4S 



THE SKELKTOX 



The blood-supply. — Arterial twigs derived from tlie middle and small menin- 

 geal arteries enter it on the eerebral, and branches from the frontal and sui)raorbital 

 arteries on the outer surface. The horizontal plate derives twigs from the ethmoidal 

 and otlier branches of the ophthalmic artery. 



Ossification. — The frontal <levelops from two eartliy spots dei)osited in the 

 outer layer ol' tlie meml)ranous wall of the cranium, in "the situations ultimately 

 known as the frontal eminences. Tliese nuclei api)ear about the eighth week, and 

 (juicklv spread througli tlie membrane. At birth the bones are quite distinct. 

 yubse("iuently they articulate with each other in the median line to form the 

 metopic suture. In a few cases, the bones remain distinct throughout life. In the 

 majority of cases the suture is obliterated; ankylosis commences about the second 



Fig. 58.— The Frontal Bone at Birth. 



year. In adult skulls, traces of the metopic suture may often be seen in the region 

 of the glabella. 



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

 the lower end of the metopic suture to form the nasal 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 bone above. Accessory nuclei are 

 sometimes seen between this bone and the lachrymal; they may persist as Wormian 

 ossicles. 



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

 anterior ethmoidal cells. Occasionally they invade the horizontal plate, and extend 

 over the roof of the orbit. 



THE EPIPTERIC AND WORMIAN BONES 



The epipterics are bones of variable size which occupy the antero-lateral fonta- 

 nelles, regions indicated in the adult skull by the name pterion. Each epipteric 

 bone is wedged between the squamosal, frontal, greater wing of sphenoid, and the 

 parietal, and is present in most skulls between the second and iifteenth year. 

 After that date it may persist as a separate ossicle, or unite with the frontal or the 

 sijuamosal. In this case it will cause a fronto-squamosal suture, and exclude 

 the parietal from the s[)henoid. More conunonly the epipteric joins the sphenoid. 

 In some skulls it is scarcely as large as a split pea, in others it is as broad as the 

 thumb-nail. The e})ipteric bone is pre-formed in membrane, and ap})ears in the 

 course of the first year. 



The W^ormian bones are small, irregular-shaped ossicles, often found in the 

 sutures of the skull, es])ecially those in relation with the )»arictal bt)nes. Wormian 

 bones sometimes occur in great mmiber; as many as a hundred have been counted 

 in one skull. They are rarely present in the facial sutures. 



