THE LOWER LIMB 579 



b}- their apposition the tunnel called the sinus tarsi. It is very 

 strong, and is to be regarded as the anterior hgament of the posterior 

 astragalo-calcaneal joint. 



The posterior ligament extends from the posterior aspect of the 

 outer tubercle on the posterior border of the astragalus to the 

 adjacent upper and inner surfaces of the os calcis. It is thin and 

 membranous, and its fibres are arranged in a radiating manner. 



The internal ligament is a narrow band which passes from the 

 inner tubercle on the posterior border of the astragalus to the back 

 of the sustentaculum tali of the os calcis. It is related to the 

 tendon of the flexor longus hallucis. 



The external ligament extends from the lower part of the outer 

 surface of the astragalus, below the facet, to be attached to the 

 adjacent part of the outer surface of the os calcis. It is under 

 cover of, and parallel with, the middle fasciculus of the external 

 lateral ligament, which, along with the internal lateral ligament of 

 the ankle-joint, contributes to the strength of this articulation. 



The synovial membrane of this joint is peculiar to it. 



Arterial Supply. — The arterial supply is derived from the posterior 

 tibial, external malleolar of the anterior tibial, posterior peroneal, 

 and external tarsal of the dorsaUs pedis artery. 



Nerve-supply. — The short saphenous and the posterior tibial, or 

 it may be the external plantar, nerves. 



Anterior Astragalo-calcaneal Joint. — This joint has a lateral liga- 

 ment at either side, and a posterior, but it is continuous anteriorly 

 with the astragalo-navicular joint. The ligaments are interosseous 

 or posterior, internal astragalo-calcaneal, and external or superior 

 calcaneo-navicular. 



The interosseous or posterior ligament has already been described 

 in connection with the posterior astragalo-calcaneal joint. 



The internal astragalo-calcaneal ligament extends from the 

 inner surface of the neck of the astragalus to the upper margin 

 of the inner border of the sustentaculum tali of the os calcis. 

 It blends posteriorly with the inner end of the interosseous 

 ligament, and anteriorly with the upper border of the internal 

 calcaneo-navicular or spring hgament. It is strengthened by the 

 internal lateral ligament of the ankle-joint. The internal or inferior 

 calcaneo-navicular or spring hgament is also an internal hgament 

 of this joint, but, inasmuch as it ranks as a ligament of the astra- 

 galo-navicular joint, it will be described in connection with that 

 articulation. 



The external or superior calcaneo-navicular ligament is placed on 

 the outer side of the joint. It also ranks as an external lateral 

 ligament of the astragalo-navicular joint. It is strong, and extends 

 from the anterior part of the upper surface of the os calcis, external 

 to the anterior facet, to a depression on the outer surface of the 

 navicular bone near its posterior margin. Inferiorly it blends with 

 the internal calcaneo-navicular ligament, and superiorly with the 

 superior astragalo-navicular hgament. 



