88 W. H. F. ADDISON AND J. L. APPLETON, JR. 
RATE OF GROWTH OF THE INCISOR TEETH 
Two methods were used for determining the rate of growth 
of the incisor teeth (a) cutting off one or more teeth at the gingi- 
val margin and (b) making marks upon the enamel. The results 
here given are based on the latter method, as giving more nearly 
the normal rate of growth. By means of a dental engine, the 
animal always having been anesthetized, a fine transverse notch 
was made on the enamel of the incisors a short distance from 
the gingival margin. The interval between this mark and 
the tip of the tooth was then measured. At the end of about 
a week the distance between these two limits was again taken, 
and the difference between the two measurements showed the 
amount of wearing away. ‘Two series of experiments were 
made by this method upon adult animals. In the first series 
six animals were used and in the second four animals, and meas- 
urements were made for several consecutive weeks. The long- 
est period that one individual was studied was six weeks. 
The two series gave very similar results. The lower tooth 
was always found to grow more rapidly than the upper. The 
upper tooth averaged 0.31 to 0.32 mm. per day, or 2.2 mm. 
per week, while the lower tooth averaged 0.4 mm. per day, or 
2.8 mm. per week. No doubt there are many variations of the 
rate of growth under different circumstances, so that these 
figures must be taken as representing the average rate under 
one particular set of conditions. The food of these animals 
was the mixed diet now in use in the rat colony of The Wistar 
Institute. A short series of trials was made with animals kept 
in a large glass jar and given only soft food. In these animals 
the rate of wearing away corresponded very closely to that 
seen in animals which have also hard food and have the oppor- 
tunity of gnawing. In these, therefore, the interaction of the 
opposing teeth must have caused the attrition. 
For assistance with these experiments we wish to thank 
Dr. Stotsenburg, who greatly aided us in carrying out our ob- 
servations. 
