54 ANATOMY 



In rats at about 30 days of age the extrusive growth of the 

 superior incisor teeth amounts to about 1.9 mm. per week; of 

 the inferior incisor teeth, to about 2.3 mm. per week. The 

 curvative growth of the superior incisor teeth amounts to about 

 1.0 mm. per week; of the inferior incisor teeth, to about 1.6 

 mm. per week (Marshall '21). 



Throughout life growth continues, and in the adult animal 

 is on the average 2.2 mm. per week in the upper and 2.8 mm. 

 per week in the lower incisor. 



In a five months animal the thickness of the enamel and its 

 constituent layers measured in the mid-line of the teeth is given 

 in table 19. 



TABLE 19 



The interstitial growth of the dentine of the persistent teeth 

 of the rat, as shown by azo-dyes under the microscope, amounts 

 approximately to 0.01 mm. per day (Marshall '21). 



Measurements of the incisors and skulls of animals of dif- 

 ferent ages were made and are shown in table 20. 



Displacement of incisors. Owing to various defects and ac- 

 cidents, one or more of the incisor teeth are sometimes dis- 

 placed so that they do not wear normally. Overgrowth of the 

 unopposed teeth follows, sometimes causing the death of the 

 animal. 



The incisor teeth are affected by parathyroidectomy (Ham- 

 mett '22). After this operation they become white, opaque and 

 fragile. Often also they overgrow as the result of displace- 

 ment. 



