POISONOUS SNAKES OP NORTH AMERICA. 371 



Natural History, May 16, 1860 (Proceedings, Vol. vii; published July, 

 1860, pp. 293-294), and is quite brief, as follows: 



The duct proper does uot reach the opening at the base of the tooth, but ends at 

 a short distance from it. The communication beyond this is made by means of the 

 sheath of the tooth, which is too loose to prevent the poison from escaping around 

 the exterior of the tooth instead of entering its canal, were it uot for the circumstance 

 that as the tooth is protruded the sheath is crowded back, and thus made to fit 

 tightly the circumference. 



Dr. Johnston's statement is much more elaborate, and includes an 

 interesting- account of the successive growth and advance of the reserve 

 fangs. It was written by him as early as October 3, 1859, and given 

 to Dr. Weir Mitchell for publication.* 



After discussing the probability of a periodical shedding of the fangs 

 and describing the successive formation and growth of the secondary 

 or reserve fangs. Dr. Johnston proceeds as follows: 



At length the prime fang is removed, if spontaneously, by the atrophy of the pulp, 

 and, I believe, by erosion of the basal anchylosed portion ; if it be broken off by vio- 

 lence the freedom of the pseudo-socket is accomplished by the same means. And 

 now the fust tooth of reserve is urged forward into a recess in the maxillary bone 

 directly adjacent to and on the inner side of the fallen fang; and the requisite 

 advancement is brought about by the developmental r(s a iergo of the remaining 

 reserve pulp, and probably also by the traction in front exerted by the cicatrizing 

 parts. It is evident that the fang emerges from its capsule, and that the point and 

 crown repose in the den, but the base is closely invested with the capsular remains 

 under the form of a periosteal expansion, which is the mediate bond of union between 

 the base and the new and shallow socket of the maxilla. 



As may be perceived upon examination at this stage, two sockets coexist in the 

 same jaw, the inner, new one, supporting the recently promoted fang, and the outer, 

 old, and now vacant (me, which is fast becoming disencumbered of the vestiges per- 

 taining to its former resident. In this maxilla the new fang occupies the innermost 

 part, having the old socket on the outer side, while in the opposite maxilla, the 

 older venom fang may be discovered in its normal situation, leaving the recess to its 

 inner side vacant for the temporary lodgment of its successor. Or, both fangs 

 being recently fixed to the jaw, the vacuities will both be formed on the outside, 

 and all the reserve fangs will appear to follow backward and outward in direct line. 



Now, let us look at the situation of the poison duct and examine into the mode by 

 which it is brought into relation with the fang. 



The venom duct arising from the gland makes a bend upward, immediately 

 beneath the eye, then advances forward under the skin as far as the crotaline fos- 

 sette [pit], and lying upon the maxilla externally, plunges downward, and pierces 

 the gum in front of the fang, where it terminates in a papilla, which projects slightly 

 into the proximal aperture of the tooth. In this position it is maintained by the 

 gum, which clasps the base laterally and in front with considerable firmness, its 

 inferior or distal edge encompassing the annular enlargement already alluded to. 

 Nor is there any other than a mediate application of the poison papilla against the 

 fang, for, as the whole venom canal of each tooth is really ujjon the outside of the 

 organ, no special membrane lines it, which might be continuous with the duct that 

 discharges into the ui)per aperture. 



Such is the condition of things in an old fang, occupying its normal exterior posi- 

 tion. But when the tooth drops out, or is broken, the gum is left entire; or, if its 



*Researche8 upon the Venom of the Rattlesnake, pp. 17-19. (Published January, 



1861.) 



