THE HEAD. 37 



Bquamous 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, is wide and deep, and 

 occupied by the wing of the sphenoid 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 cranium, very near, but external to, 

 the ethmoid fossa. Each of these borders, also, is adapted, for a limited 

 extent, to the corresponding palate 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 life ; but commence to form about the fourth month of 

 conception, by a process of resorption, which removes the spongy substance 

 interposed between the two compact tables of bone, and may even cause the 

 destruction of the internal table. The sinuses enlarge with age, and remain 

 during life separated from one another by a vertical septum. 



4. Ethmoid Bone. 



The ethmoid bone, deeply situated in the limit between the cranium and 

 the face, is inclosed between the frontal, the sphenoid, the vomer, the palate, 

 and the supermaxillary bones. It is composed of three portions : a 

 perpendicular plate, and two lateral masses. 



THE PERPENDICULAR LAMINA OF THE ETHMOID BONE. Situated in the 

 median plane, and flattened on both sides, this bone presents two faces, a left 

 and right, and four borders. 



Faces. The faces, covered by the pituitary membrane, present, pos- 

 teriorly, small sinuous crests ; elsewhere they are smooth. A very narrow 

 interval, constituting the bottom of the nasal cavities, separates them from 

 the lateral masses. 



Borders. The superior border looks towards the centre of the cranial 

 cavity, and constitutes what is called the etlimoidal ridge, or crista-galli 

 process. It is free, concave, and sharp, prolonged in front and above by the 

 median crest of the frontal bone, and confounded behind with the middle 

 portion of the inferior sphenoid. The inferior border is continuous with the 

 cartilaginous plate which separates the nasal cavities. When this plate 

 becomes ossified, which is not unfrequent, it is impossible to discover the 

 point where it begins or the ethmoid bone terminates. The middle septum 

 of the nose has been considered, and justly, as a prolongation of the perpen- 

 dicular plate (or lamina) of this bone. The anterior border is consolidated 

 with the vertical septum which separates the frontal sinuses. The posterior 

 border is joined, above, to the median plate which divides the sphenoidal 

 sinuses into two compartments. Below, it is fixed in the groove of the 

 vomcr, and soon becomes confounded with that bone, which is itself conso- 

 lidated with the inferior sphenoid. 



LATERAL MASSES OF THE ETHMOID BONE. These are two large pyriform 



