I may best begin by describing the bones as these occur in the sundry branches of the mammalian type now living. As we shall presently see, the modifications which the limbs have undergone in these sundry branches chiefly consist in the suppression of some parts and the exaggerated development of others. But, by comparing all mammalian limbs together, it is easy to obtain a generalized type of mammalian limb, which in actual life is perhaps most nearly conformed to in the case of bears. I will therefore choose the bear for the purpose of briefly expounding the bones of mammalian limbs in general—merely asking it to be understood, that although in the case of many other mammalia some of these bones may be dwindled or altogether absent, while others may be greatly exaggerated as to relative size, in no case do any additional bones appear.
On looking, then, at the skeleton of a bear (Fig. 74), the first thing to observe is that there is a perfect serial homology between the bones of the hind legs and of the fore legs. The thigh-bone, or femur, corresponds to the shoulder-bone, or humerus; the two shank bones (tibia and fibula) correspond to the two arm-bones (radius and ulna); the many little ankle-bones (tarsals) correspond to the many little wrist-bones (carpals); the foot-bones (meta-tarsals) correspond to the hand-bones (meta-carpals); and, lastly, the bones of each of the toes correspond to those of each of the fingers.
Fig. 74.—Skeleton of Polar Bear, drawn from nature (Brit. Mus.).
The next thing to observe is, that the disposition of bones in the case of the bear is such that the animal walks in the way that has been called plantigrade. That is to say, all the bones of the fingers, as well as those of the toes, feet, and ankles, rest upon the ground, or help to constitute the “soles.” Our own feet are constructed on a closely similar pattern. But in the majority of living mammalian forms this is not the case. For the majority of mammals are what has been called digitigrade. That is to say, the bones of the limb are so disposed that both the foot and hand bones, and therefore also the ankle and wrist, are removed from the ground altogether, so that the animal walks exclusively upon its toes and fingers—as in the case of this skeleton (Fig. 75), which is the skeleton of a lion. The next figures display a series of limbs, showing the progressive passage of a completely plantigrade into a highly digitigrade type—the curved lines of connexion serving to indicate the homologous bones (Figs. 76, 77).
Fig. 75.—Skeleton of Lion. (After Huxley.)