygomatic arch. This space or hollow is called the 

 temporal fossa ; bounded above and behind by the temporal lines, its inferior limit 

 is defined by the level of the zygomatic arch. Deepest opposite the angle formed 

 by the frontal and temporal processes of the zygomatic bone, the fossa becomes 

 shallow towards its circumference. 



Its floor or medial wall, which is slightly concavo-convex from before backwards 

 about mid-level, is formed above by the temporal surface of the frontal, behind 

 by the sphenoidal angle of the parietal, as well as the lower portion of that 

 bone, below the temporal line ; below and in front by the temporal surface of the 

 great wing of the sphenoid, and behind and below by the squamous portion of the 

 temporal bone. Inferiorly, the floor is limited in front by the free inferior border 

 of thft crrp.at wino- of thp, srVhp.noirl whir.h forms the utmer boundarv of the pterygo- 



