THE HEAD. 



51 



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 supra-orbital, or mperciliari/ 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 continuous, above, with the 

 median ridge of the parietal bone, and below, with 

 the crista-gaJli process. 2. On the sides, and in the 

 re-entering angle formed by the flexure of the bone, 

 there is a narrow slit, or mortise, 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 squamous portion of the temporal 

 bone. The inferior, prolonged to a point in the middle, 

 is in apposition with the nasal bones through the 

 medium of a wide external bevel ; laterally, it is very 

 thin, faintly serrated, and articulates with the lachrymal 

 bone. The lateral borders, thin and irregular, present 

 two notches : one, the superior (incisura sphenoidalis), 

 is wide and deep, and occupied by the wing of the sphe- 

 noid bone ; the other, inferior, is very narrow, and, 

 uniting with a similar notch in the sphenoid bone, forms 

 the orbital foramen, which opens into the craniiun, 



very near, but external to, the ethmoid fossa. Each of these borders, also, is 

 adapted, for a limited extent, to the corresponding palatine bone. 



Structure. — The two compact plates of the frontal bone are separated by 

 spongy texture towards the middle and in the upper part ; they separate below 

 to form the frontal sinuses. Laterally, they are very thin and consolidated with 

 each other. 



Development. — The frontal bone is developed from two lateral centres of 

 ossification, which only coalesce at a late period. In youth the cranial portion 

 of the bone forms, in front of the head, a large rounded protuberance standing 

 beyond the facial portion. This prominence disappears when the frontal sinuses 

 begin to be developed. These cavities do not exist at an early period of foetal 



HEAD OF DOG (ANT&RIOR 



face). 



1, Occipital tuberosity; '2, 

 median spur of the occi- 

 pital bone ; 3, parietal 

 bone ; 4, origin of the pa- 

 rietal crests; 5, zygomatic 

 process ot the temporal 

 i(oue ; 6, frontal bone ; 6', 

 orbital jtrocess ; 7, malar 

 bone; 8, lachrymal bone; 

 9, nasal bone ; 10, super- 

 tnaxilla ; 11, inferior ori- 

 fice of the supermaxillo- 

 dental canal ; 12, pre- 

 maxillary bone. 



