THE LOWER LIMB 573 



as in standing npon the toes. In flexion of the foot the broad anterior 

 part of the superior articular surface of the astragalus is carried backwards 

 into the narrow posterior part of the tibial socket, and lateral movement is then 

 impossible. Flexion is limited by (i) the tension of the posterior and middle 

 portions of the internal lateral Ugament ; (2) the tension of the posterior and 

 middle fasciculi of the external lateral ligament ; and (3) the locking which 

 takes place between the upper surface of the neck of the astragalus and 

 the anterior border of the lower end of the tibia. In extension of the foot 

 the narrow posterior part of the superior articular surface of the astragalus 

 is carried forwards into contact with the broad anterior part of the tibial 

 socket, and a certain amount of lateral movement can now take place. Exten- 

 sion is limited by (i) the tension of the anterior part of the internal lateral 

 hgament ; (2) the tension of the anterior and middle fascicuU of the external 

 lateral Ugament ; (3) the tension of the anterior ligament, particularly of 

 its inner part ; and (4) the locking of the posterior part of the astragcilus 

 against the posteror border of the lower end of the tibia. The range of 

 movement in the direction of flexion and extension is about 90 degrees, 

 and it takes place round a transverse axis passing through the body of the 

 astragalus in a direction forwards and outwards. At the end of extension 

 there is a tendency to abduction or turning in of the foot, due to the following 

 factors : (i) the greater length posteriorly of the inner border of the superior 

 articular surface of the astragalus ; (2) the greater depression of the corre- 

 sponding part of the outer border of the astragalus ; and (3) adduction at 

 the astragalo-calcaneal joint. 



The vertical hne of the centre of gravity falls in front of the axis of move- 

 ment at the ankle-joint. There is thus a tendency to over-flexion, which, 

 however, is counteracted by a certain amount of muscular effort on the part 

 of the sural muscles. 



Muscles concerned in the Movements — Flexion. — This is produced by the 

 tibiaUs anticus, peroneus tertius, extensor longus digitorum, and extensor 

 proprius hallucis. 



Extension. — This is produced by the gastiocnemius and soleus by means 

 of the tendo AchilUs, plantaris, tibialis posticus, flexor longus digitorum, 

 flexor longus hallucis, peroneus longus, and peroneus brevis. 



Abduction. — The foot is everted by the peroneus longus and peroneus 

 brevis. 



Adduction. — The foot is inverted by the tibialis anticus and tibialis posticus. 



Bursae and Tendon-Sheaths at Ankle-Joint. — ^The synovial borsae 



in the vicinity of the ankle-joint are as follows: 



Internal malleolar. Tendo Achlllis. 



External malleolar. Sinus tarsi. 



The malleolar bursae are not constant. There may be two mesial 

 over the internal malleolus of the tibia, and lateral over the external 

 malleolus of the fibula. 



The bursa of the tendo Achillis, which is constant, is situated 

 between the lower part of this tendon and the upper smooth hori- 

 zontal zone on the posterior surface of the tuber calcis. It Ues 

 directly above the insertion of the tendon into the middle rough 

 horizontal zone. 



The bursa sinus tarsi is situated within the sinus tarsi, between 

 the inferior surface of the astragalus and the superior surface of the 

 OS calcis. It hes within the strong interosseous hgament which binds 

 these two bones together, and it partially separates the anterior and 

 posterior laminae of which the interosseous ligament is composed. 



None of the foregoing bursae communicate with the sj'novial 

 cavity of the ankle-joint. 



