88 COMPARATIVE ZOOLOGY. 
Lying between these plates are little cavities, called lacune, 
from which radiate exceedingly fine pores, or canaliculi. 
The form of the lacunze differs in the bones of different 
animals, being angular in Fishes, and oval in other ani- 
mals. The size is related, not to the size of the animal, 
but to the size of the blood-corpuscles, and is therefore 
greatest in Reptiles. 
True bone is found only in Vertebrates, or back-boned 
animals. 
(6) Dental Tissue.— Like bone, a tooth is a combination 
of earthy and animal matter. It may be called petrified 
epithelium. In the higher animals, it consists of three 
parts: dentine, forming the body of the tooth, and always 
present ; enamel, capping the crown; and cement, covering 
the fangs (Fig. 29). The last is true bone, or osseous tis- 
sue. Dentine resembles bone, but differs in having neither 
Fie. 9.—Highly magnified section of Dentine and Cement, from the fang of a Human 
Molar: a, b, marks of the original dentinal pulp; d, dentinal tubes, terminating 
in the very sensitive, modified layer, g; h, cement. 
lacunze nor (save in Shark’s teeth) canaliculi. It shows, in- 
stead, under the miscroscope, innumerable parallel tubes, 
reaching from the outside to the pulp-cavity within. The 
“ivory” of Elephants consists of dentine.. Enamel is the 
hardest substance in the body, and is composed of minute 
six-sided fibres, set closely together. It is wanting in the 
teeth of most Fishes, Snakes, Sloths, Armadillos, Sperm- 
whales, ete. 
True dental tissue is confined to Vertebrates. 
(7) Muscular Tissue.—If we examine a piece of lean meat, 
