86 THE OSTEOLOGY AND MYOLOGY 



the gradual subsidence of the tibial tuberosity. The shaft, as a whole, is slender and weak, 

 and presents no well defined sulci or ridges, except the crest just mentioned. The distal 

 extremity enlarges gradually, and is extended into an obtuse conical process that forms 

 the inner malleolus ; the articular surface is very obliquely placed, irregularly triangular 

 in contour, and nearly flat — its only concavity resulting from the downward extension of 

 the malleolar prominence. While this malleolus is sufficiently produced to overlap and 

 confine the corresponding side of the astragalus, the general configuration of the tibial 

 articular surface insures great freedom of motion in the ankle joint. 



Fibula. — (Fig. 26.) The distal extremity forms a perfect malleolus, as in other mai'- 

 supials, and presents internally a facet for articulation with the tibia — this facet being 

 continuous with that for the astragalus. Both the crural bones enter largely into the 

 formation of the ankle-joint ; and this distribution of the articulation of the astragalus 

 between two bones, instead of its restriction mainly to one (the fibula), probably pro- 

 duces a great part of the whole difference in the relative degree of mobility enjoyed by 

 the cruro-tarsal and radio-carjjal articulations. Still the restriction of motion is not great ; 

 and the foot enjoys rotatory movements perhaps more than merely analogous to the pro- 

 nation and supination of the hand. When the foot is strongly extended — and it can be 

 extended till its longitudinal axis is directly continuous with that of the leg, — its move- 

 ments of rotation are virtually, if not actually, those of pronation and supination. Did 

 the tibia take but little less part in the formation of the ankle joint, it would become 

 merely the pivot upon which the foot, borne upon the fibula, would rotate, and would 

 exactly conform the conditions of the ankle to those of the wrist. The shaft of the fib- 

 ula is very slender, almost perfectly cylindrical, and quite straight. The groove for the 

 passage of the tendons of the peroneal muscles is but slightly marked, and scarcely trace- 

 able above the malleolus. At the upper end of the middle third the shaft begins to 

 flatten and expand laterally to support a remarkably enlarged head. This expanded sur- 

 face is posteriorly flat and smooth. Anteriorly it is transversely concave, and somewhat 

 roughened for the attachment of the large peronei. The head of the bone is partially 

 divided into two tuberosities, both irregular in contour. The inner one, which is the 

 smallest, bears the oblique, concave facet already noticed. To the outer is attached a 

 large " sesamoid " bone, to be more particularly described in speaking of the knee joint. 

 The shaft of the filmla is not in contact with that of the tibia in any part of its length ; 

 both the proximal and distal extremities of the bone have, as we have already seen, loose 

 articulation with the tibia. The decided inclination of the tibial shaft toward that of the 

 fibula may relate in some way to the forearm-like movements of the leg ; but it is not 

 easy to see how these are favoi-ed by this means. 



Knee joint. — The general mutual relations of the two boneS|Of the leg are well calcu- 

 lated to increase mobility at the expense of stability ; carrying out a purpose that began 

 to be apparent in the structure of the coxo-femoral articulation. The resulting conditions 

 are the opposite of those presented by the saltatorial marsupial genera, in which the tibia 

 and fibula are united, or closely coaptated. The structure of the knee joint evidences the 

 same plan, by allowing of other than strictly ginglymoid movement. Although this artic- 

 ulation is, as usual, feraoro-tibial only — the fibula having no actual contact Avith the 

 femur — yet the expanded filjular head, and the presence of the fibular fabella, contribute 

 in some measure to femoro-fibular relations and movements that correspond to those 



