This peculiarity may be compared with that which we have drawn attention to regarding the development of the ulna; but here the seriation is less distinct. Not only in the different species, but even in the individuals of the same species, the development of the fibula presents little regularity. In quadrupeds, the bones of the leg are directed obliquely downwards and backwards, so that they form, with the femur, which is directed obliquely downwards and forwards, an angle, the apex of which is placed at the anterior surface of the knee.
Tibia.—The tibia of quadrupeds is readily comparable with that of man; as in the case of the latter, its shaft has three surfaces—an external, which is hollowed out in its upper portion, and becomes anterior below; an internal, slightly convex and subcutaneous; the posterior, which presents, in its superior part, a crest, the oblique line of the tibia, and some rugosities. The borders separate the surfaces. The anterior border, or crest of the tibia, is prominent in its superior part; below it gradually disappears in passing towards the internal aspect of the inferior extremity. The external and internal borders separate the corresponding surfaces from the posterior one.
The superior extremity is thick, and expands in forming three tuberosities: two lateral and an anterior. The anterior tuberosity, situated at the superior part of the crest of the tibia, is very prominent; for this reason the superior extremity is very much expanded in the antero-posterior direction—hence it results that this diameter is equal to the transverse, and sometimes even greater. In man, it is the latter which is the larger. The anterior tuberosity is visible under the skin.
The inferior extremity, less thick, is prolonged internally by a prominence which corresponds to the internal malleolus of man. In animals whose fibula is but slightly developed the tibia presents, on the external part of its inferior extremity, a small prominence, which replaces the fibular malleolus. The ruminants must, however, be excepted, in which we find in this region a special bone, which certain authors look on as the inferior part of the fibula (see [p. 97]). The inferior surface of this extremity of the tibia is articular; and is in contact with one of the tarsal bones, the astragalus. Because the superior surface of this latter has the form of a pulley, a pulley much more marked than that on the human astragalus, the corresponding surface of the tibia, which has the opposite form, presents two lateral cavities, separated by a median ridge, which is directed forwards and slightly outwards; this ridge projects into the groove of the pulley.
The Fibula.—This bone, situated at the back of the external surface of the tibia, is, as we have said, more or less developed. Its superior extremity, or head, articulates with the external tuberosity of the tibia. Its inferior extremity, when it exists—it is this which disappears in animals which have the fibula incompletely developed—forms a prominence which, placed on the external surface of the inferior extremity of the tibia, articulates with the astragalus, and recalls the external malleolus of man.
We have stated above that it is the inferior extremity of the fibula which disappears when the bone is incompletely developed; it is necessary to except the bat, in which the fibula, fairly well developed at its inferior extremity, by which it articulates with the tibia, thins off in its superior portion, and does not reach the corresponding extremity of the latter. Further, as in this animal the surface of the knee, which corresponds to the anterior surface of the same region in other animals, is turned backwards, the result is that the fibula is situated on the inner side of the tibia, instead of being placed on the outer.
The Foot
The foot, in animals, as well as in man, is formed of three portions, which, as we pass from the part which articulates with the leg towards the terminal extremity, are: the tarsus, the metatarsus, and the toes. These three portions are the homologues of the carpus, the metacarpus, and the fingers, which, as we have already seen in the case of the hand, are the osseous groups which form its skeleton. The tarsus is formed of short bones, as the carpus is; these are, in man, seven in number. The bones are arranged in two rows: one, the posterior, formed of two bones superimposed—the astragalus, by which the tarsus articulates with the leg, and the calcaneum, which forms the prominence of the heel; and an anterior row formed of five juxtaposed ones—the cuboid, situated externally, and the scaphoid internally, in front of which are found the three cuneiforms. To the tarsus succeeds the metatarsus, whose form reminds us very much of that of the metacarpals.
With regard to the toes, which we enumerate in proceeding from the most internal to the most external, they are formed of phalanges, which are three in number for the four outer toes; but the number is reduced to two in the case of the first—that is, the so-called great-toe.
The bones of the tarsus are not seven in all animals; they are fewer in ruminants and solipeds. We already know that, in the latter, the metacarpals and the digits are equally reduced in number; the same is the case for the metatarsals and the toes. We will analyze these differences when dealing with the [species individually].