EXTENSOR LOXGUS DIGITORUM 389 



ui)})cr })art of the anterior annular li,iiament is included in a special sheath beneath 

 the lower part of the same lijranient. Op})osite the first rnetatarso-phalangeal 

 joint, it gives off from its sides and under surface Ijands of connective tissue which 

 unite partly with the two lateral ligaments, especially the internal one, and partly 

 with the periosteum upon the sides of the first phalanx. The rest of the tendon 

 is tiattened out and fits closely to the dorsal aspect of the first phalanx; it is finally 

 attached to the second phalanx in a transverse line which crosses the upper surface 

 of its base. 



Nerve-supply. — From the anterior tilnal Ijy filaments which enter the inner 

 and dee]icr as])ect of the muscle about the middle of the leg. 



Action. — (1) To extend the first phalanx of the great toe; (2) slightly to extend 

 the second phalanx, but this movement is chiefly performed by the small muscles 

 of the sole of the foot, which give off expansions to Ije attached to the borders of 

 the tendon at the sides of the first phalanx; (3j to .flex the ankle, and at the 

 same time it will slightly adduct the front of the foot and invert the sole. When 

 the muscle contracts strongly it will hyper-extend the first phalanx, and at the 

 same time flexion of the second phalanx will be produced by the resistance of the 

 flexor longus hallucis tendon. 



Relations. — Superficially, the deep fascia, the tibialis anticus, and the extensor 

 longus digitorum; internally, the tibialis anticus; externally, the extensor longus 

 digitorum; deeply, the interosseous membrane, the tibia, and the anterior tibial 

 vessels and nerve. The tendon lies lieneath the two portions of the anterior annular 

 ligament, and after crossing the anterior tibial artery near the ankle-joint it runs to 

 its insertion with the dorsalis pedis artery and the innermost tendon of the extensor 

 brevis digitorum on its outer side. 



Variations. — The muscle is occasionallj'^ divided, and a smaller external portion joins the 

 first tendon of the extensor brevis digitorum, or is inserted separately into the head of the first 

 metatarsal bone or the base of the first phalanx. 



8. EXTENSOR LOXGUS DIGITORUM PEDIS 



The extensor longus digitorum — named from its length and its action upon the 

 toes — is fusiform and somewhat flattened, with a four-divided tendon. 



Origin. — (1) The outer part of the under surface of the external tuberosity of 

 the tibia; (2) the upper three-fourths of the anterior or extensor surface of the 

 fibula; (3) the outer border of the anterior surface of the interosseous membrane 

 in its upper third; (4) the posterior surface of the deep fascia of the leg; (5) the 

 intermuscular septa which separate it from the upper part of the tibialis anticus 

 and from the long and short peronei. 



Insertion. — The three phalanges and the metatarso-phalangeal joints of each 

 of the four outer toes. 



Structure. — This is a penniform muscle, and its fibres arise fleshy from the 

 bones and the fascia, and pass forwards and inwards to the back and outer side 

 of the long tendon of insertion. This begins about the middle of the leg, and 

 becomes free from fleshy fibres about the level of the ankle-joint; it passes behind 

 the upper part of the anterior annular ligament, but not in a special synovial sheath; 

 then beneath the lower part of the anterior annular ligament in a special synovial 

 sheath with the peroneus tertius. At this point it divides into four tendons, which 

 diverge upon the back of the foot to the bases of the four outer toes. Each tendon 

 first gives off some strong fibres, which blend with the lateral ligaments of the 

 metatarso-phalangeal articulation, and with the periosteum along the borders of 

 the first phalanx. It then forms a Inroad expansion covering the back of the first 

 phalanx, and divides into three parts: the central part is inserted into the dorsal 

 aspect of the base of the second phalanx; and the two lateral jxirts pass forwiirds 

 with a slight convergence upon the back of the second phalanx to be inserted into 

 the dorsal aspect of the base of the third phalanx. 



Nerve-supply. — From tlie anterior tibial by filaments which enter the deep 

 aspect of the muscle in its upper third. 



Action, — (1) To extend the first phalanges of the four outer toes. It has some 



