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MECHANISM OF THE LEG AND FOOT. 43 
(d.) The principal metatarsal bone, d, representing the distance c pD, be- 
tween the lower end of the leg and the roots of the toes, really consists of 
three bones fused in one; these are partly distinct only in the penguins, 
among recent birds ; but in all birds except ostriches, the original distinction 
is indicated by three prongs or claws at the lower end of the bone: for joint- 
ing with the three principal toes. The other toe, almost always the hinder 
one, when it is present, is hinged on the metatarsus in an entirely different 
way; by means of a separate little rudimentary bone, the accESSORY META- 
TARSAL, 7, in the figure, in dotted outline. It is of various shapes and sizes, 
and variable in position up and down the lower part of the metatarsus. Or- 
dinarily it is too small, or too flat, to be seen from the outside of the 
foot at all; it has no true jointing with the main metatarsal, but is simply 
pressed flat against it, and more or less soldered, much as the lower part of 
the fibula is with the tibia. It may be wanting in some birds with no hind 
toe; in others, without hind toe, it still persists. 
(e.) In spite of the anatomical propricties involved, this part of the leg, 
from heel to bases of toes,—from c to p—represented really by the meta- 
tarsal bone and its accessory, has gained a name now so firmly established, 
that it would be finical to attempt to change it in ordinary descriptive writ- 
ings. This is THE TARSUS; we shall soon see how important a thing it is. 
(f.) The toes or digits consist of a certain number of bones placed end 
to end, all jointed upon each other, and the first series upon the metatarsal 
or its accessory. ach of these individual bones is called a phalanzx (pl. 
' phalanges) or internode (because intervening between the joints or nodes of 
the toes). The furthermost one of each toe almost invariably bears a claw. 
They are of various lengths relative to each other, and of variable number 
in the same or different toes; but these points, and others, are fully consid- 
ered farther on. We may here glance at the 
§ 73. (a.) Mecuantsm involved. The hip is a ball-and-socket joint, per- 
mitting roundabout as well as fore-and-aft movements of the thigh. The 
knee is usually a hinge-joint only, allowing back and forward motion of the 
lee; so constructed that the forward movement is never carried beyond a 
right line with the leg, while the backward is so free that the leg may be 
completely doubled under the thigh. In some birds there are also rotatory 
movements at the knee, very evident in certain swimmers. The ankle or 
heel-joint is a strict hinge, and sometimes a wonderful one, too, taken in 
connection with the action of certain muscles that move the tarsus. For in 
some birds the interior structure of the joint is such that it locks the tarsus, 
when straightened out upon the leg, in that position, so firmly that some 
voluntary muscular effort is needed to overcome the resistance ; such birds 
can sleep standing up on one leg, and this is the design of the mechanism. 
The ankle permits just the opposite bendings to those of the knee; the tar- 
sus cannot pass backward out of a straight line with the leg; but can come 
forward until the toes nearly touch the knee. The jointing of the toes on 
the metatarsal bone is peculiar; for the hinge-surfaces of the metatarsal 
