text: Vipers In vipers, which have the most highly developed venom-delivery apparatus, the venom gland is very large and is surrounded by the masseter or temporal muscle, which consists of two bands, the superior arising from behind the eye, the inferior extending from the gland to the mandible. A duct carries venom from the gland to the fang. In vipers and elapids, this groove is completely closed, forming a hypodermic needle-like tube. In other species, the grooves are not covered, or only partially covered. From the anterior extremity of the gland, the duct passes below the eye and above the maxillary bone, to the basal orifice of the venom fang, which is ensheathed in a thick fold of mucous membrane. By means of the movable maxillary bone hinged to the prefrontal bone and connected with the transverse bone, which is pushed forward by muscles set in action by the opening of the mouth, the fang is erected and the venom discharged through the distal orifice. When the snake bites, the jaws close and the muscles surrounding the gland contract, causing venom to be ejected via the fangs.
  Elapids
  In the proteroglyphous elapids, the fangs are tubular, but are short and do not possess the mobility seen in vipers.