The Ophidians (as we have seen) surpass all other Reptiles in the number of their vertebræ, with incomplete hæmal arches; these constitute the skeleton of the long, slender, limbless trunk. All these vertebræ coalesce with one another, and are articulated together by ball-and-socket joints. Besides this articulation to the centrum, the vertebræ of Ophidians articulate with each other by means of joints which interlock by parts reciprocally receiving and entering one another, like the tenon-and-mortise joint in carpentry. "The vertebral ribs have an oblong articular surface, concave above and almost flat below, in the Python. They have a large medullary cavity, with dense but thin walls, with a fine cancellated structure at their articular ends. Their lower end supports a short cartilaginous membrane, closing the hæmal arch, which is attached to the broad and stiff abdominal scute. These scutes, alternately raised and depressed by muscles attached to the ribs and integuments, aid in the gliding movement of serpents." The peculiar motion of Snakes was first noted by Sir Joseph Banks, and commented on by Sir Everard Home. Sir Joseph was observing a _Coluber_ of unusual size, and thought he saw its ribs come forward in succession, like the feet of a caterpillar. To test this, he placed his hand under the animal, the ends of the ribs were distinctly felt pressing upon the surface in regular succession, leaving no doubt that the ribs formed so many pairs of levers, by means of which it moves its body from place to place. The muscles which bring forward these ribs, according to Sir Everard, consists of five sets. One from the transverse process of each vertebra and the rib immediately behind it, which rib is attached to the next vertebra. The next set goes from the rib near the spine, and passes over two ribs, sending a slip to each, and is inserted into a third, a slip connecting it with the next muscle in succession. Under this is a third set, issuing from the posterior side of each rib, passing over two ribs, and sending a lateral slip to the next muscle, and is also inserted in the third rib behind. And so on throughout the five sets of muscles. On the inside of the chest there is a strong set of muscles attached to the anterior surface of each vertebra, and passing obliquely forward over four ribs is inserted into the fifth one only in the centre. From this part of each rib a strong flat muscle comes forward on each side, before the viscera, forming the abdominal muscles and uniting in a middle tendon, so that the lower half of each rib which is beyond the origin of this muscle, and which is only laterally connected to it by a loose cellular membrane, is external to the belly of the animal, and is used for the purpose of progressive motion, while that half of each rib which is next the spine, as far as the lungs extend, is employed in respiration.