206 EVOLUTION AND DISEASE. 
mammal. The centre of every growing tooth is occupied 
by a cavity known as the pulp chamber, which in the 2 
living tooth is filled with connective tissue, blood-vessels, 4 
nerves, and cells known as odontoblasts. The tissues — 
constituting the pulp are actively engaged in forming 
dentine ; in teeth and tusks with persistent pulps this 
pulp chamber is always relatively large, and as the tooth 
is worn at the apex the pulp adds new material at the 
base. In teeth with non-persistent pulps, like those of 
primates, carnivora, and the like, the pulp chamber 
diminishes with age, and in some the pulp becomes | 
converted into bony substance known as osteo-dentine. 
When teeth become inflamed or carious the pulp often 
calcifies, and it is no unusual event to find in a carious 
tooth extensive ossification of the pulp in the neighbour- 
hood of the carious cavity: this is salutary, for by this 
means suppuration of the pulp is prevented and often 
a tooth remains serviceable much longer than would 
otherwise be the case. 
Ivory-turners have often found in the tusks of ele- 
phants such things as bullets, iron slugs, and spear-heads, 
yet on an attentive examination no trace of injury 
could be detected on the exterior of the tusk. These 
specimens attracted the attention of investigators such 
as Blumenbach, Haller, Cuvier, Goodsir, and Owen. The 
solution of the mystery was indicated by a study of the 
material surrounding the foreign body. Cuvier detected 
the irregularity of the dentine immediately surrounding 
the bullet, and Goodsir made an elaborate study of the 
whole question, and it became clear from the researches 
of Goodsir, Nasymth, and Owen, that this irregular 
