The poison-fangs are normally covered with a fold or capsule of mucous membrane, in which they are sheathed. This fold conceals a whole series of reserve teeth in different degrees of development, which eventually become attached to the extremity of the maxillary when the principal tooth falls out or is broken ([fig. 2]).
Fig. 3.—A, Fang of one of the Viperidæ (Vipera russellii); D, transverse section of the fang.
Poison Apparatus.—The grooved or furrowed teeth in the Proteroglypha and the canaliculate teeth in the Solenoglypha are arranged, not for the purpose of seizing prey, but in order to deal it a mortal blow by injecting the venom into its flesh.
In the normal position they lie almost horizontally, and exhibit no mobility of their own. But, when the animal prepares to bite, their erection is effected by the snake throwing its jaw back; and this movement, which is always very sudden, enables it at the same time to compress its poison-glands, by the aid of special constrictor muscles.
On examining the various species of poisonous snakes, we observe very sharply marked differences in the arrangement and dimensions of the teeth. Thus, in the Viperidæ they are long, extraordinarily sharp, and capable of producing deep wounds ([fig. 2], A, and [fig. 3]). They are traversed by an almost completely closed canal, from the base, which communicates with the poison-duct, to the neighbourhood of the point, where it opens very obliquely on the convex surface ([fig. 3], A and D).
Fig. 4.—B, Fang of one of the Colubridæ (Naja tripudians); E, transverse section.
Fig. 5.—C, Fang of one of the Hydrophiinæ (Hydrophis pelamis); F, transverse section.