STRUCTURE OR DEE WVE 
CHAP. 
Fic. 
3.—Diagrammatic sections of various forms 
of teeth. J, Incisor or tusk of Elephant, 
with pulp cavity persistently open at base ; 
ZT, Human incisor during development, with 
root imperfectly formed, and pulp cavity 
widely open at base ; ///, completely formed 
Human incisor, with pulp cavity opening by 
a contracted aperture at base of root; 7V, 
Human molar with broad crown and two 
roots; V, molar of the Ox, with the enamel 
covering the crown deeply folded, and the 
depressions filled up with cement; the sur- 
face is worn hy use, otherwise the enamel 
coating would be continuous at the top of the 
ridges. In all the figures the enamel is black, 
the pulp white ; the dentine represented by 
horizontal, lines, and the cement by dots. 
(After Flower and Lydekker.) 
It has been pointed out 
that the scales of the Elas- 
mobranch fishes consist of 
a cap of enamel upon a 
base of dentine, the former 
being derived from the epi- 
dermis and modelled upon 
a papilla of the dermis 
whose cells secrete the 
dentine. The fact that 
similar structures arise 
within the mouth (ae. the 
teeth) is explicable when 
it is remembered that the 
mouth itself 1s a late in- 
ragination from the out- 
side of the body, and that 
therefore the retention by 
its tissues of the capacity 
to produce such structures 
is not remarkable. 
The . relations of the 
three constituents of the 
tooth in its simplest form 
is shown in the accom- 
panying diagram, where 
the intimate structure of 
the enamel, dentine, and 
cement (or crusta petrosa 
as it 1s sometimes called) 
is not indicated. The latter 
has the closest resemblance 
to bone. The dentine 
traversed by fine 
which run parallel to each 
other and anastomose here 
and there. The enamel is 
formed of long prismatic 
1s 
canals 
fibres, and is excessively hard in structure, containing less animal 
matter than the other tooth tissues. 
To this fact is frequently 
