THE ANKLE-JOINT. 285 



The middle fasciculus (calcaneo-fibular), the longest of the three, is a narrow, 

 rounded cord passing from the apex of the external malleolus downward and slightly 

 backward to a tubercle on the outer surface of the os calcis. It is covered by the 

 tendons of the Peroneus longus and brevis. 



The Synovial Membrane invests the inner surface of the ligaments, and sends 

 a duplicature upward between the lower extremities of the tibia and fibula for a 

 short distance. 



Relations. The tendons, vessels, and nerves in connection with the joint are, 

 in front, from within outward, the Tibialis anticus, Extensor proprius hallucis, 

 anterior tibial vessels, anterior tibial nerve, Extensor longus digitorum, and Pero- 

 neus tertius ; behind, from within outward, the Tibialis posticus, Flexor longus 

 digitorum, posterior tibial vessels, posterior tibial nerve, Flexor longus hallucis ; 

 and, in the groove behind the external malleolus, the tendons of the Peroneus 

 longus and brevis. 



The Arteries supplying the joint are derived from the malleolar branches of 

 the anterior tibial and the peroneal. 



The Nerves are derived from the anterior and posterior tibial. 



Actions. The movements of the joint are those of flexion and extension. 

 Flexion consists in the approximation of the dorsum of the foot to the front of the 

 leg, while in extension tfie heel is drawn up and the toes pointed downward. The 

 malleoli tightly embrace the astragalus in all positions of the joint, so that any 

 slight degree of lateral movement which may exist is simply due to stretching 

 of the inferior tibio-fibular ligaments and slight bending of the shaft of the 

 fibula. Of the ligaments, the internal, or deltoid, is of very great power so 

 much so that it usually resists a force which fractures the process of bone to which 

 it is attached. Its middle portion, together with the middle fasciculus of the 

 external lateral ligament, binds the bones of the leg firmly to the foot and resists 

 displacement in every direction. Its anterior and posterior fibres limit extension 

 and flexion of the foot respectively, and the anterior fibres also limit abduction. 

 The posterior portion of the external lateral ligament assists the middle portion in 

 resisting the displacement of the foot backward, and deepens the cavity for the 

 reception of the astragalus. The anterior fasciculus is a security against the dis- 

 placement of the foot forward, and limits extension of the joint. The movements 

 of inversion and eversion of the foot, together with the minute changes in form by 

 which it is applied to the ground or takes hold of an object in climbing, etc., are 

 mainly effected in the tarsal joints, the one which enjoys the greatest amount of 

 motion being that between the astragalus and os calcis behind and the navicular 

 and cuboid in front. This is often called the transverse or medio-tarsal joint, and 

 it can, with the subordinate joints of the tarsus, replace the ankle-joint in a great 

 measure when the latter has become ankylosed. 



Extension of the tarsal bones upon the tibia and fibula is produced by the 

 Oastrocnemius, Soleus, Plantaris, Tibialis posticus, Peroneus longus and brevis, 

 Flexor longus digitorum, and Flexor longus hallucis ; flexion, by the Tibialis anti- 

 cus, Peroneus tertius, Extensor longus digitorum, and Extensor proprius hallucis ;* 

 inversion, in the extended position, is produced by the Tibialis anticus and posti- 

 cus ; and eversion by the Peronei. 



Surface Form. The line of the ankle-joint may be indicated by a transverse line drawn 

 across the front of the lower part of the leg, about half an inch above the level of the tip of the 

 internal malleolus. 



Surgical Anatomy. Displacement of the trochlear surface of the astragalus from the 

 tibio-fibular mortise is not of common occurrence, as the ankle-joint is a very strong and powerful 

 articulation, and great force is required to produce it. Nevertheless, dislocation does occasionally 

 occur, both in an antero-posterior and a lateral direction. In the latter, which is the most com- 

 mon, fracture is a necessary accompaniment of the injury. The dislocation in these cases is 

 somewhat peculiar, and is not a displacement in a horizontally lateral direction, such as usually 



1 The student must bear in mind that the Extensor longus digitorum and Extensor proprius hal- 

 lucis are extensors of the toes, but flexors of the ankle, and that the Flexor longus digitorum and Flexor 

 longus hallucis are flexors of the toes, but extensors of the ankle. 



