STRUCTURE AND GROWTH OF INCISOR TEETH 89 
It is interesting to compare these results with those obtained 
on the rabbit—the only other rodent which has been carefully 
studied in respect to the growth of its incisors. MacGillavry 
(75), using a young adult rabbit, made marks upon its lower 
incisors 2.6 mm. and 3 mm. from the tip. After five to seven 
days the marks had disappeared. Evidently the rate of growth 
was about 2.5 to 3 mm. per week. Noé (’02) used a rabbit 
which happened to possess overgrown teeth. The animal 
accidentally broke off the lower incisors in the bars of its cage, 
and Noé made observations upon the rate of their growing 
out. This he found to be .615 mm. per day, or 4.3 mm. per 
week. This is larger than MacGillavry’s results and may have 
been due to the unopposed growth and to the other abnormal 
conditions which may have been present in the formative organs. 
Using MacGillavry’s figures for comparison, it would seem 
that the lower teeth of the albino rat and of the rabbit grow 
out at about the same rate. 
OVERGROWTH OF INCISORS 
Examp!es of overgrowth of the incisors of rodents, especially 
in rabbits and hares, which were hunted as food, must have 
been observed from early times. In the older literature, they 
are referred to principally as curiosities, which have excited the 
interest of whoever has found them. Later the causes of the 
malformations were also considered. Thus Jenyns (’29), to 
cite only one observer, found several examples in wild rabbits, 
and has given a good illustration of the curved aspect of the 
teeth. He also clearly states the several causes which, in his 
opinion, may give rise to the condition. In addition to the one 
usually accepted at his time—accidental breaking off of one 
tooth—he considered also as causes (a) too soft food, (b) morbid 
or too rapid secretion of the osseous matter of the teeth, and 
(c) dislocation of one of the condyles. 
Wiedersheim (’02—’03) has reported a case occurring in a rat, 
where he found an associated assymmetry of the cranium. He 
is in doubt as to which was cause and which was effect—the 
overgrowth of the teeth or the assymmetry of the cranium. 
