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; dentinal tubes, 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 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 



Fig. io.— Adipose Tissue, a; are united into masses by connective 



with fibres of connective , , J 



tissue, b. tissue, in the skin (as in the "blub- 



