896 16. CROTALID.T. 



shutting of the blade of a pocket-knife. All the motions of 

 the fangs are controlled by these two sets of antagonistic 

 muscles, one of which prepares the fangs for action, while 

 the other stows them away when not wanted. 



"The fangs, when not in use, are further protected by 

 a contrivance for sheathing them, so that they rest like a 

 sword in its scabbard. This is a fold of mucous membrane, 

 the 'vagina dentis, which envelopes the tooth like a hood, 

 enwrapping its base, and slipping down over its length, partly 

 as a consequence of its elastic texture, partly on account ot 

 its connections. Erection of the fang causes the sheath to 

 slip off, like the finger of a glove, and gather in folds around 

 the base of the tooth. This arrangement can be readily 

 examined without dissection. 



"The poisonous fluid is secreted in a gland which lies 

 against the side of the skull, below and behind the eye, of 

 a flattened oval shape, obtuse behind, tapering in front to 

 a duct that runs to the base of the tooth. Without going 

 into the minute anatomy of the gland, it may be described 

 as a sac, or reservoir, in the walls of which the numerous 

 secretory follicles are imbedded} it is invested with two 

 layers of dense, white, fibrous tissue, the outer of which gives 

 off three strong ligaments that hold it in place. In a large 

 snake, the entire gland may be nearly an inch long and one- 

 fourth as wide, weighing, empty, 1 or 1 2 grains, and having 

 a capacity of 10 or 15 drops of fluid. There is no special 

 reservoir for the venom, other than the central cavity of the 

 gland. A certain dilatation of one portion of the duct, form- 

 erly supposed to be such store-house, is due to thickening of 

 its walls, without corresponding increase of capacity, result- 

 ing from muscular fibres which serve as a sphincter to com- 

 press the canal and prevent wasteful flow of the contents. 

 There is further provision to this same end. When the tooth 

 is folded back, the duct attached to its root is submitted to 



