THE TARSAL JOINTS 269 



inner and stronger fibres of the anterior ligament. It is also limited by the pos- 

 terior portion of the astragalus meeting with the tibia. Thus the middle portion of 

 the external lateral ligament is always on the stretch, owing to its obliquely back- 

 ward direction, Avhereby it limits flexion; and its attachment to the fibula in front 

 of tlie malleolar apex, wliereby it prevents over-extension as soon as the foot begins 

 to twist inwards. This inward twisting, or adduction of the foot, is partly due to 

 the greater posterior length of the inner border of the superior articular surface of 

 the astragalus, and to the less proportionate height posteriorly of the external border 

 of that surface, but chiefly to the lateral movement in the calcaneo-astragaloid 

 joints. Flexion and extension take place round a transverse axis draAvn through 

 the Ijody of the astragalus. The movement is not in a direct antero-posterior plane, 

 but on a plane inclined forwards and outAvards from the middle of the astragalus to 

 the intermetatarsal joint of the second and tliird toes. 



5. THE TARSAL JOINTS 

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



(a) The calcaneo-astragaloid union. 



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



(c) The medio-tarsal joint. 



(a) The Calcaxeo-astragaloid Union 



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

 posterior. 



(i) The Posterior Calcaneo-astragaloid Joint 



Class . — Diartkrosis. Subdivision. — Arthrodia. 



The calcaneum articulates with the astragalus by two joints, the anterior and 

 posterior: the former communicates with the medio-tarsal; the posterior is sej^arate 

 and complete in itself. The two bones are united V)y the following ligaments : — 



Interosseous. External calcaneo-astragaloid. 



Posterior calcaneo-astragaloid. Internal calcaneo-astragaloid. 



The interosseous ligament (figs. 255 and 256) is a strong band connecting the 

 apposed surfaces of the calcaneum and astragalus along their oblique grooves. It 

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

 It is Ijetter marked, deeper, and broader externally. Strong laminae extend from 

 the rough inferior and external surfaces of the neck of the astragalus to the rough 

 superior surface of the os calcis anteriorly, forming the posterior boundary of the 

 anterior calcaneo-astragaloid joint; these have been described as the anterior 

 interosseous ligament. The posterior laminae extend from the roof of the sinus 

 pedis to the os calcis immediately in front of the external facet, thus forming the 

 anterior part of the capsule of the ])osterior joint. 



The external calcaneo-astragaloid ligament (fig. 255) extends from the 

 groove just below and in front of the external articular facet of the astragalus, to 

 the OS calcis some little distance from the articular margin. Its fibres are nearly 

 parallel with those of the middle fasciculus of the external lateral ligament of the 

 ankle, Avhich passes over it and adds to its strength. It fills up the interval ]>etween 

 the middle and anterior fasciculus of the external lateral ligament, a considerable 

 bundle of its fibres blending with the anterior border of the midtlle fasciculus. 



The posterior calcaneo-astragaloid ligament passes from the external 

 tubercle and lower edge of the groove for the flexor longus hallucis to the os calcis, 

 a variable distance from the articular margin. 



The internal calcaneo-astragaloid ligament is a narrow band of well-marked 



