342 MUSCLES AND FASCIAE, 



^ The Extensor Proprius Pollicis is a thin, elongated, and flattened muscle, 

 situated between the Tibialis anticus and Extensor longus digitorum. It arises 

 from the anterior surface of the fibula for about the middle two-fourths of its 

 extent, its origin being internal to the Extensor longus digitorum ; it also arises 

 from the interosseous membrane to a similar extent. The fibres pass downwards, 

 and terminate in a tendon, which occupies the anterior border of the muscle, 

 passes through a distinct compartment in the annular ligament, and is inserted into 

 the base of the last phalanx of the great toe. Opposite the metatarso-phalangeal 

 articulation, the tendon gives off a thin prolongation on each side, which covers 

 its surface. 



Relations. By its anterior border, with the deep fascia, and the anterior annular 

 ligament. By its posterior border, with the interosseous membrane, fibula, tibia, 

 ankle-joint, and Extensor brevis digitorum. By its outer side, with the Extensor 

 longus digitorum above, the dorsalis pedis vessels and anterior tibial nerve 

 below. By its inner side, with the Tibialis anticus, and the anterior tibial vessels 

 above. 



The Extensor Longus Digitorum is an elongated, flattened, semi-penniform 

 muscle, situated the most externally of all the muscles on the fore part of the leg. 

 It arises from the outer tuberosity of the tibia ; from the upper three-fourths of 

 the anterior surface of the shaft of the fibula ; from the interosseous membrane, and 

 deep fascia ; and from the intermuscular septa between it and the Tibialis anticus 

 on the inner, and the Peronei on the outer side. The muscle terminates in three 

 tendons, which pass through a canal in the annular ligament, with the Peroneus 

 tertius, run across the dorsum of the foot, and, the innermost tendon having sub- 

 divided into two, are inserted into the second and third phalanges of the four 

 lesser toes. The mode in which the tendons are inserted is the following ; each 

 tendon opposite the metatarso-phalangeal articulation is joined, on its outer side, 

 by the tendon of the Extensor brevis digitorum (except the fourth), and receives 

 a fibrous expansion from the Interossei and Lumbricales, which then spreads 

 into a broad aponeurosis, covering the dorsal surface of the first phalanx ; this 

 aponeurosis, at the articulation of the first with the second phalanx, divides into 

 three slips, a middle one, which is inserted into the base of the second phalanx ; 

 and two lateral slips, which, after uniting on the dorsal surface of the second 

 phalanx, are continued onwards, to be inserted into the base of the third. 



Relations. By its anterior surface, with the deep fascia, and the annular liga- 

 ment. By its posterior surface, with the fibula, interosseous membrane, ankle- 

 joint, and Extensor brevis digitorum. By its inner side, with the Tibialis anticus, 

 Extensor proprius pollicis, and anterior tibial vessels and nerve. By its outer side, 

 with the Peroneus longus and Peroneus brevis. 



The Peroneus Tertius may be considered as part of the Extensor longus digi- 

 torum, being almost always intimately united with it. It arises from the lower 

 fourth of the anterior surface of the fibula, on its outer side ; from the lower part 

 of the interosseous membrane ; and from an intermuscular septum between it and 

 the Peroneus brevis. Its tendon, after passing through the same canal in the 

 annular ligament as the Extensor longus digitorum, is inserted into the base of the 

 metatarsal bone of the little toe, on its dorsal surface. This muscle is sometimes 

 wanting. 



Nerves. These muscles are supplied by the anterior tibial nerve. 



Actions. The Tibialis anticus and Peroneus tertius are the direct flexors of the 

 tarsus upon the leg ; the former muscle, from the obliquity in the direction of its 

 tendon, raises the inner border of the foot, and the latter, acting with the Peroneus 

 brevis and Peroneus longus, will draw the outer border of the foot upwards, and 

 the sole outwards. The Extensor longus digitorum and Extensor proprius pollicis 

 extend the phalanges of the toes, and, continuing their action, flex the tarsus upon 

 the leg. Taking their origin from below, in the erect posture, all these muscles 

 serve to fix the bones of the leg in a perpendicular direction, and give increased 

 strength to the ankle-joint. 



