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.

[Illustration: 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).

[Illustration: _Fig_. 75.--Skeleton of Lion. (After Huxley.)]

[Illustration: FIG. 76.--Anterior limb of Man, Dog, Hog, Sheep, and
Horse. (After Le Conte.) _Sc_, shoulder-blade; _c_, coracoid; _