MASTICATION. 245 



while they cease to correspond on the opposite side. It is, therefore, to 

 a certain extent, a circular motion, in which the axis of the lower jaw 

 crosses that of the upper. A further peculiarity of this lateral motion 

 of the lower jaw is that it is alternative; that is, there is not a deviation 

 first to the right and then to the left, but if the motion is first a deviation 

 to the right in the process of mastication it returns again to its central 

 position and again rotates to the right. This may occur for half an hour 

 or more in solipedes, and in ruminants in both first mastication and in rumi- 

 nation ; then the motion may be reversed, and ivay occur as a left lateral 

 deviation for a similar length of time. The camel is the only animal 

 which furnishes an exception to this method of mastication. In it the 

 lateral motion is alternative ; the deviation occurring first to the right 

 and then to the left of the central position. In solipedes this motion is 

 apparently not as marked as in the ruminants, but this difference is 

 merely apparent, it being to a certain extent concealed by the long lips 

 of these animals. It is, therefore, more evident but not actually greater 

 in the ruminants than in solipedes. Lateral motion of the lower jaw is 

 produced by alternate contractions of the pterygoids, especially the in- 

 ternal, the external being very small in ruminants, and by the masseters ; 

 when the deviation occurs to the right the motion is produced by con- 

 traction of the right masseter and left internal pterygoid muscles. When 

 the deviation occurs to the left it is produced by contractions of the left 

 masseter and right internal pterygoids, the action of the pterygoids being 

 more marked in ruminants than in solipedes from the fact that in the 

 former animals the palatine ridges are nearer together ; therefore, the 

 lateral power of the pterygoid muscles is more marked. 



2. The Action of the Teeth in Mastication. — The teeth are passive 

 organs of mastication, which are imbedded in the alveoli of the jaws. 

 Teeth may be divided into three different parts : the crown, or the part 

 which projects into the mouth above the gum ; the neck of the tooth, where 

 it passes through the gum ; and the root, which is imbedded in the alveolus. 

 Teeth may be of two different kinds,— either simple, where the entire 

 external surface of the tooth is covered by enamel; or compound, where 

 two different substances, enamel and dentine, compose the free surface. 

 When a tooth is divided longitudinally it is found to consist of three 

 different substances ; the hardest, and that which in simple teeth covers 

 the crown, is termed the enamel, and passes over the neck of the tooth, 

 becoming gradually thinner, and only partially covering the fang. The 

 enamel (Fig. 93) is composed of pentagonal or hexagonal prisms, or 

 enamel fibres, of about one five-thousandth of an inch in diameter, 

 closely packed together and arranged in a radiating manner from the 

 surface of dentine below. The enamel contains no nutrient vessels, and 

 when destroyed is not renewed. The bulk of both crown and fang of 



