Page:Natural History, Reptiles.djvu/154

146 another purpose in the animal’s economy. The ribs are immensely numerous and encircle a great part of the trunk; and the free extremities of these form, as we shall presently describe, the ordinary instruments of progressive motion in this Order. The vertebræ, or joints of the spine, are connected not by two hollow facets, filled up with cartilage, but by one convex face, filling that which is concave.

With a few exceptions, which form the connecting links with the Saurians, and are but a step removed in structure from the species last noticed, the Serpents are characterised by a remarkable looseness of the bones of the skull, and in particular by the mode in which the jaws are articulated. These reptiles are ordained to prey upon animals whose size much exceeds that of any part of their own bodies, or heads, and yet they are not furnished with any apparatus either of teeth or claws, by which their food may be divided; it must be swallowed whole. The elasticity of membranous viscera, such as the gullet or the stomach, we readily conceive, might be sufficiently great to allow of the passage of large masses of food; but the fixed and unyielding nature of the bones of the mouth in most animals would prevent the possibility of this. In the reptiles before us, however, a singular and beautiful deviation from this ordinary condition of fixity in the bones of the mouth meets the necessity of the case, and allows of an immense expansion of the parts. The structure may be thus familiarly explained. The lower jaw, which is much longer, and extends much farther back than the skull, is not hinged to the upper jaw,