THE TARSAL JOINTS 



301 



medialward. This medial twisting, or adduction of the foot, is partly due to the greater pos- 

 terior length of the medial border of the superior articular surface of the talus, and to the less 

 proportionate height posteriorly of the lateral border of that surface, but chiefly to the side to 

 side movement in the talo-calcaneal joints. Flexion and extension take place round a transverse 

 axis drawn through the body of the talus. The movement is not in a direct antero-posterior 

 plane, but on a plane inclined forward and laterally from the middle of the astragalus to the 

 intermetatarsal joint of the second and thu-d toes. 



Muscles which act on the ankle-joint. — Flexors.— Tihisdis anterior, extensor hallucis longus, 

 extensor digitorum longus, peroneus tertius. Extensors.- — Tibialis posterior, flexor digitorum 

 longus, flexor hallucis longus, peroneus longus, peroneus brevis, soleus, gastrocnemius, plantaris. 



Fig. 333. — Anterior Aspect of the Lower Extremity of the Tibia. 

 In A, the articular surface is prolonged upward in front, forming a "squatting facet" which 

 a,rticulates with a corresponding facet on the neck of the talus. In B (the usual condition) the 

 articular surface is confined to the lower aspect of the bone. 



5. THE TARSAL JOINTS 



These may be divided into the following sub-groups : — 



(a) The talo-calcaneal union. 



(b) The articulations of the anterior portion of the tarsus. 



(c) The medio-tarsal joint. 



(a) The Talo-calcaneal Union 



There are two joints which enter into this union — viz., an anterior and a 

 posterior. 



(i) The Posterior Talo-calcaneal Joint 



Class.^ — Diarthrosis. Subdivision. — Arthrodia. 



The calcaneus articulates with the talus by two joints, the anterior and 

 posterior: the former communicates with the medio-tarsal; the posterior is 

 separate and complete in itself. At the latter joint the two bones are united by 

 an articular capsule with the following ligaments: — 



Interosseous. 



Posterior talo-calcaneal. 



Lateral talo-calcaneal. 

 Medial talo-calcaneal. 



The interosseous ligament (figs. 334 and 335) is a strong band connecting 

 the apposed surfaces of the calcaneus and talus along their oblique grooves. It 

 is composed of several vertical laminae of fibres, with some fatty tissue in between. 



