56 TEE BONES. 



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

 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. — Yery 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 ex- 

 tremity of the cells, and advances progressively from below upwards. The per- 

 pendicular 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 transforma- 

 tion having scarcely been achieved when the animal is six or eight mouths old. 



Differential Characters in the Ethmoid Bone of other Animals. 



A. Ox, Sheep, Goat. — In Ruminants, the great ethmoidal cell is enormously developed, 

 and looks like a third turbinated bone prolonged beyond tlie usual two; it has been named 

 the olfactory antrum. The bone is closely imprisoned between the adjacent bones, in con- 

 sequence of the slight development of the sinuses around it. This character otherwise belongs 

 to all the domesticated animals, except Solipeds (Fig. 36). 



B. Camel. — The crista-galli process is very thick, and the ethmoidal fossae are narrow 

 and deep. 



C Pig.— The superior turbinated bone is very long, and the papyraceous plate appears in 

 the orbital cavity. 



D. Dog, Cat. — The ethmoidal fossa is very deep, and the cells very developed and 

 diverticulated. The perpendicular lamina is at a late period consolidated with the sphenoid 

 bone. 



5. Sphenoid Bone (Fig. 34). 



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

 ethmoidal, palatine, vomer, pterygoid, frontal, and temporal bones. It is formed 

 by the union of two pieces, which have been sometimes described as distinct 

 bones — the anterior or inferior sphenoid, and the posterior or superior sphenoid. 

 It is now supposed, in the description, that this union has been completed. 



It is a bone flattened before and behind, curved from one side to the 

 other, thick in its middle part, named the hody, and thin on the sides, which, in 

 their inferior half, are prolonged in the form of alev, 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 muscular imprints 

 superiorly. 2. On the sides and from within outwards : (a) the Vidian (or 

 ptenjjjoid) fissure, directed from above downwards, and continued by the Vidian 

 canal, a very small foramen which opens into the orbital hiatus ; {li) the sub- 

 sphenoidcd, or pterygoid process, a long eminence, flattened on both sides, inclining 

 downwards, articulating with the palatine and pterygoid bones, and traversed at 

 its base by the Vidian canal ; (r) a little behind and above this eminence, the 

 superior orifice of the sub-sphenoidtd {or pterygoid) foramen — a large canal which 

 bifurcates inferiorly ; (d) more in front, the orbital hiatus, a kind of vestibule into 



