THE TEMPORAL BOXES. 



173 



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 different persons, are larger in men than 

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

 commonly the larger. They are subdivided by a bony lamina, which is often dis- 

 placed to one side. 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. 137). 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 two 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 fe\v years after birth ; but 

 it occasionally remains throughout life. Occasionally secondary centres of ossifica- 

 tion 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 sometimes 

 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. 



Attachment of Muscles. To three 

 pairs : the Corrugator supercilii, Orbicu- 

 laris palpebrarum, and Temporal, on 

 each side. 



FIG. 137. Frontal bone at birth. Developed by 

 two lateral halves. 



The Temporal Bones. 



The Temporal Bones (tempus, time) are situated at the sides and base of the 

 skull, and present for examination a squamous, mastoid. and petrous portion. 



The squamous portion (squama, a scale), the anterior and upper part of the 

 bone, is scale-like in form, and thin and translucent in texture (Fig. 138). Its outer 

 surface is smooth, convex, and grooved at its back part for the deep temporal 

 arteries ; it affords attachment to the Temporal muscle and forms part of the 

 temporal fossa. At its back part may be seen a curved ridge part of the temporal 



