The progressive motion of serpents is executed by the assistance of the ventral scales. The body is raised up into one or more arches, and then suddenly unbent is pushed forwards, the posterior edges of the scales, op posed to the ground at the hinder extremity of the circle, offering sufficient resistance. By thus alternately. ele vating and unbending the body, serpents move in their characteristic manner, backwards or forwards, with an astonishing degree of rapidity. When at their utmost speed, they appear scarcely to touch the ground. In their more ordinary movements, the ventral piwes cr scales serve as feet. These scales slide under each other by a kind of /no/us/on, so as to permit the ventral surface to shorten or lengthen at the will of the animal.
\Viten some of the foremost scales are pressed on the ground, those behind ate brought forward, and in their tuin supporzing the body, enable the fore part to ad vance. To assist the scales to do this with greater ad vantage, they are counected with one another by means of muscular threads and a longitudinal band, and arc likewise aided by the peculiar mechanism of the ribs.. These last, we have seen, are connected with the ven tral scales by a flexible cartilage. They are capable of moving on their vertebral joint either ventrally or dor. sally, antetiorly or posteriorly. These motions arc aided by the muscles which are inserted into them, and origi nate in ribs contiguous to those to be moved, and in the vertebrx. The body in general is of a rounded form ; but when preparing for progressive motion, the ribs are drawn somewhat dorsally, so as to flatten the scales of the belly, and, by moving anteriorly or posteriorly, give to the scales with which they are connected a corres ponding degree of motion. The ribs in this case act as limbs to the scales, which may be compared to feet. This singular use of the ribs of snakes in assisting. pro gressive motion was detected by the acute Tyson, in his admirable anatomical examination of the rattlesnake. Phil. Trans. No. 144, copied by Ray into his Synopsis Aninzalium, p.291 ; and it was still farther illustrated by Sir Everard Honte, in his dissections of the boa con strictor. Phil. Trans. 1812, p. 163.
Besides the capabilities of executing progressive mo tion, many serpents can twist their bodies round the branches of trees, or suffer a considerable portion to hang clown. In this attitude, the larger kinds are ready to fall down upon their piey passing beneath, such as deers ancl antelopes. Such animals are not only retard
ed by thcir weight, but incommoded by the foe twisting itself in wreaths round their body, and by contractile ef forts crushing it to death.
This method of seizing their prey is confined to the larger kinds. The smaller sorts are able by their mouth and teeth to seize and retain their victims. There is no mastication, the food being swallowed entire. To facili tate the deglutition, the under jaws consist of two bones, as in birds, and, like these animals, they are joined to the cranium by the intervention of a bone similar to the os quadratum. The upper jaw is also loosely connect ed with the bead, and in some species admits of consi derable motion at the point of junction. The mouth can thus be opened very wide, and larger animals ad mitted, than, from the ordinary size of the devourer, one would be led to suppose.
The armature of the mouth is of two kinds ; com mon teeth, and poison fangs. The common teeth form a single row on each side in the lower jaw, and usually a double row in the upper. The external row, above, is connected with the maxillary bonc; the inner row is supported, as in many fishes, on the palatine bones. These both are subulate and recurved.
The toison fangs, vulgarly termed the sting, are con fined to the upper jaw, and occur on each side towards the extremity. In some the fang is the largest tooth, the foremost in the row, and is followed by several com mon teeth in the maxillary bone. In others, the maxil lary bone is abbreviated, and the deficiency at its proxi mal end is supplied by a long moveable peduncle clesti tute of teeth. The large fang is placed in this short maxillary bone, and is followed by a few other teeth of similar form. These fangs arc to be viewed as the os seous openings of the ducts from the poison bags which are situated at the base. They cohtain a tubular cavity from their base, passing through the tooth on its convex side to the apex, which ends in a narrow slit. \Viten the serpent hites an animal, the po son flows from the bag through this slit into the bottom of the Wound, where to most advamage it can produce its deleterious effects. 'Whether this dreadful apparatus of venom is to be con sidered as connected with the digestive system, as an in strument for conquering the prey, and procuring food, or for defensive purposes, has not been satisfactorily de termined. The latter conjecture is the most plausible.