204 E VOL UTION 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 



FIG. no. 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. 1 10 ; 

 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 



