Three distinct edges separate these surfaces. The anterior edge (vista ) is very promi nent and sharp in its three superior fourths, but rounded off below : in its upper part it is quite subcutaneous, and may be felt under the skin. The external edge forms a very distinct line of demarcation between the internal and posterior surfaces; it gives attachment to the interosseous ligament, and at its inferior extremity it bifur cates and encloses a concave triangular surface, in which the fibula rests. The internal edge is more rounded than either of the others; more distinct inferiorly than superiorly. At its up per end it gives insertion to the internal lateral ligament of the knee-joint and the popliteus muscle, and lower down to the solceus and the common flexor of the toes.
The inferior or tarsal extremity of the tibia is of larger dimensions than the shaft, although much smaller than the superior. On its infe rior surface we notice a quadrilateral articular cavity, of greater dimensions transversely than from before backwards, concave in this latter direction, and slightly convex transversely, in consequence of the existence of a slight ridge in the centre, which passes from before backwards. This surface is for articulation with the supe rior part of the body of the astragalus to form the ankle-joint.
The anterior surface of the inferior extremity of the tibia is convex and rough; it gives in sertion to the anterior ligamentous fibres of the ankle-joint, and the tendons of the extensor muscles pass over it. The posterior surface is very slightly convex; sometimes a very super ficial groove exists upon it for lodging the ten don of the flexor pollicis longus ; and internal to that, and lying behind the internal malleolus, a more distinct and constant groove, which passes obliquely downwards and inwards, and lodges the tendons of the tibialis posticus and flexor eommunis.
On the inside of the inferior extremity, we observe that the bone is prolonged downwards and slightly inwards, forming a thick and flat tened process, quadrilateral in form, called malleolus intern/is. The internal surface of this process is rough and convex; it is quite subcutaneous; its external surface is smooth, and exhibits a triangular articular facet, which is united at a little more than a right angle with the articular surface on the inferior extremity of the tibia; by this facet the internal malleolus moves on the inner surface of the body of the astragalus. The apex of the mal lcolus has the internal lateral ligament of the ankle-joint inserted into it ; the anterior edge gives insertion to ligamentous fibres, and the posterior edge, much thicker than the anterior, is closely connected with the posterior surface of the inferior extremity of the tibia, and has upon it the oblique groove already referred to. In comparing the position of the malleolus in ternus with that of the internal tuberosity of the tibia, (which may best be done by laying the bone on its posterior surface on a horizontal plane,) it will be observed that the malleolus is considerably anterior to the tuberosity, a fact which is attributable to the same cause which occasions the change of aspect in the inferior part of each of the three surfaces of the shaft, namely, a torsion of the bone similar to that already noticed in the other long bones of the extremities. This torsion is manifest at the junction of the inferior and middle thirds, the lower part having the appearance of being twisted inwards, and the upper part outwards.
The outer side of the tarsal extremity of the tibia is excavated so as to form a triangular surface, rough in its entire extent, to which the fibula is applied, and into which are implanted the strong ligamentous fibres by which that bone is tied to the tibia.
Structure.—The cancellated texture is accu mulated in large quantity at the extremities, where, especially at the superior, a line is very frequently apparent on the whole circumference, indicating the place of junction of the epiphysis and shaft. The medullary canal is large, ap proaching the cylindrical form, and surrounded by a dense compact tissue.
Fibula (Fr. perong; Germ. Wadenbein).— This bone is situated on the outer and posterior part of the tibia. It is about the same length as that bone, but as its upper extremity is ap plied to the under surface of the external tube rosity, its inferior extremity projects below that of the tibia. There is a slight obliquity in its direction, and in consequence, its inferior extre mity advances more forwards than its superior.
The fibula is a very slender bone in its entire extent, however its extremities are a little enlarged. The superior extremity or head of the fibula (capitulum) is somewhat rounded on its inner side, flattened on its external surface, terminating superiorly in a point into which the external lateral ligament of the knee-joint is inserted, anterior and posterior to which the edge of the bone receives the tendon of the biceps muscle. At the upper and anterior part of its internal surface there is a small sur face nearly plane, which is articulated with a similar one on the external tuberosity of the tibia. On the shaft of the fibula we may dis tinguish three surfaces, but in consequence of the great extent to which the fibula appears to have undergone torsion, it is at first difficult to detect the lines of demarcation between these surfaces. The external surface is very narrow and convex in its upper third, gradually ex pands as it descends, and becomes hollowed out in its middle third, where it receives the peroncei muscles ; in both these portions the aspect of this surface is outwards and slightly forwards. In the inferior third it is quite flat, and its aspect is outwards and backwards. The internal surface has a longitudinal sharp ridge upon it, which gives insertion to the interosse ous ligament. This crest divides the internal surface into two portions ; the anterior, very small, in some cases not exceeding two or three lines, gives attachment to the extensor muscles of the toes and the peronmus tertius; the pos terior, much more considerable and slightly concave longitudinally for about its two supe rior thirds, has the tibialis posticus inserted into it. This surface, which above looks nearly directly inwards, looks forwards in its inferior third. The posterior surface is also very nar row above, and expands as it descends; upon it the twist in the bone is very obvious. In its superior third this surface looks outwards and backwards ; in its middle third, where it is much more expanded, it looks directly back wards; and in its inferior third its aspect is inwards, and here it terminates in forming a rough surface which is adapted to the similar one on the fibular side of the inferior extremity of the tibia. Superiorly the posterior surface of the tibia gives attachment to the solceus muscle, and lower down to the flexor pollicis proprius. The orifice of the nutritious canal, directed down wards and forwards, is found here.