36 THE BONES. 



which is added a single centre to form the parietal eminence. 1 In early 

 life the parietal ridges are absent. 



3. Frontal Bone. 



The frontal is a flat quadrilateral bone, whose sides are bent in the 

 middle at an acute angle, and are carried back, and a little inwards, to meet 

 the wings of the sphenoid bone. It assists in forming the cranial roof and 

 part of the face. It is bordered : above, by the parietal bone ; below, by the 

 nasal and lachrymal bones; and on each side, by the temporal bones. It 

 offers for study an external and an internal face, and four borders. 



Faces. The external face is divided, by the double flexure of the bone, 

 into three regions : a middle and two lateral. The first, nearly plane, is 

 lozenge-shaped, is covered by the skin, and constitutes the base of the fore- 

 head. It gives rise on each side, at the point where it is inflected, to a long 

 process, flattened above and below, which curves backward, forming the orbital 

 arcli. The superior or external face of this process is convex and slightly 

 roughened ; the internal face is smooth and concave, and forms part of the 

 orbital fossa. Its posterior border, thick and concave, is continued, in- 

 wardly, with the corresponding parietal ridge, and outwardly with the 

 superior border of the zygomatic process. It limits, in front, the temporal 

 fossa. The anterior border, also concave, but thin, concurs in the forma- 

 tion of the orbital margin ; the summit, thickened and denticulated, rests 

 upon, and is united to, the zygomatic process of the temporal bone ; the base 

 is wide, and is traversed by an opening termed the supraorbital, or super- 

 ciliary foramen. The two lateral regions of the external face of the frontal 

 bone are slightly excavated, and assist, for the greater portion of their 

 extent, to form the orbits. They often show, near the base of the orbital 

 arch, a small depression corresponding to the flexure described by the 

 great oblique muscle of the eye in passing 'through its pulley. 



The . internal face of the frontal bone is concave, and divided into two 

 unequal parts by a transverse ridge, corresponding to the anterior border of 

 the cribriform plate of the ethmoid bone. The superior, the most extensive, 

 is covered with digital impressions, and belongs to the cranial cavity. It 

 exhibits : 1, On the median line, a slight furrow, or a crest which is con- 

 tinuous, above, with the median ridge of the parietal bone, and below, with 

 the crista- galli process ; 2, On the sides, and in the re-entering angle formed 

 by the flexure of the bone, there is a narrow slit, or mortice, which receives 

 the wing of the sphenoid bone. The inferior part is united, on the median 

 line, to the perpendicular plate of the ethmoid. It assists in forming the 

 bottom of the nasal cavities, and presents laterally two large openings 

 which lead to the frontal sinuses vast anfractuous spaces excavated 

 between the two plates of the bone. 



Borders. The superior border is denticulated, and cut obliquely, in its 

 middle portion, at the expense of the internal plate, and on the lateral parts 

 at the expense of the external table ; it is in contact with the parietal and 



1 This centre is described as the interparietal bone by those anatomists who consider 

 the two lateral centres as two distinct parietals. (Leyh is one of the veterinary anatomists 

 who describe this nucleus as a separate bone, designating it the /aZci/brm or interparietal 

 bone. He also describes the parietal as a pair or double bone ; whereas the majority of 

 French anatomists include the interparietal bone as the median protuberance of the 

 parietal, which they look upon as a single or impair bone. Percivall names it a pair bone, 

 but follows the example of the French hippotomists with regard to the interparietal.) 



