FRONTAL BONE. 



Fisj. 25 * 



inferior angle is thick, and presents 

 a broad and shallow groove for the 

 lateral sinus. 



Development. By a single centre. 

 Ossification commences at the parie- 

 tal eminence at the same time with 

 the bodies of the vertebra?. 



Articulations. With Jive bones : 

 with the opposite parietal bone, the 

 occipital, frontal, temporal, and sphe- 

 noid. 



Attachment of Muscles. To one 

 only, the temporal. The occipito- 

 frontalis glides over its upper sur- 

 face. 



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 construction 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 internal surface, borders, and 

 processes. 



External surface. At about the middle of each lateral half of the fron- 

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

 the superciliary ridges, large towards their 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 foramen, the supra-orbital notch, 

 which gives passage to the supra-orbital artery, veins, and nerve. Be- 

 tween the two superciliary ridges is a rough projection, the nasal tuber osi- 

 ty ; 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, arid of an intervening 

 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, 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 



* The internal surface of the left parietal bone. 1. The superior or sagittal border. 

 2 The inferior, or squamous border. 3. The anterior, or coronal border. 4. The poste- 

 rior, or lambdoidal border. 5. Part of the groove for the superior longitudinal sinus. 

 6. The internal termination of the parietal foramen. 7. The anterior inferior angle of 

 the bone, on which is seen the groove for the trunk of the arteria meningea media. 8 

 The posterior inferior angle, upon which is seen a portion of the groove for the latera. 

 itvud. 



