girdle-- are shown in Figure 2. The limb skeleton corresponds
closely with that of the fore-limb. The femur (fe.) answers to the
humerus, and is to be distinguished from it by the greater
distinctness of its proximal head (hd.) and by the absence of an
olecranon fossa from its distal end. The tibia (ti = the radius) is fused
for the distal half of its length with the fibula (fb. = ulna). A tarsus
(tarsalia) equals the carpus.* Two of the proximal tarsalia may be
noted: one working like a pulley under the tibia, is the astragalus
(as.); one forming the bony support of the heel, is the calcaneum
(ca.). There is a series of metatarsals, and then come four digits of
three phalanges each.
* Such a resemblance as exists between one vertebra and another in
the rabbit, or between the humerus and the femur, is called serial
homology; the two things correspond with each other to the extent of
imperfect reduplication. "Homology" simply is commonly used to
indicate the resemblance between any two structures in different
animals, in origin and position as regards other parts. Thus the heart
of the rabbit and of the frog are homologous structures, corresponding
in position, and resembling each other much as two memory
sketches of one picture might do.