The vertebral column or backbone is a chain of bony rings or vertebrae which runs dorsally (p. 217) from the hinder end of the skull to the tail, and forms a long tube containing the spinal cord—a backward continuation of the brain. On the anterior face of the first vertebra are two hollows into which a pair of knobs on the skull fit. Although, with the exception of the first and second, all the vertebrae are formed on essentially similar lines, there is considerable variation in shape and size in the different regions of the spine, as may be seen from Fig. 161. There are seven neck vertebrae, and this number is remarkably constant in mammals (p. 220). Following these are the chest vertebrae, which bear pairs of ribs. The majority of the ribs curve round and join on to a ventral bar of bone called the sternum or breast bone; so that the cavity of the chest, containing those very important organs the heart and lungs, is enclosed [Pg 228] in a protective bony cage. The vertebrae of the abdominal region of the body are very large and stout. Between them and the tail-bones are four fused vertebrae, forming a mass which on each side gives attachment to the large hip-bone. The bones of the rabbit’s limbs.—The fore and hind-limbs of the rabbit are obviously comparable: the upper arm corresponding to the upper leg, the elbow to the knee, the fore-arm to the “leg,” the wrist to the ankle, and the fore-foot to the hind-foot. This similarity becomes even more apparent when the limb-skeletons are examined. Commencing in each limb at the end nearest the body we find a single long bone. In the fore-limb the rounded, upper end of this bone works in a socket at the anterior angle of the shoulder-blade, a triangular plate on each side which overlies the chest dorsally; in the hind-limb the rounded end of the corresponding bone works in a cup in the hip-bone. Again, between the elbow and the wrist are two bones lying side by side; and between the knee and the ankle are two corresponding bones, although here they are only separate in their upper parts. Similarly, the wrist bones correspond to the ankle bones, and the bones of what may be called the fingers to those of the toes. Certain of the ankle bones are, however, much elongated: obviously a great advantage to an animal which progresses by a series of hops—owing to the increased leverage which is thereby given to the hind-foot. The rabbit’s fore-foot bears five toes, the hind-foot four.