STRUCTURE AND GROWTH OF INCISOR TEETH Sill 
The mandibular symphysis is formed of fibrous tissue and 
allows independent rotation of either ramus with its contained 
tooth. This lateral movement of the lower incisors appears 
to be under the control of the will of the animal. According 
to the observations of Jolyet and Chaker (’75) this mobility has 
a definite purpose in mastication. They observed a rapid alter- 
Fig. 4 Cross-sections of the (a) upper and (b) lower incisor teeth of a 5-month 
albino rat, taken near the alveolar margins. These show the arrangement of 
the enamel and the dentine, and the difference in contour of the enamel in the 
upper and lower teeth. The mesial surface cf each tooth is towards the right 
side. >< 15: 
nate separation and approximation of the tips of the lower in- 
cisors in the act of attempting to bite into a match or other 
slender object offered to the animal. At the same time the 
upper incisors were held stationary. 
Mention may be made here of a point of variation among 
the Rodentia in the relation of the angle of the lower jaw to the 
sheath of bone around the lower incisor. In the Myomorphi 
and Sciuromorphi the angle arises from the lower surface of the 
incisive sheath, while in Hystrix the angle arises entirely on 
the outer side. 
