66 
COMPARATIVE ZOOLOGY. 
in rows from the gums of the upper jaw. In some Whales 
there are about 300 plates on each side. 25 
True teeth, consisting mainly of a hard, calcareous sub¬ 
stance called dentine , are found only in back-boned ani¬ 
mals. They are distinct from the skeleton, and differ 
from bone in containing more min¬ 
eral matter, and in not showing, 
under the microscope, any minute 
cavities, called lacunce. A typical 
tooth, as found in Man, consists of 
a central mass of dentine , capped 
with enamel and surrounded on 
the fang with cement . The first 
tissue is always present, while the 
others may be absent. It is a mixt¬ 
ure of animal and mineral matter 
Fig. 31.—Section of Human Mo- . „ „ 
iar, enlarged: k, crown; n, disposed in the form ot extremely 
demiuc- f rc C meut; ! '^ e p^P- fil5e t,lbeS and Cells > S0 nlillnte aS to 
cavit y- prevent the admission of the red 
particles of blood. One modification of it is ivory, seen 
in the tusks of Elephants. Enamel is the hardest tissue 
of the body, and contains not more than two per cent, of 
animal matter. It consists of six-sided fibres set side by 
side, at right angles to the surfaces of the dentine. Ce¬ 
ment closely resembles bone, and is present only in the 
teeth of the higher animals. 
Teeth are usually confined to the jaws; but the num¬ 
ber, size, form, structure, position, and mode of attachment 
vary with the food and habits of the animal. As a rule, 
animals developing large numbers of teeth in the back 
part of the mouth are inferior to those having fewer teeth, 
and those nearer the lips. The teeth of Mammals only 
have fangs. 
The teeth of Fishes present the greatest variety. In 
number, they range from zero to hundreds. The Hag 
