64 



THE SKELETON. 



edges transversely ; they are converted into canals by articulation with the ethmoid, 

 and are called the anterior and posterior ethmoidal canals : they open on the inner 

 wall of the orbit. The anterior one transmits the nasal nerve and anterior 

 ethmoidal vessels, the posterior one the posterior ethmoidal vessels. In front of 

 the ethmoidal notch, on either side of the nasal spine, are the openings of the 

 frontal sinuses. These are two irregular cavities, which extend upward and 

 outward, a variable distance, between the two tables of the skull, and are 

 separated from one another by a thin, bony septum, which is often displaced to 

 one side. They give rise to the prominences above the supraorbital arches called 

 the superciliary ridges. In the child they are generally absent, and they become 

 gradually developed as age advances. These cavities vary in size in\lifferent 

 persons, are larger in men than in Avomen, and are frequently of unequal size on 

 the two sides, the right being commonly the larger. They are lined by mucous 

 membrane, and communicate with the nose by the infundibulum, and occasionally 

 with each other by apertures in their septum. 



The internal surface of the horizontal portion presents the convex upper 

 surfaces of the orbital plates, separated from each other in the middle line by the 

 ethmoidal notch, and marked by eminences and depressions for the convolutions of 

 the frontal lobes of the brain. 



Borders. The border of the vertical portion is thick, strongly serrated, bevelled 

 at the expense of the internal table above, where it rests upon the parietal bones, 

 and at the expense of the external table at each side, where it receives the lateral 

 pressure of those bones ; this border is continued below into a triangular rough 

 surface which articulates with the great wing of the sphenoid. The border of the 

 horizontal portion is thin, serrated, and articulates with the lesser wing of the 

 sphenoid. 



Structure. The vertical portion and external angular processes are very thick, 

 consisting of diploic tissue contained between two compact laminae. The hori- 

 zontal portion is thin, translucent, and composed entirely of compact tissue ; hence 

 the facility with which instruments can penetrate the cranium through this part of 

 the orbit. 



Development (Fig. 30). The frontal bone is formed in membrane, being devel- 

 oped by two centres, one for each lateral half, which make their appearance about 

 the seventh or eighth week, above the orbital arches. From this point ossification 

 extends, in a radiating manner, upward into the forehead and backward over the 

 orbit. At birth the bone consists of tAvo pieces, which afterward become united, 

 along the middle line, by a suture which runs from the vertex to the root of the 

 nose. This suture usually becomes obliterated within a few years after birth ; but 



it occasionally remains throughout life, 

 constituting the metopic suture. Secondary 

 centres of ossification appear for the nasal 

 spine one on either side at the internal 

 angular process where it articulates with 

 the lachrymal bone; and sometimes there is 

 one on either side at the lower end of the 

 coronal suture. This latter centre some- 

 times remains ununited, and is known as 

 the pterion ossicle, or it may join with the 

 parietal, sphenoid, or temporal bone. 



Articulations. With twelve bones: 

 two parietal, the sphenoid, the ethmoid, 

 two nasal, two superior maxillary, two 

 lachrymal, and two malar, 

 pairs : the Corrugator supercilii, Orbicu 



FIG. 30. Frontal bone at birth, 

 two lateral halves. 



Developed by 



Attachment of Muscles. To three 

 laris palpebrarum, and Temporal, on each side. 



