Each gland is surrounded by a thick capsule of fibrous tissue, two prolongations of which, the one anterior, the other posterior, keep it in its place beneath the masseter muscle. A portion of the latter is inserted in the capsule itself, in such a way that when the snake closes its jaws to bite, the gland is forcibly compressed and the contained liquid is squeezed into its excretory duct.
Between the muscle and the envelope of the gland there is a serous pouch, which enables the one to slide over the other.
The excretory duct runs along the outer side of the upper jaw, and opens by a slit at the base of the poison-fang, with which it inosculates at right angles in a little muscular mass forming a sphincter.
In the normal position of repose, the poison-fang is always concealed by a gingival fold of mucous membrane, in the substance of which are buried a few fibres of the tendon of the internal pterygoid muscle. When the latter contracts, the tooth is almost completely exposed, and the efferent duct of the gland then assumes an oblique position, which allows of the direct discharge of the venom through the canal which runs along the greater portion of the length of the tooth.
When the poison-fangs are folded back in their sheath, the poisonous secretion can escape freely into the buccal cavity by the slit situated at the base of the fangs.
At the moment when the animal is about to bite, when it throws back its head and opens its jaws, directing its fangs forwards, the muscles that come into action (masseters, temporals, and pterygoids) compress the glands on each side, and cause the venom to be expelled in a sudden jet, as if by a sort of ejaculatory process. In the case of certain species the venom may be projected to a distance of more than a yard.
The quantity of venom secreted by the glands varies greatly, according to the length of time which has elapsed since the animal took its last meal, and in accordance with a number of other conditions not very easy to determine.
The Common Viper of Europe yields scarcely 10 centigrammes of poison, while an adult Indian Cobra may excrete more than 1 gramme.
Freshly collected venom is a syrupy liquid, citron-yellow or slightly opalescent white in colour.