310 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1951 



PREFRONTAL , SQUAMOSAL 



QUADRATE 



MAXILLA 



RESERVE 

 FANG 



OPERATIVE 

 FANG 



LOWER JAW 



Figure 2.— Skull of rattlesnake, showing the mechanism of the jaw bones in operating the 



poison fangs. 



bones of the cranium (see fig. 2) . At rest the maxillaries are so placed 

 that the fangs, which are firmly fixed to them, point backward and lie 

 along the roof of the mouth. Protected in this way when not in use, 

 they may grow to considerable length, sometimes not far short of 3 

 inches, as in the Gabun viper of Africa. 



With the mouth open the viper or rattlesnake brings into operation 

 a set of muscles that puts the highly mobile jawbones through a series 

 of lever actions, in such a way that the short maxillary bones are ro- 

 tated through an angle of about 90°. This brings the fangs into posi- 

 tion for the lightning thrust that follows (fig. 4) . 



The erection of a viper's fangs is an independent action, not neces- 

 sarily used only during striking. I have watched an adder yawn to 

 ease its facial muscles, slowly raising each fang in turn, where no 



POISON 

 DUCT 



Figure 3. — Sketch of partly dissected head of rattlesnake, showing the poison apparatus. 



(After Boulenger,) 



