ON THE TEETH OF MAMMALIA. 



125 



glenoid cavity while the jaw is at rest. The motion allowed by this arti- 

 culation is a rolling movement from side to side ; but the jaw can neither 

 be advanced nor drawn back, the bridge-like margin, c, preventing a for- 

 ward movement, and the long process, d, a posterior retraction. 



The following figure (129) gives a lateral view of the same parts: 

 a, is the condyle of the lower jaw of a Horse ; 6, the glenoid cavity. 



129 



Lateral view of the articulating coudyle of the lower jaw in the Horse, a, and of the glenoid cavity, 6. 



In the Rodentia, another and still different modification prevails, by 

 which a movement backward and forward is permitted, as well as 

 laterally. The glenoid cavity, instead of being transverse, is longitu- 

 dinal, without an anterior or posterior elevated margin : internally, it is 

 bounded by the flat surface of the squamous portion of the temporal 

 bone ; externally, by a portion of the zygomatic process, at the root of 

 which it is situated. In the Rat, Squirrel, &c., the condyle of the lower 

 jaw is a small convex surface surmounting an elevated branch. In the 

 Hare, however, this condyle is oval, anteriorly, and is continued back- 

 ward, becoming almost linear, along the broad margin of this branch 

 of the jaw : the anterior oval portion only is fitted into a small glenoid 

 cavity, while the rest is closely applied to the side of the temporal bone, 

 to which it is secured by ligament : hence, in the Hare, a lateral motion 

 of the jaw only is afforded ; the condition of the glenoid cavity, and 

 the fixedness of the condyles of the lower jaw, preventing an anterior and 

 posterior movement: whilst, in the Squirrel, Rat, &c., the movement 

 is obliquely lateral, first to one side, then to the other, the jaw alternately 

 advancing and retreating. 



In the Cetacea, the articulation of the lower jaw is such as only 

 to permit a simple hinge-like movement, and its condyles are rigidly con- 

 fined, by fibrous ligaments, to the glenoid cavity. The teeth of the carni- 

 vorous Cetacea (the Dolphin, Porpoise, &c.) do not oppose each other, 

 but alternate with each other, so that those of one jaw pass into the inter- 

 spaces between those of the other. These animals do not masticate their 



