482 THE MUSCULATURE 



ankle. Usually at the distal margin of the transverse (anterior annular) ligament the tendon 

 divides into two parts which pass between the two layers of the cruciate (lower part of an- 

 terior annular) Ugament, and then each divides again into two parts, thus giving rise to four 

 sUps, one for each of the four lateral toes. 



Insertion. — Each tendon on the dorsal surface of the toe to which it goes divides into three 

 fasciculi: an intermediate, which is attached to the dorsum of the base of the second phalanx; 

 and two lateral, which converge to the dorsum of the base of the third phalanx. The margins 

 of the tendon are also bound by fibrous tissue to the sides of the back of the first phalanx. 



Nerve-supply. — Most frequently two branches of the deep peroneal (anterior tibial) enter 

 the deep surface of the muscle, one near its tibial origin, one about the centre of the beUy. 



Relations. — In the proximal half of the leg it lies on the interosseous membrane, and beneath 

 the fascia of the leg, and adjoins medially the tibialis anterior, laterally the peroneus longus. 

 Distallj' it lies over the extensor hallucis longus and adjoins laterally the peroneus brevis. The 

 tendon passes beneath the transverse crural and the superficial layer of the cruciate (anterior 

 annular) ligaments and over the extensor digitorum brevis muscle. The superficial peroneal 

 (musculo-cutaneous) nerve runs in the septum between it and the peroneal muscles; the anterior 

 tibial artery and deep peroneal nerve pass beneath the head of the muscle, and then between 

 it and the tibialis anterior. 



The peroneus tertius (fig. 415). — Origin. — From the distal third of the medial surface 

 of the fibula, the neighbouring interosseous membrane, and the anterior intermuscular septum. 



Structure. — It is essentially a fasciculus of the extensor digitorum longus, from which it is 

 seldom completely differentiated. The fibre-bundles descend obUquely forward to be inserted 

 in a penniform manner on a tendon which runs along the lateral margin of the tendons of the 

 extensor digitorum. The attachment of fibre-bundles continues to the cruciate ligament (lower 

 part of anterior annular ligament). 



Insertion. — On the base of the fifth metatarsal and often also on the base of the fourth. 



Nerve-supply. — The more distal nerve to the extensor digitorum continues into this muscle. 



Relations. — It lies lateral to the extensor digitorum longus. Its tendon passes into the foot 

 beneath the transverse crural and the superficial layer of the cruciate ligament in the same 

 compartments with those of the extensor longus. 



The extensor hallucis longus (fig. 415). — Origin. — From the middle two-fourths of the 

 medial surface of the fibula near the interosseous crest, and from the distal half of the interos- 

 seous membrane. 



Structure. — Penniform. The fibre-bundles are attached as far as the cruciate ligament 

 to the back and sides of a tendon which begins on the antero-medial margin of the distal 

 third of the muscle. 



Insertion, — On the base of the second phalanx of the big toe. On the back of the first 

 phalanx the margins of the tendon are attached to the bone by bands of fibres. 



Nerve-supply. — As a rule, a branch from the deep peroneal (anterior tibial) nerve enters 

 the deep surface of the muscle near the junction of the upper and middle thirds, and passes 

 distally across the middle of the obliquely running muscle fibre-bundles. 



Relations. — It hes on the distal half of the interosseous membrane, partly covered by the 

 extensor digitorum longus and the tibialis anterior muscles. Its tendon passes over the front 

 of the distal extremity of the tibia and the medial side of the dorsum of the foot and is held in 

 place by the transverse and cruciate ligaments and by a strengthening band in the fascia over 

 the base of the first metatarsal. In the distal part of the leg the anterior tibial artery and the 

 deep peroneal (anterior tibial) nerve pass beneath the muscle to enter the foot on the lateral 

 side of its tendon. 



Actions. — The muscles of this group all flex the ankle. The tibialis anterior and extensor 

 hallucis longus evert the foot at the talo-calcaneo-navicular joints, and invert it at the talo- 

 navicular, calcaneo-cuboid joints. In certain positions the tibialis anterior may, however, 

 invert the foot at these joints. The peroneus tertius and the long extensor evert the foot. 

 The force of the extensor hallucis longus is exerted powerfully on the first phalanx and weakly 

 on the second. The short muscles of the big toe aid in extending the second phalanx. The 

 extensor digitorum longus extends the first phalanx of each toe powerfully, but exerts less 

 force on the second and third. The lumbrical muscles assist in extending the last two phalanges. 



Variations. — The origin of the tibialis anterior may extend to the femur. Its tendon of 

 insertion may give accessory slips to the cuneiforms, metatarsals, and phalanges. More rarely 

 its belly is divided into two portions, one of which sends a tendon to the first cuneiform and one 

 to the finst metatarsal. A slip, the tensor fasciae dorsalis pedis (Wood), may pass to the dorsal 

 fascia of the foot. Another, the tibio-astragalus anticus (Gruber), to the talus (astragalus) 

 or calcaneus. The bellies or the tendons of the extensor hallucis and extensor digitorum may 

 be more or less completely fused, or tendon slips may pass from the tendon of one muscle to that 

 of the other. Tendon slips may pass to the metatarsal bones or from the tendon of one toe to 

 that of a neighbouring toe. The tendon to each toe may be doubled. The belly of the extensor 

 digitorum longus may be more or less completely subdivided to correspond with the tendons 

 to individual toes. The peroneus tertius is frequently fused with the long extensor. It may 

 be doubled. More often its tendon may })ifurcatc or trifurcate and be inserted into the extensor 

 tendons of the fifth toe or into the fourth or third metatarsal. It is absent in about 8.5 per cent, 

 of bodies (Le Double). 



Abnormal Muscles. — The abductor hallucis longus is rarely found as a completely in- 

 dependent muscle. It usually arises as a fasciculus of the extensor digitorum longus, extensor 

 hallucis longus, or the tibiali.s anterior. It is inserted into the base of the first metatarsal. The 

 extensor primi internodii hallucis (extensor hallucws brevis) has an origin similar to that of the 

 long abductor above dcsciribed. It is inserted into the dorsum of the base of the first phalanx 

 of the big toe. It is not to be confounded with that portion of the extensor digitorum brevis 

 connected with the great toe and also sometimes called the extensor hallucis brevis. 



