HOW ANIMALS EAT. 65 
Whale. In the former, the plates consist of closely set 
vertical hollow tubes; in the latter, the baleen, or whale- 
bone, plates, triangular in shape, and fringed on the inner 
side, hang in rows from the gums of the upper jaw. In 
some Whales there are about 300 plates composing the 
outer row in each jaw.” 
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 mineral 
matter, and in not showing, under 
the microscope, any minute cay- 
ities, called dacune. <A typical 
tooth, as found in Man, consists 
of a central mass of dentine, cap- 
ped with ename? and surrounded 
with cement. The first tissue is 
always present, while the others 
may be absent. It is a mixture 
of animal and mineral matter dis- 
Fia. 29.—Section of Human Mo- 
posed in the form of extremely lar, enlarged: k, crown; 7, 
. - neck; f, fang; e, enamel; d, 
fine tubes and cells, ‘so minute as dentine; c, cement; 9, palp- 
to 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 
tissne 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. 
Cement closely resembles bone, and is present only in the 
teeth of the higher animals. 
Teeth are usually confined to the jaws; but the number, 
size, form, structure, position, and mode of attachment vary 
with the food and habits of the animal. As a rule, ani- 
mals developing large numbers of teeth in the back part 
of the mouth are inferior to those having fewer teeth, and 
5 
