242 sTRUoTtmE and adai'tation of teeth 



The lower jaw is so much narrower than the 

 upper, that the surfaces of the molars are not in 

 apposition when the jaws are closed, and at rest ; 

 and their crowns are all placed obliquely ; the 

 grinding surfaces of the upper teeth sloping in- 

 wards, while in the lower jaw they incline out- 

 w^ards and downwards. As the crescentiform 

 layers oi*^ enamel in the compound molar teeth 

 assume a longitudinal direction, they are thus 

 brought into efficient service by the great lateral 

 action of the lower jaw. If we watch the sheep 

 or ox when ruminating, the jaw is seen to take a 

 protruding position, describing the segment of a 

 circle as it is alternately swept from side to side. 

 Instead of the transverse hinge-like condyles of 

 the cat tribe, admitting of motion in the vertical 

 plane only, the condyles of the ruminants are 

 flattened at their articular surfaces, and the glenoid 

 cavities are very flat and shallow, and are much 

 wider than the apposed surface of the condyles ; 

 rendering them capable of free horizontal motion, 

 whether laterally or in the antero-posterior direc- 

 tion. As we have noticed, the temporal bone and 

 the zygomatic arch are remarkably small, when 

 compared with the corresponding parts in the 

 camivora, and the deep excavations in the lower 

 jaw for the insertion of the temporal and masseter 

 muscles are not observed, those muscles being very 

 feebly developed. But to meet the very different 

 requirements of the animal, the angle of the lower 



