‘Snakes work their prey down through the collapsed pharynx,’ says Günther. That is, the muscles of the throat seize upon what is presented to them, and do their part, as in other animals. Only, in most other animals there is the action of swallowing, one mouthful at a time; whereas in serpents the action is continuous, the throat going on with the work begun by the teeth, which in a snake is only grasping and working the food in with a motion so gradual as to simulate suction. The reason why the head and jaws have been so enormously stretched and distorted, is because all the bones are, in common language, loose; that is, they are not consolidated like the head-bones of higher animals, but united by ligaments so elastic as to enable them to separate in the way we have seen. This extends to the jaws, and even to the palate, which is also armed with teeth, two rows extending backwards. The lower jaw or mandible being extremely long, the elastic ligament by which the pair of bones is connected in front, forming the chin, enables them to separate widely and move independently. This is the case in a lesser degree with the palate bones, and the upper jaw-bones, all six being furnished with long, fine, recurved, close-set teeth, adapted for grasping and holding, but not for dividing or for mastication in any way.
For, as we have seen, if a snake were to open its mouth one moment for the purpose of what we call biting, the prey would escape. In addition to a very unusual length, the lower jaw is joined to the skull by an extra bone,—one which is not found in mammals, but only, I think, in birds,—a long ‘tympanic’ bone, which forms an elbow, and permits of that wide expansion of the throat necessary for the passage of such large undivided prey.
The illustration of the skeleton of a cobra, on p. 33, will enable the student to distinguish the principal head-bones. There is so much similarity of construction throughout the whole ophidian families that a cobra is chosen here, because the unusually long anterior ribs which form the hood can be observed, and the expansion of which is described elsewhere. The longer teeth in the upper jaw are here fangs; the inclination of the other rows of teeth and the bones sufficiently illustrate those of the non-venomous kinds generally, such as the little ring snake that has just swallowed his frog. A few of the larger constricting snakes possess an additional bone—an intermaxillary in front between the upper jaws, very small, yet sometimes furnished with two or four teeth, thus facilitating the expansion of the jaws as well as the retention of the food.
It is this adaptive development of head-bones that enabled Coluber natrix to turn his frog round to a more convenient position, and then draw it into his mouth so gradually that we scarcely comprehended how it disappeared. The six rows of small teeth form six jaws so to speak, each one of which advanced a very little, while the other five were engaged in holding firmly. In those largest pythons which have the little bone in front between the two upper jaw-bones (intermaxillary) we may say there are seven jaws. As those gigantic snakes have to deal with proportionately large and strong prey, they are thus enabled to retain and manage it.
In the graphic language of Professor Owen let me recapitulate.
The mouth can be opened laterally or transversely, as in insects, as well as vertically, as in other vertebrates. The six jaws are four above and two below, each of which can be protruded or retracted independently of the others. ‘The prey having been caught and held, one jaw is then unfixed by the teeth of that jaw being withdrawn and pushed forward, when they are again unfixed farther back upon the prey; another jaw is then unfixed, protruded, and re-attached, and so with the rest in succession. This movement of protraction, being almost the only one of which they are susceptible, while stretched apart to the utmost by the bulk of the animal encompassed by them: and thus by their successive movements, the prey is slowly introduced into the gullet.’[3]
Skeleton of a Cobra (from Owen’s Anatomy of the Vertebrates).