896 16. CROTALIDA 
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 deutis, 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. Ina large 
snake, the entire gland may be nearly an inch long and one- 
fourth as wide, weighing, empty, 10 or 12 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 
