MECHANISM OF THE LEG AND FOOT. 43 



(d.) The principal metatarsal bone, d, representing tlie distance c d, 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 Ijone, the accessory meta- 

 tarsal, m, in the figure, in dotted outline. It is of various shapes and sizes, 

 and variable in position up and down the lower p:u-t 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 proprieties involved, this part of the leg, 

 from heel to bases of toes, — from c to d — 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 accessor}^ Each of these individual bones is called a phalanx (pi. 

 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 diflerent toes ; but these points, and others, are fully consid- 

 ered farther on. We may here glance at the 



§ 73. (a.) Mechanism 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 

 leg ; 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, veiy 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 eflbrt is needed to overcome the resistance ; such birds 

 cjiu sleep standing up on one leg, and this is the design of the mechanism. 

 The ankle permits just the op]Dosite 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-sui'faces of the metatarsal 



