COUES AND YARROW ON HERPETOLOGY. 265 



the construction of the tube is not as if a hole had been bored through 

 a solid tooth. It is in effect a flat tooth, with the edges rolled over 

 together till they meet, converting an exterior surface, first into a groove^ 

 finally into a tube. This is shown, on microscopic examination of a sec- 

 tion of the tooth, by the arrangement of the dentine. Unlike an ordi- 

 nary tooth, the fang is movable, and was formerly supposed to be hinged 

 in its socket, since it is susceptible of erection and depression. But 

 the tooth is firmly socketed, and the source of this movement is the 

 maxillary bone itself, which rocks to and fro by a singular contrivance. 

 The maxillary is a small, stout, triangular bone, movably articulated 

 above with a smaller one, tlie lachrymal, which is itself hinged upon the 

 frontal. Behind, the maxillary articulates with the palatal and ptery- 

 goid, both of which are of rod-like shape, and are acted upon by the 

 spbeno- pterygoid muscle, the contraction of which pushes them for- 

 ward. This forward impulse of the palatal and pterygoid is communi- 

 cated to the maxillary, against which they abut, causing the latter to 

 rotate upon the lachrymal. In this rocking forward of the maxillary ,^ 

 the socket of the fang, and with it the tooth itself, rotates in such man- 

 ner that the apex of the tooth describes the arc of a circle, and finally 

 points downward instead of backward. This protrusion of the fang is 

 not an automatic motion, consequent upon merfe opening of the mouth, 

 as formerly supposed, but a volitional act, as the reverse motion, namely, 

 the folding back of the tooth, also is 5 so that, in simply feeding, the 

 fangs are not erected. The folding back is accomjilished by the ecto- 

 pterygoid and sphenopalatine muscles, which, arising from the skull 

 behind as a fixed point of action, in contracting draw upon the jaw-bones 

 in such a way that the maxillary, and with it of course the fang, are 

 retracted, when the tooth is folded back with an action comparable to the 

 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 of 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 fol- 

 licles are imbedded ; it is invested with two layers of dense, white, fibrous 



