ORGANIZATION 



235 



structure, and finally merge (in the long bones) into the 

 central cavity containing the marrow. Under the micro- 

 scope, each canal appears to be the center of a multitude 

 of lamince, or plates, arranged around it. Lying between 

 these plates are little cavities, called lacunce, which are 

 connected by exceedingly fine tubes, or canaliculi. The 

 two represent the spaces occupied by the original cells 

 of the bone, and differ in shape and size in different 

 animals. 



True bone is found only in vertebrates, or backboned 

 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 : 



FIG. 206. 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 y cement. 



dentine, forming the body of the tooth, and always pres- 

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

 the fangs (Fig. 229). The last is true bone, or osseous 

 tissue. Dentine resembles bone, but differs in having 

 neither lacunae 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 fibers, set closely together. It is 



