Snake venom is a highly modified saliva containing zootoxins that facilitate the immobilization and digestion of prey, and defense against threats. It is injected by unique fangs during a bite, and some species are also able to spit their venom. The glands that secrete the zootoxins are a modification of the parotid salivary glands found in other vertebrates, and are usually situated on each side of the head, below and behind the eye, and encapsulated in a muscular sheath. The glands have large alveoli in which the synthesized venom is stored before being conveyed by a duct to the base of channeled or tubular fangs through which it is ejected. Venoms contain more than 20 different compounds, mostly proteins and polypeptides. A complex mixture of proteins, enzymes, and various other substances with toxic and lethal properties serves to immobilize the prey animal, enzymes play an important role in the digestion of prey, and various other substances are responsible for important but non-lethal biological effects. Some of the proteins in snake venom have very specific effects on various biological functions including blood coagulation, blood pressure regulation, and transmission of the nervous or muscular impulses, and have been developed for use as pharmacological or diagnostic tools, and even useful drugs.