EXTERNAL PARTS OF BIRDS. — THE FEET. 121 



The next segment of the limb, C to D, or the ft)ot proper, is represented by the principal 

 metatarsal bone, mt. This corresponds to the linnian instep or arch of the foot, nearly from 

 the ankle-joint quite to the roots of the toes. The metatarsal bone, like the metacarital of 

 the hand, VA'hich 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 tripai-tite. Among 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 toes. 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 th(^ 

 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 articu- 

 lation with the hind toe. ly In spite of anatomical ])roprieties, the metatarsal part of a bird's 

 foot — from heel to base of toes — from C to D, is inordinary descriptive ornithology invaviahly 

 called "T/ie 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 

 vulgariy called " the shank." These points must be ingrained in the student's mind to 

 ])revent confusion. (See fig. 112 his, 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 (imgnis). 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. yiyykvyios, 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 

 merZio-tarsal, as in reptiles. 



