The gland which secretes the poison is a modification of the parotid salivary gland of other Vertebrates, and is usually situated on each side of the head below and behind the eye, invested in a muscular sheath. It is provided with large alveoli in which the venom is stored before being conveyed by a duct to the base of the channelled or tubular fang through which it is ejected. In the Vipers, which furnish examples of the most highly developed poison apparatus, although inferior to some in its toxic effects, the poison gland is very large and in intimate relation with the masseter or temporal muscle, consisting of two bands, the superior arising from behind the eye, the inferior extending from the gland to the mandible. When the snake bites, the jaws close up, causing the gland to be powerfully wrung, and the poison pressed out into the duct. From the anterior extremity of the gland the duct passes, below the eye and above the maxillary bone, where it makes a bend, to the basal orifice of the poison fang, described above (p. 55), which is ensheathed in a thick fold of mucous membrane, the _vagina dentis_. By means of the movable maxillary bone (_supra_, p. 49) hinged to the prefrontal, and connected with the tranverse bone which is pushed forward by muscles set in action by the opening of the mouth, the tubular fang is erected and the poison discharged through the distal orifice in which it terminates. [Illustration: FIG. 13--POISON APPARATUS OF RATTLESNAKE: VENOM GLAND AND MUSCLES (LATERAL VIEW). (After Duvernoy) _a_, Venom gland; _a´_, venom duct; _b_, anterior temporal muscle; _b´_, mandibular portion of same; _c_, posterior temporal muscle; _d_, digastricus muscle; _e_, posterior ligament of gland; _f_, sheath of fang; _g_, middle temporal muscle; _h_, external pterygoid muscle; _i_, maxillary salivary gland; _j_, mandibulary salivary gland.] In some of the Proteroglyphous Colubrids, as we have seen, the poison fangs are not tubular, but only channelled and open along the anterior surface; and as the maxillary bone in these snakes is more or less elongate, and not or but slightly movable vertically, the poison duct runs above the latter, making a bend only at its anterior extremity, and the tranverse bone has not the same action on the erection of the fangs. Otherwise the mechanism is the same. In the Opisthoglyphous Colubrids, with grooved teeth situated at the posterior extremity of the maxilla, a small posterior portion of the upper labial or salivary gland is converted into a poison-secreting organ, distinguished by a light yellow colour, provided with a duct larger than any of those of the labial gland, and proceeding inward and downward to the base of the grooved fang; the duct is not in direct connexion with the groove, but the two communicate through the mediation of the cavity enclosed by the folds of mucous membrane surrounding the tooth, and united in front.