A knowledge of the edges which separate these surfaces will assist the student in understanding the position of the surfaces themselves. The anterior edge begins just below the head, passes down in front of the bone as far as the middle, then becomes exter nal and bifurcates, enclosing a triangular sur face on the outside of the inferior extremity of the bone, which is quite subcutaneous. The external edge is at first external, and about the commencement of the inferior third it begins to wind round so as ultimately to become posterior. The internal edge, which is the most acute, and is more prominent in the centre than at its extremities, passes forwards inferiorly, and terminates in front of the inferior extre mity of the bone : below it gives attachment to the interosseous ligament.
The inferior extremity is long and flat, and terminates in a point; it extends entirely below the inferior articular surface on the tibia, and, as Cruveilhier aptly. remarks, it forms exter nally the pendant to the malleolus internus, which it exceeds in length and thickness ; it is consequently called the malleolus externas. The internal surface of the external malleolus presents in its anterior two-thirds a plane triangular surface for articulation with the astragalus; behind this surface there is an excavation, which is rough, and gives insertion to the posterior external lateral ligament. The external surface is convex and subcutaneous, and the posterior surface is grooved for the passage of the tendons of the peronwi muscles. The apex of the inalleolus is directed down wards, and is the point of attachment of the middle external lateral ligament.
Structure. —This bone is very light and elastic, a property rendered necessary by the antagonist musclt.s which are inserted into its opposite surfaces. Its extremities are composed of eancellated structure, which extends some way to the shaft of the bone. The medullary canal, very narrow and irregular, is found only in its middle third.
Development Qf the bones of the leg.—The tibia begins to ossify somewhat earlier than the fibula. Both bones begin to ossify in their shafts ; the ossifie point of the shaft of the tibia appears about the middle of the second month. According to Ikleckel, in the embryo of ten weeks, the fibula is not above half the length of the tibia; after the third month the two bones are nearly equal. Both bones have an ossifie point for each extremity. The superior extremity of the tibia begins to ossify towards the termination of the first year after birth. The inferior extre
mity is ossified in the course of the second year : the external malleolus is a prolongation of the inferior extremity. The union of the extremities with the shaft commences by the inferior, and is completed from the eighteenth to the twenty-fifth year. The ossification of the fibula follows nearly the same course, excepting that the superior extremity does not begin to ossify till the fifth year.
The tibia constitutes the principal pillar of support to the leg. It is placed perpendicu larly under the femur, and as the latter bone is inclined inwards, it follows that there must he an angle formed between these two bones at the knee-joint, a very obtuse one, with its apex inwards.* It is then by the strength and direction of the tibia that the leg firmly sup ports the body in the erect attitude; the fibula seenis not to contribute at all to the solidity of thelimb, but is chiefly employed to increase the surface of attachment for the muscles of the leg.
The developement of the tibia and fibula in the inferior mammal ia is pretty similar to that of the radius and ulna. The tibia is always fully developed, and, as in man, is the prin cipal bone of the leg, its size being pro portionate to the weight and strength of the animal. Admitting the fibula to be the ana logue of the latter bone, we find that, as it is rudimentary in the Solipeds and Ruminants, so the fibula is in a similar condition in these animals. In the former animals this bone is applied to the external side of the head of the tibia in the form of an elongated stilet, termi nating less than half way down in a fine point. On the other hand, in Ruminants it is only the inferior part of the fibula that is developed ; it appears under the form of a small narrow bone, extending a very little way upwards, and form ing the external malleolus.
In Pachydermata the fibula is fully deve loped and quite distinct from the tibia, and very small in proportion. In Edentata the two bones are fully developed, and in the Sloths the inferior extremity of the fibula con tributes to form the articular surface fur the astragatus. In Rodentia the two bones are united together in the inferior half, as also with the Insectivora, particularly in the Mole. In many Carnivom these bones are fully developed and detached : this is particularly manifest in the Phocidw and the Felidre. In the Dogs, however, the fibula is attached to the posterior part of the tibia.