326 INFERIOR EXTREMITY 



which passes from the distal end of the lateral malleolus to the 

 lateral surface of the calcaneus. 



Ligamentum Deltoideum (O.T. Internal Lateral Lig.). 

 This is an accessory band on the medial side of the joint. 

 It is strong and of triangular form. Its apex, which is directed 

 proximally, is attached to a shallow pit on the distal end 

 of the medial malleolus. . Its fibres diverge from the apex, 

 and are attached in a continuous layer from before backwards 

 to the navicular bone, the talus, sustentaculum tali, and 

 posterior to this to the talus again. 



Synovial Membrane. The synovial membrane lines the 

 ligaments above described, and in some cases it sends a small 

 process proximally between the tibia and fibula. It is thrown 

 into a transverse fold anteriorly when the joint is flexed, and 

 into a similar fold posteriorly when the joint is extended. 



Movements. The movements which take place at the ankle-joint are 

 (i) flexion (dorsal-flexion); (2) extension (plantar-flexion); and (3) a 

 very limited degree of lateral movement (abduction and adduction) when 

 the foot is fully extended. The two principal movements (flexion and 

 extension) take place around a horizontal axis, which is not transverse, but 

 which is directed laterally and posteriorly, so that it is inclined to the 

 median plane of the body at an angle of about 60 (Krause). This 

 horizontal axis passes through or near the interosseous canal between the 

 calcaneus and talus (Henle). As the articular cavity formed by the tibia 

 and fibula, and also the part of the talus which plays in it, are broader in 

 front than behind, it follows that the more completely the ankle-joint is 

 flexed, the more tightly will the talus be grasped between the two malleoli. 

 In the erect position the talus is held firmly in the bony socket, and portions 

 of its articular surface project both in front of and behind the tibia. The 

 centre of gravity is placed anterior to the ankle-joint, and in this way the 

 bones are kept firmly locked. When, on the other hand, the ankle-joint 

 is fully extended (as when we rise on tip-toe) the narrower posterior part 

 of the talus is brought into the socket, and thus a limited amount of lateral 

 movement is allowed. In flexion the calcaneo-fibular and posterior talo- 

 fibular bands, greater part of the deltoid ligament, and the posterior part 

 of the capsule are put on the stretch. In extension the anterior talo- 

 fibular ligament, the anterior fibres of the deltoid ligament, and the anterior 

 part of the capsule are rendered tense. 



The Muscles principally concerned in producing dorsi-flexion of the 

 foot at the ankle-joint are the tibialis anterior and the peronaeus tertius ; 

 those which operate as plantar-flexors are the superficial muscles of the 

 calf, the tibialis posterior, and the peronseus longus and brevis. 



TlBIO-FIBULAR JOINTS. 



The fibula articulates with the tibia by both its proximal 

 and distal extremity. The upper of the two, the articulatio 

 tibiofibulariS) is a diarthrodial joint possessing a cavity and a 



