The Viperidae are a family of venomous snakes found all over the world, except in Antarctica,
Australia, Ireland, Madagascar, Hawaii, various other isolated islands, and above the Arctic Circle. Four subfamilies are currently recognized. All have relatively long, hinged fangs that permit deep penetration and injection of venom.
Each of the two fangs is at the front of the mouth on a short maxillary bone that can rotate back and forth. When not in use, the fangs fold back against the roof of the mouth and are enclosed in a membranous sheath. The left and right fangs can be rotated together or independently. All viperids have a pair of relatively long solenoglyphous (hollow) fangs that are used to inject venom from glands located towards the rear of the upper jaws.
Viperids use this mechanism primarily for immobilization and digestion of prey. Secondarily it is used for self-defense, though in most cases with non-prey items such as humans they are more likely to give a dry bite (not inject any venom). During a strike, the mouth can open nearly 180° and the maxilla rotates forward, erecting the fangs as late as possible so as the fangs do not become damaged. The jaws close on impact and powerful muscles that surround the venom glands contract to inject the venom as the fangs penetrate. This action is very fast; in defensive strikes it can be more a stab than a bite.
This is in contrast to elapid venoms that generally contain neurotoxins that disable muscle contraction and cause paralysis. Death from elapid bites usually results from asphyxiation because the diaphragm can no longer contract. However, this rule does not always apply: some elapid bites include proteolytic symptoms typical of viperid bites, while some viperid bites produce neurotoxic symptoms. Viperid venoms typically contain an abundance of protein degrading enzymes, called proteases, that produce symptoms such as pain, strong local swelling and necrosis, blood loss from cardiovascular damage complicated by coagulopathy, and disruption of the blood clotting system. Death is usually caused by collapse in blood pressure.