26 FRONTAL BONE. 



completed by the opposite parietal bone, which serves to contain the 

 superior longitudinal sinus. Some slight pits are also observable near 

 this groove, which lodge the glandulae Pacchioni. 



The anterior inferior angle is thin and lengthened, and articulates 

 with the greater wing of the sphenoid bone. Upon its inner surface 

 it is deeply channelled by a groove for the trunk of the arteria menin- 

 gea media. This groove is frequently converted into a canal. The 

 posterior inferior angle is thick, and presents a broad and shallow 

 groove for the lateral sinus. 



Development. By a single centre. Ossification commences at the 

 parietal eminence at the same time with the bodies of the vertebrae. 



Articulations. With five bones ; with the opposite parietal bone, 

 the occipital, frontal, temporal, and sphenoid. 



Attachment of Muscles. To one only, the temporal. The occi- 

 pito-frontalis glides over its upper surface. 



FRONTAL BONE. The frontal bone bears some resemblance in form 

 to the under valve of a scallop shell. It is situated at the anterior 

 part of the cranium, forming the forehead, and assists in the construc- 

 tion of the roof of the orbits and nose. Hence it is divisible into a 

 superior or frontal portion, and an inferior or orbito-nasal portion. 

 Each of these portions presents for examination an external and inter- 

 nal surface, borders, and processes. 



External Surface. At about the middle of each lateral half of the 

 frontal portion is a projection, the frontal eminence. Below these points 

 are the superciliary ridges, large towards tH&fr inner termination, and 

 becoming gradually smaller as they arch outwards : they support the 

 eyebrows. Beneath the superciliary ridges are the sharp and prominent 

 arches which form the upper margin of the orbits, the supra-orbital 

 ridges. Externally the supra-orbital ridge terminates in the external 

 angular process, and internally in the internal angular process; at 

 the inner third of this ridge is a notch, sometimes converted into a fo- 

 ramen, the supra-orbital notch, which gives passage to the supra-orbi- 

 tal artery, veins, and nerve. Between the two superciliary ridges is a 

 rough projection, the nasal tuberosity ; this portion of the bone denotes 

 by its prominence the situation of the frontal sinuses. Extending 

 upwards and backwards from the external angular process is a sharp 

 ridge, the commencement of the temporal ridge, and beneath this a 

 depressed surface that forms part of the temporal fossa. 



The orbito-nasal portion of the bone consists of two thin processes, 

 the orbital plates, which form the roof of the orbits, and of an inter- 

 vening notch which lodges the ethmoid bone, and is called the etltmoidal 

 fissure. The edges of the ethmoidal fissure are hollowed into cavities, 

 which, by their union \vith the ethmoid bone, complete the ethmoidal 

 cells ; and, crossing these edges transversely, are two small grooves, 

 sometimes canals, which open into the orbit by the anterior and posterior 

 ethmoidal foramina. At the anterior termination of these edges are 

 the irregular openings which lead into the frontal sinuses; and be- 



