204 EVOLUTION AND DISEASE. 
metres round the curve. In the museum of the Odonto- 
logical Society of Great Britain the incisor of a hippo- 
potamus (a huge pig) is preserved which has described a 
complete circle, the point of the tooth re-entering its 
own pulp chamber. The circle formed by this overgrown 
tooth has a diameter of forty centimetres. 
The remarkable tusks of the babirussa, especially 
when the animal is confined in zoological gardens, are 
exceedingly prone to take an abnormal course, and 
instead of forming graceful curves beside the head, may 
Fic. 110.—The head of a Babirussa. The upper canines are 
re-entering the skull. 
deviate towards the middle line and enter the skull. 
Such an example taken from life is sketched in fig. 110 ; 
in order to prevent disaster the babirussa was thrown and 
ten centimetres of the tusks removed ; they had, how- 
ever, penetrated to the depth of twelve millimetres. 
This deviation of the upper canines may be in part 
accounted for by the fact that, like tusks in general, 
they are slightly movable in their sockets, hence by 
rubbing them against the sides of the dens or cages, 
a false direction is impressed upon them. Careful 
