THE BONES OF THE HEAD 



69 



wards for a variable distance within the roof of the orbit. Each 

 sinus is lined by mucous membrane continuous with that of the 

 corresponding nasal fossa, wdth which it communicates by a passage 

 called the infuadibulum. 



The bone derives its blood-supply from the middle meningeal, 

 meningeal branches of the internal carotid, and anterior and 

 posterior ethmoidal arteries internally, and the supra-orbital and 

 frontal arteries externally. 



Articulations.— These are twelve in number, as follows : pos- 

 teriorly with the two parietals above, and the sphenoid (great and 

 small wings) below ; by the external angular processes with the 

 two malars ; between the orbits with the two nasals, two superior 

 maxillae, and two lachr^nnals ; and in the middle line vdth the 

 lateral masses and vertical plate of the ethmoid. 



Structure. — It is a tabular bone. The orbital plates, being 

 destitute of diploe, are thin and translucent, except in those cases 

 where extensions of the frontal air sinuses invade them. 



Varieties. — (i) There may be a persistent frontal suture, called the metopic 

 suture, this condition being known as metopism. (2) Wormian bones are 

 sometimes met vdth at the centre 

 of the supero-lateral border in the 

 region of the anterior fontanelle, 

 and, if these remain permanent, they 

 give rise bj- their union to a breg 

 matic bone. 



Ossification. — The frontal ossifies 

 in membrane from two centres, one 

 for each half, which appear about 

 the seventh week of intra-u ferine Ufe 

 in the situation of the future frontal 

 eminences. At birth the bone con- 

 sists of two halves united by mem- 

 brane, and in the course of the 

 first year they become united by a 

 vertical frontal or metopic suture. 

 This suture gradually becomes ob- 

 literated from below upwards, and 

 usually disappears about the sixth 

 year, though shght traces may persist above and below, especially in the latter 

 situation. Three pairs of secondary centres are described, two mesiaUy 

 placed for the nasal spine, one at either side in the region of the future trochlear 

 fossa, and one for each external angular process. The frontal air sinuses begin 

 to appear about the seienth year, but ihey do not attain any size till after 

 puberty. They axe rather larger in the male than in the female. 



Fig. 



Appears about the 7th Week 

 (iDtia-uterine) 



47. — Ossification of the 

 Frontal Bone. 



The Temporal Bones. 



The temporal bones (ossa temporis) are so named because the hair 

 over the temple is the first to become gray, thus indicating advance 

 in life Each bone is situated on the lateral aspect of the head 

 below the parietal. For convenience of description each is divided 

 into three parts, namely : the squamous portion, which bears the 



