398 ANATOMY OF VERTEBRATES. 



sixths of the tooth, the section, of which a magnified view is given 

 in fig. 270, shows the dentine of the fang inclosing the poison- 

 canal, and having its own centre or pulp-canal, p, p, in the form 

 of a crescentic fissure, situated close to the concave border of the 

 inflected surface of the tooth. The pulp-cavity disappears, and 

 the poison-canal again resumes the form of a groove near the apex 

 of the fang, fig. 269, A, b, v', and terminates on the anterior surface 

 in an elongated fissure. 



The venom-fangs of the viper, rattle-snake, and the Fer-de- 

 lance are coated only with a thin layer of a subtransparent and 

 minutely cellular cement. The disposition of the dentinal tubes 

 is obedient to the general law of verticality to the external 

 surface of the tooth ; it is represented as seen in the trans- 

 verse section from the middle of the fang in fig. 270. Since 

 the inflected surface of the tooth can be exposed to no other 

 pressure than that of the turgesccnt duct with which it is 

 in contact, the tubes which proceed to the central surface, 

 while maintaining their normal relation of the right angle to 

 it, are extremely short ; and the layer of dentine separating the 

 poison-tube from the pulp-cavity is proportionally thin. The 

 dentinal tubes that radiate from the opposite side of the pulp- 

 cavity to the exposed surface b of the tooth are disproportionately 

 long. 



The teeth of Ophidians are developed and completed in that 

 part which forms the original seat of the tooth-germs in all 

 animals ; viz. the mucous membrane or gum covering the alveolar 

 border of the dentigerous bones. The primitive dental papilla in 

 the common harmless snake very soon sinks into the substance of 

 the gum, and becomes enclosed by a capsule. As soon as the 

 deposition of the calcareous salts commences in the apex of the 

 papilla, the capsule covering that part becomes ossified and 

 adherent to the dentine, and the tooth begins to pierce and 

 emerge from the gum before its mould, the pulp, is half com- 

 pleted. Fresh layers of cells are successively added to the base 

 of the pulp, and converted, by their confluence and calcification, 

 into the tubular dentine, until the full size of the tooth is 

 attained, when its situation in the gum is gradually changed, 

 and its base becomes anchylosed to the shallow cavity of the 

 alveolar surface of the bone. In the posterior part of the large 

 mucous sheath of the poison-fang, the successors of this tooth 

 are always to be found in different stages of developement ; 

 the pulp is at first a simple papilla, and when it has sunk into 

 the gum the succeeding portion presents a depression along its 



