In most of the C'olubri, each maxillary and premandibular bone includes from twenty to twenty-five teeth. They are less numerous in the genera Tortrix and Honzalopsis, and are reduced to a still smaller number in the poisonous serpents, in the typical genera of which the short maxillary bone supports only a single perforated fang.
Poisonous Serpents. — The transition to these serpents, which was begun in the Bucephali and allied genera with grooved maxillary teeth, is completed by the poisonous serpents of the genera Pcianzis, Hydrophis, Elaps, Bon,garus, and Hamadryas.
The superior maxillary bone diminishes in length with the decreasing number of teeth which it supports. The ecto-pterygoid bone elongates in the same ratio, so as to retain its position as an abutment against the shortened maxillary, and the muscles implanted into this external pterygoid bone communicate through it to the maxillary hone. the hinge-like movements backwards and forwards upon the ginglymoid articulations connecting that bone with the prefrontal and palatine bones. As the fully-developed poison-fangs are at tached by the same firm basal anchylosis to maxillary sockets, which forms the charac teristic mode of attachment of the simple or solid teeth, they necessarily follow all the movements of the superior maxillary bone. When the external pterygoid is retracted, the superior maxillary rotates backwards, and the poison-fang is concealed in the lax mucous gum, with its point turned backwards. When the muscles draw forward the external ptery goid, the superior maxillary bone is pushed forwards, and the recumbent fang withdrawn from its concealment and erected.
In this power of changing the direction of a large tooth, so that it may not impede the passage of food through the mouth, we may perceive an analogy between the viper and the Lophius; but in the fish the movement is confined to the tooth alone, and is de en dent on the mere physical property of the elastic medium of attachment; in the serpent the tooth has no independent motion, but rotates with the jaw, whose movements are governed by muscular actions. In the fish the great teeth are erect, except when pressed down by some extraneous force.' In the ser pent the habitual position of the fang is the recumbent one, and its erection takes place only when the envenomed blow is to be struck.
A true idea of the structure of a poisons fang will be formed by supposing the crown of a simple tooth, as that of a boa, to be pressed flat, and its edges to be then bent towards each other, and soldered together so as to form a hollow cylinder, or rather cone, open at both ends. The flattening of the fang and its inflection around the poison-duct commences immediately above the base, and the suture of the inflected margins runs along the anterior and convex side of the recur*, ed fang, as shown in fig. 567, A.: the poison
canal is thus in front of the pulp-cavity, as shown in the longitudinal section of the fang B. The basal aperture of the poison-canal v is oblique, and its opposite outlet v' is still more so, presenting the form of a narrow elliptical longitudinal fissure terminating at a short distance from the apex of the fang. The relative position of the two apertures of the poison-canal is shown in the figure of the fang of the large Cobra in my " Oclonto giaphy " (pl. 65.fig. 9., and in Vol. IV. p. 290. fig. 210., art. REPT1LIA), where a fine hair is represented as passing through the poison canal.
The poison-glands occupy the sides of the posterior half of the head ; each gland con sists of a number of elongated narrow lobes, extending from the main duct, which runs along the lower border of the gland upwards and slightly backwards : each lobe gives off lobules throughout its extent, thus presenting a pinnatifid structure; and each lobule is subdivided into smaller secerning cmca, which constitute the ultimate structure of the gland. The whole gland is surrounded by a double aponeurotic capsule, of which the outermost and strongest layer is in connection with the muscles by whose contraction the several caeca and lobes of the gland are compressed and emptied of their secretion. This is then con veyed by the duct (see REPT1LIA, Vol. IV. p. 291.,fig. 211. e) to the basal aperture of the poison-canal of the fang f. We may suppose, that as the analogous lachrymal and salivary glands in other animals are most active during particular emotions, so the rage which stimu lates the venom-snake to use its deadly weapon must be accompanied with an increased secre tion and great distension of the poison-glands ; and as the action of the compressing muscles is contemporaneous with the blow by which the serpent inflicts the wound, the poison is at the same moment injected with force into the wound from the apical outlet of the per forated fang, The duct which conveys the poison, al though it runs through the centre of a great part of the tooth, is really on the outside of the tooth, the canal in which it is lodged and protected being formed by a longi tudinal inflection of the dentinal parietes of the pulp-cavity. This inflection corn longitudinal indentation on the convex side of the fang; as it proceeds it sinks deeper into the substance of the tooth, and the sides of the groove meet and seem to coalesce, so that the trace of the inflected fold ceases, in some species, to be perceptible to the naked eye ; and the fang appears, as it is commonly described, to be perforated by the duct of the poison-gland. In the Hydrophis the groove remains permanently open, as in fig 567. c.