700 



STRUCTURE OF TEETH. DERMAL SKELETON. 



Fig. 3C1. 



Pelrosa, has the characters of true Bone ; possessing its distinctive 

 stellate Lacunae and radiating Canaliculi. Where it exists in small 

 amount, we do not find it traversed 

 by Medullary Canals ; but, like den- 

 tine, 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 conse- 

 quently pass towards those which 

 radiate from the central cavity in 

 the direction of the surface of the 

 Dentine, where this possesses a 

 similar vascularity, — as was re- 

 markably the case in the teeth of the 

 extinct Megatherium. In the Human 

 tooth, however, the Cementum has 

 no such vascularity ; but forms a 

 thin layer (Fig. 361, 6), which en- 

 velopes the root of the tooth, com- 

 mencing near the termination of the 

 capping 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 Rep- 

 tiles and Fishes, it forms a thick 

 continuous envelope over the whole 

 surface, until worn-away at the 

 crown. 



547. Dermal Skeleton. — The Skin 

 of Fishes, of most Reptiles, and 

 of a few Mammals, is strengthened by plates of a Horny, Car- 

 tilaginous, Bony, or even Enamel-like texture; which are some- 

 times 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 they are essentially-different structures ; 

 the former being developed in the substance of the true Skin, with 

 a layer of which in addition to the Epidermis they are always 

 covered, and bearing a resemblance to Cartilage and Bone in their 

 texture and composition ; whilst the latter are formed upon the 

 surface of the true Skin, and are to be considered as analogous to 

 Nails, Hoofs, &c, and other 'Epidermic appendages.' In nearly 

 all the existing Fishes, the Scales are flexible, being but little con- 



Vertical Section of Human 

 Molar Tooth :—a, Enamel ; b, 

 Cementum or crusta petrosa ; 

 c, Dentine or ivory ; d, osseous 

 excrescence, arising from hy- 

 pertrophy of cementum ; e, 

 pulp-cavity ; /, osseous lacunae 

 at outer part of Dentine. 



