THE FRONTAL BONE 79 



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 w r hich articulates with the greater wing of the sphenoid. 



Horizontal or Orbital Portion. This portion of the bone consists of two thin 

 plates, the orbital plates, which form the vault of the orbit, separated from one 

 another by a median gap, the ethmoidal notch. 



Surfaces. Orbital Surface. The surface of each orbital plate (fades orbitalis) 

 consists of a smooth, concave, triangular lamina of bone, marked at its front 

 and external part (immediately beneath the external angular process) by a shallow 

 depression, the lacrimal fossa (fossa glandulaelacrimalis), occupied by the lacrimal 

 gland; and at its anterior and internal part by a depression (sometimes a small 

 tubercle), the trochlear fossa (fovea trochlearis^), for the attachment of the carti- 

 laginous pulley of the Superior oblique muscle of the eye. These plates are united 

 in front by a roughened uneven surface called the nasal process, which articulates 

 in front with the nasal bones, laterally with the nasal process of each maxilla. 

 From the middle of the nasal process a thin lamina of bone (the nasal spine} 

 projects downward and forward; on either side of this is a shallow groove, which 

 enters into the formation of the nasal fossa. The nasal spine forms part of the 

 septum of the nose, articulating in front with the nasal bones and behind with 

 the perpendicular plate of the ethmoid. The ethmoidal notch (indsura ethmoidalis) 

 separates the two orbital plates; it is quadrilateral, and occupied, when the bones 

 are united, by the cribriform plate of the ethmoid. The margins of this notch 

 present several half cells, which, when united with corresponding half cells on the 

 upper surface of the ethmoid, complete the ethmoidal cells ; two grooves are also seen 

 crossing these edges obliquely; they are converted into canals by articulation with 

 the ethmoid, and are called the anterior and posterior ethmoidal canals (foramen 

 ethmoidale anterius and foramen ethmoidale postering); they open on the inner wall 

 of the orbit. The anterior one transmits the nasal nerve and anterior ethmoidal 

 vessels, the posterior one the posterior ethmoidal vessels. In front of the ethmoidal 

 notch, on each side of the nasal process, is the opening of the frontal air sinus 

 (sinus frontalis). These are two irregular cavities, which extend upward and 

 outward, a variable distance, between the two tables of the skull, and are separated 

 from each other by a thin bony septum (septum sinuum frontalium), which is 

 often displaced to one side. Within the sinuses imperfect trabeculae of bone 

 often exist. The sinuses are beneath and give rise to the prominences above 

 the supraorbital arches called the superciliary ridges (arcus super dliares). The 

 frontal air sinuses are absent at birth, become apparent about the seventh year of 

 life, and from this period until the age of twenty gradually increase in size. Some- 

 times, however, the sinuses remain very small or never develop at all or one side 

 may be large and the other small or one may exist on one side and be absent 

 on the other. 1 The right sinus is usually the larger. These cavities are larger 

 in men than in women. The floor of each sinus is very thin and is over the orbit 

 and the upper border of the lateral mass of the ethmoid. The thinnest portion 

 of the floor is at the upper and inner angle of the orbit, and at this point pus is 

 liable to point in cases of empyema of the sinus. The frontal sinuses are lined by 

 mucous membrane and each sinus communicates with the middle meatus of the 

 nose by the infundibulum. In some cases the sinuses ymmunicate with each 

 other by means of an aperture in the septum and occasionally join the sinus in 

 the crista galli of the ethmoid. 2 



The internal surface (cerebral surface, fades cerebralis) of the horizontal portion 

 presents the convex upper surfaces of the orbital plates, separated frdm each other 



1 Dr. D. Kerfoot Shute. Article on the Skull, in Reference Handbook of the Medical Sciences. 2 Ibid. 



