62 



THE SKELETON 



Borders.— The articular border of the frontal portion (parietal margin) forms a little more 

 than a semicircle. It is thick, strongly serrated, and bevelled so as to overlap the parietal 

 above and to be overlapped by the edge of that bone below. The border is continued inferiorly 

 into a triangular rough surface on either side, which articulates with the great wing of the sphe- 

 noid. The posterior border of the orbital portion is thin and articulated with the lesser wing 

 of the sphenoid. 



Blood-supply. — The blood-vessels for the supply of the vertical portion are derived 

 from the frontal and supra-orbital arteries, which enter on the outer surface, and from the 

 middle and small meningeal, which enter on the cerebral surface. The horizontal portion 

 receives branches from the ethmoidal, and other branches of the ophthalmic, as well as from 

 the meningeal. 



Articulations. — -The frontal articulates with the parietal, sphenoid, ethmoid, lacrimal, 

 zygomatic (malar), maxilla, and nasal bones: Also, with the epipteric bones when present, 

 and occasionally with the squamous portion of the temporal, and with the sphenoidal concha 

 when it i-eaches the orbit. 



Fig. 78. — Unusually Large Frontal Sinuses. 



Ossification. — The frontal is ossifietl from two nuclei deposited in the outer layer of the 

 membranous wall of the cranium, in the situations ultimately known as the frontal eminences. 

 These nuclei appear about the eighth week, and ossification spreads quickly through the mem- 

 brane. At birth the bones are quite distinct, but subsequently they articulate with each other 

 in the median line to form the metopic suture. In the majority of cases the suture is obliter- 

 ated by osseous union, which commences about the second year, though in a few cases the 

 bones remain distinct throughout life. 



After the two halves of the bone have united, osseous material is deposited at the lower 

 end of the metopic suture to form the frontal spine, which is one of the distinguishing features 

 of the human frontal bone. The spine appears about the twelfth year, and soon consolidates 

 with the frontal bone above. Accessory nuclei are sometimes seen between this bone and the 

 lacrimal and may jirrsist as Wormian ossicles. 



The frontal sinuses appear about the seventh year as prolongations upward from the hiatus 

 semihuuiris and increase in size up to old age. As they grow they extend in three directions, 

 viz., upward, laterally, and backward along the orbital roof. A bony septum, usually com- 

 plete, separates the sinuses of the two sides, and they are larger in the male than in the female. 

 The superciliary arches are not altogether reliable guides as to the size of the sinuses, since 

 examples are seen in which the arches are low and the sinuses large. In fig. 78 an example 

 of unusually large sinuses is figured, illustrating the extension upward, laterally, and backward. 



THE SPHENOID 



Tli(! sphenoid [os sphenoidale] (figs. 79, 80, 81, 82) is situated in the base of 

 the skull and takes part in the formation of the floor of the anterior, middle, and 

 posterior eranial fossa;, of the temporal and nasal fossae, and of the cavity of the 

 orbit. It is very irregular in shape and is described as consisting of a central 

 part or body, two pairs of lateral exj)ansi()ns called the great and small wings, 

 and a pair of processes which project downward, called tiie pterygoid processes. 



The body, irregularly cuboidal in shape, is hollowed out into two large cavities 

 known as the sphenoidal sinuses, sei)arated by a thin sphenoidal septum and 

 opening in front by two large apertures into the nasal fossa). The superior sur- 



