EXTENSOR B RE VIS DIGITORUM 391 



4. PEROXEUS TERTIUS 



The peroneus tertius — named from -e/ji'i>r,, the fibula, and caWed tertius because 

 it is the third of the muscles which jjass from the fibula to the metatarsus — is a 

 small triangular sheet which is closely blended with the preceding muscle. 



Origin. — (1) The lower fourth of the anterior (or extensor) surface of the 

 fibula; (2) the front of the interosseous membrane for a short distance above the 

 ankle-joint; (3) the external intermuscular septum and the deep fascia of the leg. 



Insertion. — The upper part of the base of the fifth metatarsal bone. 



Structure. — Arising fieshy, the muscular fibres pass downwards and inwards 

 in penruform fashion to a tendon which appears on the inner border of its anterior 

 surface. It becomes free from fleshy fibres at the level of the ankle-joint, and 

 after passing beneath the upper part of the anterior annular ligament, it is in- 

 cluded with the extensor longus digitorum in a special synovial sheath beneath the 

 lower part of the ligament, and finally diverges from it to be inserted into the inner 

 part of the upper surface of the base of the fifth metatarsal bone. 



Nerve-supply. — Unlike the other peronei, which are supplied by the musculo- 

 cutaneous nerve, it receives filaments from the anterior tibial Avhich enter the 

 inner and deep aspect of the muscle in the upper part of its course. 



Action. — (1) To flex the ankle-joint; (2_) to abduct the anterior part of the 

 foot; (3) slightly to elevate the outer border of the foot and so to produce eversion 

 of the sole. 



Relations. — Superficially, the anterior annular ligament and branches of the 

 musculo-cutaneous nerve; on the inner side, the extensor longus digitorum, oi 

 which it is really a subdivision; on the outer side, the peroneus brevis; deeply, 

 the ankle and outer tarsal joints with the extensor brevis digitorum. 



Variations. — The peroneus tertius is often closely blended with the extensor longus digi- 

 tomm. It is sometimes wanting, and replaced by a slip of tendon from the extensor longus 

 digitorum. Occasionally it sends slips of tendon to the expansion of the extensor longus 

 digitorum on the first phalanx of the two outer toes, or to the fourth dorsal interosseous. 



MUSCLE OX THE DORSUM OF THE FOOT 



This consists of the four bellies of one muscle — the extensor brevis digitorum, 



EXTEXSOR BREVIS DIGITORUM PEDIS 



The extensor brevis digitorum — named from its being the shorter of the two 

 muscles which extend the toes — is a triangular sheet which breaks up in front into 

 four small divisions. 



Origin. — (1) The outer part of the upper surface of the great process of the 

 calcaneum; (2) the interior of the loop of fascia Avhich forms the outer jiart of the 

 lower anterior annular ligament. 



Insertion. — By four tendons into the four inner toes; the innermost is attached 

 to the outer border of the upper surface of the first ]ihalanx of the great toe near 

 its base; the three other tendons to the outer l)order of the tendons of the extensor 

 longus digitorum just in front of the bases of the first phalanges. 



Structure. — Arising superficially by fleshy, and on the deep surface by short 

 tendinous iil)res, the muscle diverges inwards and forwards, and soon divides into 

 four fleshy bellies, of which that to the great toe is the largest and most separate. 

 Each portion has a bipenniform arrangement, with its central tendon upon the 

 dorsal surface, and becoming free opposite the middle of the metatarsus. 



