Cranial Skeleton.—The cranium is composed of a certain number of bones, the homologues of which are found in the mammalian skeleton; but the bones are complex, and subject to modifications according to the structure and habitat of each species.
The special arrangement of the bones of the face is above all characteristic of the poisonous snakes. Those forming the upper jaw, the palate and the mandibles or “inter-maxillaries” are movable upon each other and on the cranium. The upper and lower maxillaries are united by an extensile ligament and articulated with the tympanic bone, which permits the mouth to be opened very widely when the animal swallows its prey.
Dentition.—The non-poisonous snakes have two rows of teeth in the upper jaw—one external, the maxillary, usually composed of from 35-40 small, backwardly curved teeth; the other internal, the palatine, which only numbers from 20-22 teeth, having the same curvature ([fig. 1], A).
Fig. 1.—A, Cranial skeleton of one of the non-poisonous Colubridæ (Ptyas mucosus); B, cranial skeleton of one of the poisonous Colubridæ (Naja tripudians); C, cranial skeleton of one of the poisonous Colubridæ (Bungarus fasciatus); D, cranial skeleton of one of the Viperidæ (Vipera russellii); E, cranial skeleton of one of the Viperidæ Crotalinæ (Crotalus durissus); F, cranial skeleton of one of the Colubridæ Hydrophiinæ (Hydrophis pelamis).
In the poisonous snakes the maxillary bones are shorter, and the outer row is represented by a single long and tubular or grooved tooth (the fang), fused with the maxillary bone, which is itself movable ([fig. 1], B, C, D, E, F).
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.