244 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



The pteiygoid muscles, especially the internal, which is usually the 

 largest, are also elevators of the lower jaw, and are most developed in 

 herbivorous animals ; they are also levers of the third class, and are, to 

 a certain extent, concerned in the production of lateral and antero- 

 posterior motion of the lower jaw in animals where these motions are 

 possible. Propulsion of the lower jaw, or antero-posterior motion, is 

 most marked in the rodents, although it is also present to a less degree 

 in solipedes and ruminants, but is impossible in carnivora on account of 

 the shape of the articulation of the jaws. In this motion the maxillary 

 condyles slide forward on the glenoid fossae of the temporal bone in ani- 

 mals where there may be a subcondyloid apophysis posteriorly and no 

 restricting surface anteriorly. This motion is quite marked in the pig, 

 which has a triangular condyle, but attains its maximum development in 

 the rodent. This motion is accomplished by means of the masseter 

 muscle, aided by the external pterygoids ; for in the rodents, and in a less 

 degree in ruminants, the origin of the most posterior fibres of the mas- 

 seter muscle are in advance of their insertion in the lower jaw. This 

 obliquity in direction of the masseter fibres, in contraction of this muscle, 

 therefore serves to move the jaw forward. 



The retraction of the lower jaw, which of course occurs only in ani- 

 mals in which forward motion is possible, is accomplished by means of 

 the temporal muscle, the digastric being not concerned in the process, 

 since backward motion of the jaws only occurs in closing the mouth, 

 while the digastric in its contraction opens the mouth and even tends to 

 advance the lower jaw somewhat. 



Lateral movement of the lower jaws is more or less pronounced in 

 all herbivora, but is most marked in the ruminants ; it never occurs in 

 the carnivora, as already explained, on account of the shape of the con- 

 dyles and overlapping molars, and the crossing of the canine teeth. The 

 lateral motion of the lower jaw is not a simple lateral displacement 

 parallel to the axis of the lower jaw; that is, it is not equal at both ex- 

 tremities of the jaw, but is an angular deviation very marked at the in- 

 cisor teeth, and is a rotation of the lower jaw around one condyle of the 

 inferior maxillary bone, the incisor teeth describing an arc of a circle, 

 whose centre is one condyle, which thus moves very little, while the oppo- 

 site condyle advances and partly leaves the articular surface, as may be 

 determined by placing the finger in the temporal fossa, when the coronoid 

 process on the side opposite to that toward which rotation is taking place 

 may be felt to move forward. In this lateral motion of the lower jaw 

 the axes of the molar teeth cease to be parallel, the incisor arch passing 

 one-third to one-half its extent to one side of the upper arch or pad 

 which represents it in the ruminants ; the molar teeth of the upper and 

 lower jaw are in contact on the side toward which rotation is occurring, 



