[Illustration: FIG. 2.--A, Maxillary bone and fangs of one of the _Viperidæ_ (_Vipera russellii_); B, maxillary bone and fangs of one of the _Colubridæ_ (_Naja tripudians_); C, maxillary bone and fangs of one of the _Colubridæ_ (_Bungarus fasciatus_); D, maxillary bone and teeth of one of the non-poisonous _Colubridæ_ (_Ptyas mucosus_). (After Sir Joseph Fayrer.)] Certain species (_Dipsas_) have maxillary teeth which increase in size from front to rear; the longest teeth are _grooved_ and serve for the better retention of prey, and also to impregnate it with saliva; but they are not in communication with the poison-glands. 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). [Illustration: 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). [Illustration: FIG. 4.--B, Fang of one of the _Colubridæ_ (_Naja tripudians_); E, transverse section.] [Illustration: FIG. 5.--C, Fang of one of the _Hydrophiinæ_ (_Hydrophis pelamis_); F, transverse section.] [Illustration: FIG. 6.--Three transverse sections of a poison-fang of one of the _Colubridæ_, through A A, B B, C C; P P, pulp cavity; V V V, poison-canal (groove). (After C. J. Martin.)] * * * * * In the _Elapinæ_ sub-family of the _Colubridæ_, and especially in the _Hydrophiinæ_, the teeth are much shorter and simply grooved or canaliculate; that is to say, the canal communicates with the exterior throughout its extent by a narrow slit, which traverses the entire convex surface of the tooth (figs. 4, 5, and 6).