POISONOUS SERPENTS. 231 



situated close to the concave border of the inflected surface of the 

 tooth. It is such a section of wliich a magnified view is given in 

 PL 65 A. The pulp-cavity disappears, and the poison-canal again 

 assumes the form of a groove near the apex of the fang, and' termi- 

 nates on the anterior surface in an elongated fissure. 



If the end of each inflected fold of cement in the tooth of the 

 Labyrinthodon were dilated sufficiently to contain a tube, that tooth 

 might convey the ducts of fifty poison-glands deeply imbedded in 

 its substance and yet all of them actually on the outside of the tooth 

 itself: it is the existence of a single fold of the same kind, but more 

 simple, inasmuch as it is straight instead of wavy, which forms the 

 complication of the viper's fang subservient to the completion of its 

 peculiar offensive weapon. 



The venom-fangs of the viper, rattle-snake and fer-de-lance are 

 coated only with a thin layer of a subtransparent and minutely cellular 

 cement : the disposition of the calcigerous tubes is obedient to the 

 general law of verticality to the external surface of the tooth ; 

 it is represented as seen in a transverse section from the middle 

 of the fang in Plate 65 a. Since the inflected surface of the tooth 

 can be exposed to no other pressure than that of the turgescent 

 duct with which it is in contact, the tubes which proceed to that 

 surface, d, while maintaining their usual relation of the right 

 angle to it, are extremely short, and the layer of dentine sepa- 

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

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

 cavity to the exposed surface of the tooth, are disproportionately long. 



The pulp-cavity following the form of the tooth itself, presents 

 in a transverse section of this part the form of a fissure describing 

 four-fifths of a circle : the fissure is widest at the middle and at the 

 two extremities : the exterior calcigerous tubes, in quitting the pulp- 

 cavity, form a graceful curve, the convexity being turned towards the 

 nearest horn of the crescent : at the middle of the pulp-fissure the 

 tubes proceed straight to the opposite surface ; and at the two extre- 

 mities of the crescent the central tubes are nearly straight, while the 

 lateral ones radiate in graceful curves which become bolder as they 

 diverge from the central and straighter tubes. Throughout the greater 



