THE HEAD. 39 



very extensive and convex, looks outwards in front and behind, and is covered 

 by an osseous plate traced with shallow furrows, which correspond internally 

 with the small crests to which the cells are attached. This lamella is 

 prolonged, inferiorly, a little beneath the inferior extremity of these latter, 

 and turns outwards to articulate with the palate and superior maxillary 

 bones ; superiorly, it coalesces with the sphenoid and the orbital portion of 

 the frontal bone. 



Internally, the lateral masses are hollowed from above to below by 

 extremely diverging canals, which open inferiorly into the nasal cavities, 

 and separate the cells from one another. The latter are so incurvated that 

 the internal cells communicate with each other. There are some, however, 

 which are completely closed ; the anterior, or great cell, is frequently so. 1 



Base. The base of each lateral mass looks upwards, and is formed by the 

 transverse septum between the cranium and the nasal cavities. This 

 septum is perforated by openings which give passage to the ethmoidal nerves ; 

 it is named the cribriform plate of the ethmoid bone. It is concave on 

 the superior surface, which constitutes the ethmoidal fossa, and convex on 

 the opposite face, where attachment is given to the superior extremities of 

 the cells. It is consolidated internally with the perpendicular plate ; the 

 other points of its circumference are attached to the sphenoid bone, and to 

 the transverse ridge on the internal face of the frontal bone. 



Summit. The summit of each lateral mass is formed by the inferior 

 extremity of the ethmoidal cells, which is directed downwards towards the 

 nasal cavities. One, more voluminous than the others, is carried much 

 lower, and terminates by a rounded protuberance. It corresponds to the 

 middle cornu (concha media) of Man. 



Structure of the ethmoid bone. Very little spongy tissue enters into the 

 composition of this bone, and this is only found near the anterior border of 

 the perpendicular plate. 



Development. The ethmoid bone is late in attaining its development, 

 and the adjoining bones are nearly completely ossified when it is yet 

 entirely cartilaginous. The bony transformation commences in it at the 

 inferior extremity of the cells, and advances progressively from below 

 upwards. The perpendicular plate is only ossified in part when the cells 

 have passed through the first half of the process; at the same time it 

 coalesces with the inferior sphenoid. The cribriform plate is the last to 

 become ossified; this transformation having scarcely been achieved when 

 the animal is six or eight months old. 



5. Sphenoid Bone. 



The sphenoid bone is situated behind the cranium, between the occipital, 

 ethmoidal, palate, vomer, pterygoid, frontal, and temporal bones. It is 

 flattened from before to behind, curved from one side to the other, thick in 

 its middle part, named the body, and thin on the sides, which, in their 

 inferior half, are prolonged in the form of a Zee or wings. It has two surfaces 

 and four borders. 



Surfaces. The external surface is convex, and presents: 1, On the 

 median line, the external surface of the body, rounded from one side to the 

 other, is continued with that of the basilar process, and has marked mus- 

 cular imprints superiorly ; 2, On the sides and from within outwards : a, 

 the vidian (or pterygoid) fissure, directed from above downwards, and con- 

 tinued by the vidian canal, a very small foramen which opens into the 

 1 It is not rare to find it opening into the frontal sinus. 



