OSTEOLOGY. 



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 is the nasal spine, a sharp-pointed eminence which projects downwards and 

 forwards, and articulates in front with the crest of the nasal bones ; behind, it is 

 marked by two grooves, separated by a vertical ridge : the ridge articulates with 

 the perpendicular lamella of the ethmoid, the grooves form part of the roof of the 

 nasal fossae. On either side of the base of the nasal spine are the openings of the 

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

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

 from one another by a thin bony septum. They give rise to the prominences 

 above the root of the nose, called the nasal eminences and superciliary ridges. In 

 the child they are generally absent, and they become gradually developed as age 

 advances. These cavities vary in size in different persons, are larger in men than 

 in women, and are frequently of unequal size on the two sides, the left being 

 commonly the larger. Occasionally they are subdivided by incomplete bony 

 Iamina3. They are lined by mucous membrane ; and communicate with the nose 

 by the infundibulum, and occasionally with each other by apertures in the 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 anterior 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, 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 laminas. 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. 29). The frontal bone is formed in membrane, being 



developed by two centres, one for each 

 lateral half, which make their appearance, 

 at an early period of foetal life, in the situ- 

 ation of the orbital arches. From this 

 point, ossification extends, in a radiating 

 manner, upwards into the forehead, and 

 backwards over the orbit. At birth, it 

 consists of two pieces, which afterwards 

 become united along the middle line, by 

 a suture which runs from the vertex to 

 the root of the nose. This suture be- 

 comes obliterated within a few years after 

 birth ; but it occasionally remains through- 

 out life. 



Articulations. "With twelve bones : two 

 parietal, sphenoid, ethmoid, two nasal, 

 two superior maxillary, two lachrymal, and two malar. 



Attachment of Muscles. To three pairs: the Corrugator supercilii, Orbicularis 

 palpebrarum, and Temporal. 



Fig. 29. Frontal Bone at Birth. 

 Developed by two lateral Halves. 



