XXXI 
IVORY 
401 
smaller the area of the secondary dentine. In the spot 
ball, the bead is usually let into the ‘"nerve.” 
On one occasion a billiard ball received a smart 
stroke from a cue; to the astonishment of the player a 
large core of secondary dentine as thick as a thumb fell 
out. This core traversed the complete axis of the ball. 
Transverse section through an elephant’s 
tusk showing its elliptical outline, 
remains of the pulp chamber, the con¬ 
centric rings and bark. 
The specimen is preserved in the museum of the Royal 
College of Surgeons. 
The billiard ball is turned in the rough and kept in a 
warm room at an equable temperature for two years : it is 
then turned true. A well-seasoned billiard ball should 
be a perfect sphere with a diameter of 2^^^ inch, and 
weigh five ounces. Its contour is always changing; it 
tends to become ovoid. In addition to this inherent 
change, an ivory ball contracts with cold and expands 
with heat. During a match, expert players take care to 
avoid the balls becoming chilled by currents of cold air, 
as they are then irresponsive to fine strokes. 
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