FRONTAL BONE. 35 



The orUto-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 

 ethmoidal fissure. The edges of the ethmoidal fissure are hollowed 

 into cavities, which, by their 

 union with the ethmoid bone, Fi g- 12.* 



complete the ethmoidal cells ; 

 and, crossing these edges trans- 

 versely, are two small grooves, 

 sometimes canals, which open 

 into the orbit by the anterior 

 and posterior ethmoidal fora- 

 mina. At the anterior termi- 

 nation of these edges, are the 

 irregular openings which lead 

 into the frontal sinuses ; and be- 

 tween the two internal angular 

 processes is a rough excavation 

 which receives the nasal bones, 

 and a projecting process, the 

 nasal spine. Upon each orbital 

 plate, immediately beneath the 

 external angular process, is a shallow depression which lodges the 

 lachrymal gland ; and beneath the internal angular process a small 

 pit, sometimes a tubercle, to which the cartilaginous pulley of the 

 superior oblique muscle is attached. 



Internal Surface. Along the middle line of this surface is a 

 grooved ridge, the edges of the ridge giving attachment to the falx 

 cerebri and the groove lodging the superior longitudinal sinus. At 

 the commencement of the ridge is an opening, sometimes completed 

 by the ethmoid bone, the foramen ccecum. This opening lodges a 

 process of the dura mater, and occasionally gives passage to a 

 small vein which communicates with the nasal veins. On each side 

 of the vertical ridge are some slight depressions which lodge the 

 glandule Pacchioni, and on the orbital plates a number of irre- 

 gular pits called digital fossa;, which correspond with the convolu- 

 tions of the anterior lobes of the cerebrum. The superior border is 

 thick and strongly serrated, bevelled at the expense of the internal 

 table in the middle, where it rests upon the junction of the two 

 parietal, and at the expense of the external table, on each side, 

 where it receives the lateral pressure of those bones. The infe- 



*The external surface of the frontal bone. 1. The situation of the frontal emi- 

 nence of the right side. 2. The superciliary ridge. 3. The supra-orbital ridge. 4. 

 The external angular process. 5. The internal angular process. 6. The supra-orbital 

 notch for the transmission of the supra-orbital nerve, and artery ; in the figure it is 

 almost converted into a foramen by a small spiculum of bone. 7. The nasal tubero- 

 sity ; the swelling around this point denotes the situation of the frontal sinuses. 8. 

 The temporal ridge commencing from the external angular process (4). The depres- 

 sion in which the figure 8 is situated is a part of the temporal fossa. 9. The nasal spine. 



