58 



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 alwaj'S pres- 

 ent; enamel, capping the crown ; and cement, covering the 

 fangs (Fig. 31). The last is true bone, or osseous tissue. 



Fi&. 9. — Highly ranguifled section of Dentine and Cement, from tile fang of a Haman 

 Molar: a, b, mai-ks of the original dentinal pulp; d, dentinal tube;, terminating 

 in the very sensitive, modified layer, g ; h, cement. 



Dentine resembles bone, but differs in having neither la- 

 cunae 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 witliin. 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-membi-ane. These cells 



^':it"fl,::.eX'f ^nS; ^™ ""'*'** "'*•' ""^^^•^^ ^y connective 

 tiMue.6. tissue, in the skin (as in the "blub- 



