OF THE VENOM OF THE RATTLESNAKE. 17 



as well as his peculiar knowledge of dental structures, entitle his views to great 

 respect. 



At my request, Dr. Johnston has repeated his observations, and most kindly 

 placed them at my disposal. The following statement is in his own language : — 



" The study of heads prepared by me, leads me to entertain the following opinion 

 as to the manner in which secondary fangs establish a communication with the 

 poison duct. There appears to me good reason to believe that there is a periodical 

 fall of the venom fangs, quite independent of violence ; and this is to be regarded 

 as a physiological circumstance agreeing with what takes place in fishes, as Pike 

 or Gar, and in reptiles, as Iguana, as well as with what occurs in the jaws of 

 Megalosaurus, Iladrosaurus, etc. For I have not observed in the jaws of Crotali of 

 the same species a constant relation between the size of the serpent and of its 

 fangs; and when I couple with the disproportion alluded to, the fact, that at nearly 

 all seasons, reserve fangs and germs are found in every degree of advancement or 

 development, I cannot suppose that the almost mature secondaries are awaiting an 

 accident to effect their promotion. We know that in Alligator especially, the 

 secondaries occasion by their development the erosion of the base, and the con- 

 sequent displacement of their predecessors, and assume positions from which they 

 are to be in turn expelled. 



" In Crotalus, the secondary fangs lie in separate capsules at the bottom of the 

 dens in the mucous membrane, where the fangs crouch when not erected. Their 

 pulps are arranged in order upon a gum which lies at the base and to the inner 

 side of the functioning fang ; and each developing fang is inclosed in a separate 

 caj^sule and points directly backwards. 



" Now a transverse section of a pulp in any part of its extent, gives a crescentic 

 figure, hardly perceptible as a crescent at the point, evidently lunate with separate 

 horns on either side of the distal aperture of the poison canal, and again a crescent, 

 but with closely approximated cornua, throughout the rest of the crown, where the 

 two longitudinal folds of dentine meet along the median line and are fused together. 



" As the growth of the tooth advances, a slight annular enlargement marks its 

 neck, or at least the limit of the crown, and then the two horns of the pulp diverge 

 widely, at the upper part of the base, which is in process of formation, but again 

 approach each other, yet without meeting, as the base goes on to completion. It 

 may here be remarked, that the pulp acquires greater volume at this part of the 

 fang, which is more extended along the posterior edge than in front, and is marked, 

 both internally and externally, with conspicuous longitudinal flutings. 



"The dentine of the basal portion in front, necessarily follows the modelling 

 pulp ; and as this, by the separation of the anterior lamellte, leaves an ovoidal 

 hiatus, so the tooth substance investing the matrix shows the free edge of its folds 

 on either side as the margin of a large aperture, the inlet of the poison canal. All 

 this occurs while yet the tooth-capsule is entire. 



" In this condition the secondary fang remains until the fang in use falls sponta- 

 neously or by violence ; and the secondaries behind it will be found to exhibit 

 successive inferior stages of development. At length the prime fang is removed, if 

 spontaneously, by the atrophy of the pulp, and, I believe, by erosion of the basal 

 3 



