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 correspc >nd internally 

 with tin 1 small crests to which the colls are attached. This lamella is 

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

 iiinl turns outwards to articulate with the palate and superior maxillary 

 l.oiirs ; superiorly, it coalesces with the sphenoid and the orbital portion of 

 the frontal hone. 



Int< i-milly, 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 

 tlu- 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 

 septum between the cranium and the nasal cavit>es. This 

 septum is perforated by openings which give passage to the ethm/ . .1 nerves ; 

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

 the superior surface, which constitutes the ethmoidal fossa., nd convex on 

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

 tin- 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 

 interior 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 

 sees 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 Done. 



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 

 interior half, are prolonged in the form of alx or tciiujs. It has two surfaces 

 and four borders. 



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

 median line, the external surface of the body, rounded fr.uu one side to tlie 

 other, is eontinued with that of the basilur process, and lias marked mus- 

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

 the vtdittn (or f>l>ri/<ii<l ) jixsiti >. din cted from above downwards, and con- 

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

 1 It is not rare t" fiii'l it <i|miiii;r into tli-- 



