1026 VEKTEBRATED ANIMALS 



in the embryonic tooth ; and he has further shown that it is much 

 more frequently present than used to be supposed. The cementum, 

 or crusta petrosa, has the characters of true bone, possessing its dis- 

 tinctive stellate lacunse and radiating canaliculi. Where it exists 

 in small amount we do not find it traversed by medullary canals ; 

 but, like dentine, it is occasionally furnished with them, and thus 

 resembles bone in every particular. These medullary canals enter 

 its substance from the exterior of the tooth, and consequently pass 

 towards those which radiate from the central cavity in the direction 

 of the surface of the dentine, where this possesses a similar vascu- 

 larity, as was remarkably the case in the teeth of the great extinct 

 Megatherium. In the human tooth, however, the cementum has no 

 such vascular ity, but forms a thin layer (fig. 757, b), which envelopes 

 the root of the tooth commencing near the termination of the cap 

 of enamel. In the teeth of many herbivorous mammals it dips 

 down with the enamel to form the vertical plates of the interior of 

 the tooth; and in the teeth of the Edentata, as well as of many 

 reptiles and fishes, it forms a thick continuous envelope over the 

 whole surface, until worn away at the crown. 1 



Dermal Skeleton. The skin of fishes, of a few amphibians, of 

 most reptiles, and of few mammals, is strengthened by plates of a 

 horny, cartilaginous, bony, or even enamel-like texture, which are 

 sometimes fitted together at their edges, so as to form a continuous 

 box-like envelope ; whilst more commonly they are so arranged as 

 partially to overlie one another, like the tiles on a roof; and it is 

 in this latter case that they are usually known as scales. Although 

 we are accustomed to associate in our minds the * scales ' of fishes 

 with those of reptiles, yet essentially different structures have been 



included under this name, 

 those of the former and of 

 many of the latter being 

 developed in the substance 

 of the true skin (with a 

 layer of which, in addition 

 to the epidermis, they are 

 always covered), and bear- 

 ing a resemblance to car- 

 tilage and bone in their 

 FIG. 758. Portion of skin of sole, viewed as an texture and composition ; 

 opaque object. whilst others, such as the 



scales of snakes or the tor- 

 toise-shell, are formed upon the surface of the true skin, and are 

 to be considered as analogous to nails, hoofs, &c. and other 'epi- 

 dermic appendages.' In nearly all the existing fishes the scales are 

 flexible, being but little consolidated by calcareous deposit ; and in 

 some species they are so thin and transparent that, as they do not 

 project obliquely from the surface of the skin, they can only be 

 detected by raising the superficial layer of the skin and searching 



1 The student is recommended to consult Mr. C. S. Tomes's Manual of Dental 

 Anatomy, Human and Comparative. 



