In venomous serpents some of the glands opening into the mouth secrete a
poisonous fluid, which is introduced into the blood of a bitten victim.
The largest amount of specialization is found among the vipers, where
the teeth are reduced to a pair of hollow “fangs” in the front of the
upper jaw, and there are two large poison-glands, one on either side of
the head, giving it a characteristic resemblance to the ace of spades.
In a state of rest, when the mouth is shut, the poison-fangs are pressed
against the roof of the mouth, with their tips directed backwards. But
when the snake opens its mouth and “strikes,” the fangs are rotated
forward so that their sharp tips can be brought into action. The poison
flows into the upper end of the tooth-canal and, in vipers, enters the
wound by a small hole on the side of the tip. Were it at the end a
blockage might result. We have, in fact, an anticipation of the device
used in the construction of the needles employed with hypodermic
syringe.