118 



OSTEOLOGY. 



orbital parts bulge into the floor of the fossa, so that the ethmoidal notch appears 

 in a depression between them. On each side of the notch faint grooves for the 

 meningeal branches of the ethmoidal vessels may be seen. The circumference of the 

 fossa is formed by the serrated edges of the bone which articulate with the parietals 

 above, and on each side below with the great and small wings of the sphenoid. 



Connexions. The frontal articulates with twelve bones, viz., posteriorly, with the parietals 

 and sphenoid ; laterally, with the zygomatic bones ; inferiorly and medially, with the nasals, 

 maxillse, lacrimals, and ethmoid. 



Ossification. Ossification begins in membrane from one centre for each half. This 

 makes its appearance about the sixth or seventh week in the region above the processus 



zygomaticus. From these the two halves of the frontal 

 part of the bone are developed, and by extension 

 medially and posteriorly from their lower part the 

 orbital parts are also formed. Serres, Rambaud 

 and Renault, and v. Ihering describe the occurrence 

 of three pairs of secondary centres somewhat later : 

 one pair for the frontal spine, on either side of the 

 foramen caecum ; a centre on either side in cor- 

 respondence with the position of each trochlear 

 pit ; and a centre for each zygomatic process. 

 Fusion between these secondary and the primary 

 centres is usually complete about the sixth or seventh 

 month of foetal life. At birth the two symmetrical 

 halves of the bone are separated by the metopic 

 suture, obliteration of which, commencing as a rule on 

 a level with the frontal tubera, gradually takes place, 

 FIG. 130. OSSIFICATION OF THE FRONTAL so that about the fifth or sixth year it is more 



or less completely closed, traces only of the suture 



, Metopic suture still open, b, Position of b emg i e ft above and below. In about 8 per cent: 

 ^l t^*ft^JS3 <* Europeans however, the suture persists in the 

 e, Centres for nasal spine. adult (see ante). At birth the supraorbital notches 



lie near the middle of the supraorbital margins. 



Traces of the frontal sinuses may be met with about the second year, but it is only 

 about the age of seven that they can be definitely recognised. From that time they 

 increase in size till the age of puberty, subsequent' to which time they attain their 

 maximum development. 



lOssa Parietalia. ] L 



The parietal bones, two in number, are placed one on each side of the vault 

 of the cranium. Each articulates with its fellow of the opposite side, the frontal 

 anteriorly, the occipital posteriorly, and the temporals and sphenoid inferiorly. 

 Each bone possesses a parietal and cerebral surface, four borders, and four angles. 



The parietal surface, convex from above downwards and from before backwards, 

 displays towards its centre a more or less pronounced elevation, the tuber parietale 

 (parietal tuberosity). This marks the position of the primitive ossific centre, and 

 not infrequently corresponds to the point of maximum width of the head. At a 

 variable distance from the inferior border of the bone, and more or less parallel to 

 it, two curved lines can usually be distinguished. The linea temporalis superior 

 (superior temporal line) serves for the attachment of the temporal fascia ; the linea. 

 temporalis inferior (inferior temporal line) defines the attachment of the temporal 

 muscle, the extent and development of which necessarily determine the position of 

 the line. The surface below the lines enters into the formation of the floor of the 

 temporal fossa, and is called the planum temporale; it also affords origin to the 

 temporal muscle, and is often faintly marked by grooves which indicate the 

 course of the middle temporal artery. 



Above the superior temporal line the bone is covered only by the tissues of the 

 scalp. Near its superior border, and about an inch from its occipital angle, 

 is the small parietal foramen, through which pass a small arteriole and an 

 emissary vein. 





