42 BONES OF LEG AND FOOT. 
(b.) Now a bird’s legs are not like ours, separate from the body from the 
hip downward, but are for a variable distance inclosed within the general 
skin of the body. The freedom is greatest among the higher birds, and es- 
pecially rapacious birds, that use the feet for grasping, and least in the low- 
est swimming birds: the entire range of enclosure of the leg, is from part 
way up the thigh down almost to the very point c, as in+he case of the loon 
and other diving swimmers. And in no birds, is the knee, B, seen outside 
the general contour of the plumage; it must be looked or felt for among the 
feathers, and in most prepared skins will not be found at all. Practically, 
it is a landmark of no consequence in determining genera and species, 
though of the utmost importance in primary classification; the student may 
for awhile ignore its existence if he chooses. The first joint that sticks out 
from the plumage is the HEEL, C; and this is what, in loose popular terms, 
is called “knee,” upon the same erroneous notion that the wrist of a horse’s 
foreleg is called “ knee.” Just so people call a bird’s crus the “thigh,” and 
disregard the thigh altogether. There is no need of this confusion; and 
even without the slightest anatomical knowledge, any one can tell knee from 
heel at a glance, whatever their position relative to the body; for knees 
ALWAYS bend forward, and heels anways bend backward. 
(c.) This point c corresponds to the point c in fig. 6 of the wing. There 
we found two little carpal bones, or wrist-bones, intervening between fore- 
arm and hand, or metacarpus; but adult birds have no such actual bones in- 
tervening between tibia and the next bone, d, the MrraTarsus. So there is 
no tarsus proper; metatarsus hinges directly upon tibia, or foot upon leg, 
without true ankle-bones; that is, the foot-bone itself makes the ankle-joint, 
with the leg, at the point c, heel. (Theoretically, however, there are tarsal 
bones: for there is an epiphysis (§ 56*) at the lower end of the tibia, and 
an epiphysis at the upper end of the metatarsal bone ; afterwards fused with 
these bones respectively. One or the other, or both of these are held by 
different anatomists to be tarsal bones; more particularly, the one that fuses 
with the metatarsus ; which last, therefore, represents both tarsus and meta- 
tarsus, and is on this account called ¢aso-metatarsus.*) 
*This is as usually taught. But Gegenbaur has shown that these so-called epiphyses are true tarsal 
bones. He represents, in the chick at the ninth day of embryonic life, two bones, an upper and an under, the 
former afterward anchylosing with the tibia, the latter with the metatarsus, leaving the ankle-joint between 
them, as in reptiles. Morse, who has studied the embryos of several species, goes still further: he shows that 
the upper tarsal bone of Gegenbaur is really two bones, corresponding to the tibiale and fibulare, or astrag- 
alus and calcaneum; these subsequently co-ossify to form the upper one seen by Gegenbaur, and finally 
co-ossify with the tibia to form the bitrochlear condyle characteristic of this bone in Aves. The distal tarsal 
ossicle he believes to be the centrale of reptiles. Wyman discovers that the so-called process of the astraga- 
lus has a distinct ossification, and Morse interprets it as the intermedium. (dm. Nat. vy, 1871, 524.) In the 
light of these late discoveries, the homologies of the bird’s carpus and metacarpus become clearer. We have 
seen (§ 55, 56, fig. 6) that birds retain throughout life two distinct proximal carpal bones (called scapholunar 
and cuneiform, but better named simply radiale and ulnare), and that in early life they have a distal bone, 
that was mentioned as the magnum, but appears to be centrale, corresponding to the distal tarsal ossicle, 
Just as the ulnare and radiale do to the proximal tarsal ossicles. Morse has eyen found in the carpus of 
birds, two more ossicles, the homology of which remains undetermined. But what we now know, renders it 
almost certain, that the so-called epiphyses upon the proximal ends of the metacarpals, are not epiphyses, 
any more than the so-called tarsal epiphyses; and that the metacarpus of birds is really carpo-metacarpus, 
just as the metatarsus is actually tarso-metatarsus. This view is strengthened by the fact that the metacarpal 
bones of higher vertebrates, except the first, ordinarily lack epiphyses. 
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