•270 THE ARTICULATIONS 



fibres i)assing obliquely downwards and Ijaekwards from the depression on the 

 astragalus, just behind the inner end of the sinus pedis, to the os ealeis behind the 

 sustentaeulum tali, thus eompleting the floor of the groove for the flexor longus 

 hallueis tendon. 



The synovial sac is distinct from any other. 



Tlu' nerve-supply is from the })osterior tibial or one of its plantar branches. 



The arteries are, a branch from the posterior tibial, Avhich enters at the inner 

 end of the sinus i)edis; and twigs from the tarsal, external malleolar, and the 

 peroneal, which enter at the outer end of the sinus. 



(ii) The Anterior Calcaneo-astragalold Joint 

 Class. — Diarthrosis. Subdivision. — Arthrodia. 



This joint is formed liy the anterior facet on the upper surface of the os calcis 

 and the facets on the lower surface of the neck and head of the astragalus; it is 

 bounded laterally and behind by ligaments, and communicates anteriorly Avith the 

 astragalo-scaphoid joint. The ligaments are: — 



Interosseous. Internal (or antero-internal) calcaneo-astragaloid. 

 External calcanco-scaphoicl. 



The interosseous ligament by its anterior laminie limits this joint posteriorly. 

 It has been already descrilji-d (p. 269). 



The antero-internal calcaneo-astragaloid ligament consists of short fibres 

 attached above to the rough depression on the internal surface of the neck of the 

 astragalus, and below to the upper edge of the free border of the sustentaculum 

 tali, l)lending posteriorly with the inner extremity of the interosseous ligament, and 

 anteriorly with the ujjper border of the inferior calcaneo-scaphoid ligament. It is 

 strengthened by the internal lateral ligament, the anterior fibres of which are also 

 attached to the inferior calcaneo-scaphoid ligament. 



The external calcaneo-scaphoid (superior calcaneo-scaphoid, Gray) (figs. 255 

 and 256 ) limits this, as well as the astragalo-scaphoid joint, on the outer side. It is a 

 strong, well-marked band, extending from the rough upper surface of the calcaneum, 

 external to the anterior facet, to a slight groove on the outer surface of the scaphoid 

 near the posterior margin. It blends below with the inferior calcaneo-scaphoid, 

 and alcove with the astragalo-scaphoid ligament. Its fibres run obliquely forwards 

 and inwards. The internal lateral, and middle fasciculus of the external lateral 

 ligaments of the ankle-joint also add to the security of these two j(nnts, and assist 

 in limiting movements between the bones by passing over the astragalus to the os 

 calcis. 



The movements of which these two joints are capable are adduction and abduc- 

 tion, with some amount of rotation. Adduction, or inclination of the sole inwards, 

 being combined with some rotation of the toes inwards, and of the heel outwards; 

 while abduction, or inclination of the foot outwards, is associated with turning of 

 the toes outwards and the heel inwards. Thus the variety and the range of move- 

 ments of the foot on the leg, which at the ankle are almost limited to flexion and 

 extension, are increased. The cuboid moves with the calcaneum, while the scaphoid 

 revolves on the head of the astragalus. 



In walking, the heel is first placed on the ground; the foot is slightly adducted; 

 but as the body swings forwards, first the outer then the inner toes touch the ground, 

 the astragalus presses against the scaphoid and sinks upon the spring ligament; 

 the foot then becomes slightly aliducted. When the foot is firmly placed on the 

 ground, the weiglit is transmitted to it obliquely downwards andinwards, so that 

 if the ligaments between the calcaneum and astragalus did not check abduction, 

 inward displacement of the astragalus from the til:)io-fibular arch would only be 

 prevented liy the tendons passing round the inner ankle (especially the tibialis 

 po>iticiis). If the ligaments l)e too weak to limit abduction, the weight of the body 

 increases it, and forces the inner malleolus and astragalus downwards and inwards, 

 givins rise to flat foot. 



