Meanwhile, another way in which the snake will escape from your hold unless you grasp it tightly, is by wriggling in all directions, so that you do not know where to expect it next; for the whole of the joints of its backbone are joined by a succession of cups-and-balls, the ball of one joint fitting into the cup in the one behind it. It is easy to see how such joints can move almost every way, since the ball can twist freely in the cup wherever the muscles pull it (except where checked by the spines on the top of the backbone), and can even turn so much to one side that the snake can coil itself round or tie itself into a knot. A creature that can glide along so smoothly, twist about so freely round trees, through narrow openings and tangled brushwood, and even swim in the water, has no small advantage in life; and the snake can also coil itself up under a heap of dead leaves or in a hollow trunk of a tree for safety, or to watch for its prey when no animal would suspect it was near. But even the harmless snakes have something besides this, namely, the power of swallowing animals much broader and thicker than themselves. You will see on looking at the lizard’s skull (p. 103) that its bottom jaw is not joined at once to the top one, but there is a bone (_q_) between, which enables it to open its mouth wider than if the two jaws touched each other. Now this bone (_q_) in the snake’s jaw is so loosely hung that it moves very easily, and the lower jaw also stretches back far behind the upper one, so that when the snake brings the jaw forward it can open its mouth enormously wide. Nor is this all; it can actually stretch the bones of its jaws apart, for they have not their pieces all firmly fixed together. In the front of the mouth each jaw has elastic gristle in the place of bone, and the two halves of the jaw can thus be forced apart from each other, making room for a very large mouthful indeed. [Illustration: Fig. 28. The Boa Constrictor in the Forests of South America.] Now the snake’s teeth are all curved towards the back of his mouth, and they are never used for chewing or tearing, but only for holding and packing down its food. So when he seizes a creature too large to be easily swallowed, he fastens his front teeth into it and then brings forward _one_ side of his jaws. He then fixes the teeth of this side into the animal, and holds it fast while he brings forward the jaws on the _other_ side, fixes these teeth, then loosens and brings forward the others, and so on. In this way he keeps his mouth stretched over the prey and gradually forces it down his elastic throat, moistening it well all the time with slime from two glands, one on each side of his mouth, and when it is swallowed he lies down and rests while the stomach digests its heavy load.