124 INTRODUCTION. 



mutual action of which, on each other, resembles that of the blades of 

 shears, are inconsistent with the liberty of motion which the lower jaw of 

 Man and of the Simiae possesses, and, still more so with that of the lower 

 jaw of the ruminants. In these latter is seen a modification of the glenoid 

 cavity, the reverse of what obtains in the Carnivora ; one which admits 

 of a free, lateral, rotatory motion, in accordance with the structure 

 of the molars, which have to grind to a pulp the mass of herbage sub- 

 jected to their action. The glenoid surface for cavity it can scarcely be 

 called is somewhat in the shape of a half-moon, its concave edge (the 

 edge of the zygomatic process) being the anterior : its posterior margin is 

 elevated, but not very boldly ; and its face is flat, or rather, slightly 

 convex, with a shallow furrow, or trench, internally, and, posteriorly, at 

 the root of the elevated margin : to this surface the condyle of the lower 

 jaw is accurately fitted, so as to work upon it with great freedom of mo- 

 tion. The peculiar lateral movements of the jaw, by which the Cow 

 grinds the herbage to a pulp, are familiarly known ; and it is this lateral 

 motion, which the flat and open character of the glenoid surface allows. 

 In the Kangaroo, the glenoid surface very closely resembles that of the 

 Sheep and Goat, but not that of the rodents. The Llama and Camel 

 approach the Horse, in the character of the glenoid cavity. In the 

 Horse, this cavity is remarkable (fig. 128). 



128 



Full view of the articulating condyle of the lower jaw of the Horse, a, and glenoid cavity, b, shewing their surfaces. 



Its anterior margin consists of a broad, smooth ridge, with a convex 

 surface, and arched somewhat like a bridge ; the central depression 

 is oblong, and externally shelves upward and backward, as though its 

 margin had been smoothly cut away ; while a bold process, sweeping 

 down from this oblique surface, just before the auditory foramen, bounds 

 the central depression posteriorly, and is applied to a depressed surface 

 on the inner part of the condyle of the lower jaw, a : from the outer part of 

 the condyle the oblique surface of the glenoid cavity, 6, retreats, so that the 

 greater portion of the condyle is not in contact with any part of the 



