38 
COMPARATIVE ZOOLOGY. 
True bone is found only in Vertebrates, or back-boned 
animals. 
(5) Dental Tissue.— Like bone, a tooth is a combination 
of earthy and animal matter. It may be called petrified 
skin. In the higher animals, it consists of three parts: 
dentine , forming the body of the tooth, and always pres¬ 
ent; enamel , capping the crown; and cement , covering the 
fangs (Fig. 31). The last is true bone, or osseous tissue. 
Fig. 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. 
Dentine resembles bone, but differs in having neither la- 
cunse nor (save in Shark’s teeth) canaliculi. It shows, in 
place of the former, 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 want¬ 
ing in the teeth of most Fishes, Snakes, 
Sloths, Armadillos, Sperm-whales, etc. 
True dental tissue is confined to 
Vertebrates. 
(6) Adipose Tissue.— Certain cells be¬ 
come greatly enlarged and filled with 
fat, so that the original protoplasm oc¬ 
cupies a very small part of the space 
_ within the cell-membrane. These cells 
& are united illto “S by connective 
tissue, 6 . tissue, in the skin (as in the “ blub« 
