THE LOWER LIMB 575 



The external malleolar tendon-sheath is situated behind the 

 external malleolus. It is a common sheath for the tendons of the 

 peroneus longus and peroneus brevis, as they lie one upon the other 

 in the peroneal groove. It is situated beneath the external annular 

 hgament, which extends between the posterior border of the ex- 

 ternal malleolus and the outer border of the tuber calcis, and it com- 

 mences about 2 inches above that ligament. After escaping from 

 beneath the hgament the single sheath now forms two independent 

 sheaths, one for the peroneus longus tendon, and the other for the 

 peroneus brevis tendon, as these two tendons pass fonvards over 

 the outer surface of the os calcis. The two sheaths terminate 

 about I inch behind the level of the tuberosity on the outer side of 

 the base of the fifth metatarsal bone. 



Teno-synovitis is not uncommon in the sheath behind the external 

 malleolus. 



All the synovial bursae and tendon-sheaths in the vicinity of the 

 ankle-joint are quite distinct from the synovial cavity of that 

 joint. 



THE TIBIO-FIBULAR JOINTS. 



The superior and inferior extremities of the tibia and fibula form 

 direct articulations, and the shafts are connected by means of an 

 interosseous membrane and inferior interosseous ligament. 



Superior Tibio-fibular Joint. — This joint belongs to the class 

 diarthrosis, and to the subdivision arthrodia. The articular sur- 

 faces are the facets on the head of the fibula and on the external 

 tuberosity of the tibia, and the ligaments are two in number, 

 namely, anterior and posterior. Their fibres pass downwards and 

 outwards from the outer tuberosity of the tibia to the head of the 

 fibula, and they completely cover the joint in front and behind. 

 Superiorly and inferiorly they meet, and thus construct a capsule 

 for the joint. The anterior division of the tendon of the biceps 

 femoris is closely related to the anterior ligament, and contributes 

 materially to the strength of the joint, the more so because that 

 division has an insertion into the outer tuberosity of the tibia as 

 well as into the upper surface of the head of the fibula. 



The synovial membrane is usually distinct from that of the knee- 

 joint. Occasionally, however, it is in communication with it 

 posteriorly by means of the synovial investment which surrounds 

 the tendon of the popliteus. 



Arterial Supply. — ^The arterial supply is derived from the inferior 

 external articular, posterior tibial recurrent (inconstant), and anterior 

 tibial recurrent arteries. 



Nerve-supply. — ^The inferior external articular and recurrent 

 articular, both branches of the external popliteal, and the nerve to 

 the popliteus muscle, which is a branch of the internal p)opliteal. 



Movements. — The movements are extremely limited, and are of a gliding 

 or to-and-fro nature in an upward and downward direction. The knee being 



