EXTERNAL PARTS OF BIRDS.— THE FEET. 121 
The next segment of the limb, C to D, or the foot proper, is represented by the principal 
metatarsal bone, mt. This corresponds to the human instep or arch of the foot, nearly from 
the ankle-joint quite to the roots of the toes. The metatarsal bone, like the metacarpal of 
the hand, which it represents in the foot, is a compound one. Besides including the evanes- 
cent tarsal element or elements already specified, it consists of three metatarsal bones con- 
solidated in one, just as the metacarpal is tripartite. Arffong recent birds, the three are 
partly distinct only in the penguins; but in all, excepting ostriches, the original distinction is 
indicated by three prongs or stumps at the lower end of the bone, forming as many articular 
surfaces for the three anterior tces: The other toe most birds possess, the hind toe, is hinged 
upon the metatarsus in a different way, by means of a small separate metatarsal bone, quite 
imperfect ; this is the accessory metatarsal, am. It is situated near the lower end toward the 
inner side of the principal metatarsal bone, and is of various shapes and sizes; it has no true 
jointing with the latter, but is simply pressed close upon it, much as the fibula is applied to the 
tibia, or partly soldered with it. Above, it is defective: below, it bears a good facet for artieu- 
lation with the hind toe. 2" In spite of anatomical proprieties, the metatarsal part of a bird’s 
foot — from heel to base of toes —from C to D, is in ordinary descriptive ornithology invariably 
called ‘ The Tarsus” ; a wrong name, but one so firmly established that it would be finical 
and futile to attempt to substitute the correct name. In the ordinary attitude of most birds, 
it is held more or less upright, and seems to be rather “leg” than a part of the ‘ foot.” It is 
vulgarly called “the shank.” These points must be ingrained in the student’s mind to 
prevent confusion. (See fig. 112 bis, p. 229.) 
The digits of the foot, or toes, upon which alone most birds walk or perch, consist of 
certain numbers of small bones placed end to end, all jointed upon one another, and the basal 
or proximate ones of each toe separately jointed either with the principal or the accessory meta- 
tarsal bone. Like those of the fingers, these bones are called phalanges (Lat. phalanx, a 
rank or series) or internodes (because coming between any two joints or nodes of the toes). 
The furthermost one of each almost invariably bears a nail or claw (wnguis). The phalanges 
are of various relative lengths, and of a variable number in the same or different toes. But all 
these points, being matters of descriptive ornithology rather than of anatomy proper, are fully 
treated beyond, as is also the special horny or leathery covering of the feet usually existing 
from the point C outward. We may here glance at the 
Mechanism of these Bones. — The hip is a ball-and-socket joint, permitting round-about 
as well as fore-and-aft movements of the whole limb, though more restricted than the shoulder- 
joint. The knee is usually a strict ginglymus (Gr. yiyyAupos, gigglumos, hinge) or hinge-joint, 
allowing only backward and forward motion ; and so constructed that the forward movement of 
the leg is never carried beyond a right line with the femur, while the backward is so extensive 
that the leg may be quite doubled under the thigh. In some birds there is a slight rotatory 
motion at the knee, very evident in certain swimmers, by which the foot is thrown outward, so 
that the broad webbed toes may not ‘‘ interfere.” The heel or ankle-joint is a strict hinge; its 
bendings are just the reverse of those of the knee; for the foot cannot pass back of a right line 
with the leg, but can come forward till the toes nearly touch the front of the knee. In some 
birds the details of structure are such that, with the assistance of certain muscles, the foot is locked 
upon the leg when completely straightened out, so firmly that some little muscular effort is re- 
quired to overcome the obstacle; birds with this arrangement sleep securely standing on one leg, 
which is the design of the mechanism. The jointing of the toes with the prongs of the meta- 
tarsus is peculiar; for the articular surfaces are so disposed in a certain obliquity, that when 
end of the metacarpus includes carpal elements; and that a bird’s ankle-joint is not tibio-tarsal or between 
leg-bone and foot-bones, as in mammals, but between proximal and distal series of tarsal bones, and therefore 
medio-tarsal, as in reptiles. 
