Now the tips of every pair of ribs in a snake's body are fastened to one of these long abdominal scales in such a manner that when the snake moves the ribs forward the edge of the scale is raised--very much as you can raise the laths of the Venetian blind by pulling the cord at the side; and the snake travels by moving forward its ribs in turn, and catching hold of the ground with the edges of the scales, using first the ribs of one side and then of the other. When a snake is crawling, however, it does not curve its body into upright loops as inaccurate pictures sometimes represent, but keeps it pressed flat upon the ground, so that the scales may be able easily to take hold of any little roughness upon the surface. And when it climbs a tree it does not twine its body round and round the trunk, but crawls straight up it, just as it crawls along the ground. The mouth of a snake is very curiously made. We are not speaking now of the fangs of the poisonous serpents; we will tell you about these by and by. But remember that the mouth must be made in a very strange way, in order to allow these creatures to swallow their victims, which are often a good deal larger round than their own throats. It sounds impossible, yet the snake can swallow an animal larger in diameter than its own throat, because the bones of its jaws, instead of being firmly fastened together as ours are, can be forced a long way apart, so as to make room for the carcass to pass. Besides this, it has no less than six separate jaw-bones, four in the upper part of the mouth and two in the lower, every one of which is set with sharp, hooked teeth; and the points of these teeth are directed toward the throat. Now every one of these jaw-bones can be moved backward and forward at will. So when a snake wishes to swallow the body of a victim, it first of all seizes it in its mouth, and then pushes one of the jaw-bones forward and takes a firm hold with the teeth. Then it pushes another forward, and then a third, and then a fourth; and so it goes on, each time taking a fresh hold with the hooked teeth, till at last the carcass is forced into the mouth. Then the bones separate, so as to make plenty of room for it to pass, and the alternate action of the jaws goes on as before till the carcass is forced into the throat. And then the flesh of the throat, which is very elastic, stretches out too, till before very long the carcass disappears altogether. Then the eyes of snakes are made in a very curious way, for the eyelids, which are quite transparent, do not open and shut as ours do, but cover the eyes altogether. So a snake cannot blink; and it looks at you _through_ its own eyelids, which are very much like little spectacle-glasses fastened into the skin!