Action. To extend the great toe, and to bend the ankle.
By its fibular side this muscle is in relation with the extensor digitorum communis ; by its inner side with the tibialis anticus and anterior tibial vessels. The anterior border is covered by these two muscles, as low as about the middle of the leg, and inferiorly by the anterior annular ligament, under which it passes in a separate groove, and by the integuments. The posterior border rests upon the fibula and in terosseous ligament, and it crosses in its course over the lower end of the tibia the ankle-joint, the anterior tibial vessels, and dorsum of the foot.
4. Peroneus tertius.—This, which is in fact a mere slip of the extensor digitorum com munis, and is situated on its fibular side, is so closely connected with it at its origin that it can with difficulty be separated. It arises from the lower third of the fibula, being at tached to the anterior border and inner surface of the bone ; also from the interosseous liga ment, and from an aponeurosis which connects it on the outer side with the peroneus brevis. It is inserted by a flat tendon into the fibular side of the base of the metatarsal bone of the little toe. Its action is to assist in flexing the foot upon the leg.
It is in contact with the fascia of the leg anteriorly, with the fibula and interosseous ligament posteriorly, with the peronens brevis on the fibular side, and with the extensor corn munis on the tibial side. Its tendon passes in the same sheath with that of the common ex tensor, under the annular ligament.
A very slight effort of the extensor corn munis and extensor proprius pollicis extends the digital phalanges, and, if their action be continued, they will be made to bend the foot upon the leg. This they are enabled to do by the manner in which their line of direction is altered by the annular ligament of the ankle joint, as it gives them all the mechanical ad vantage of a pulley. The tibialis anticus and the peroneus tertius are the direct flexors of the foot on the leg, and if either act separately, it will give a' slight inclination towards the cor responding side, and thus the last-named muscle forms one of that important set whose action is, by elevating the outer side of the foot, to throw the weight of the body on the inner side.* In the erect position these muscles
take their fixed point below, and, by drawing on the bones of the leg, keep them perpen dicular on the foot.
The external muscles of the leg are two, the peroneus longus and brevis. They occupy the whole length of the outer side of the fibula, and are placed between the extensors and flexors.
1.,Pcronens longus is a long powerful muscle, arising from a small portion of the fibular side of the head of the tibia, from the upper third of the outer side of the fibula, and from the fascia of the leg and its intermuscular pro cesses. Proceeding obliquely downwards, the fibres are attached to a strong tendon, which passes, in contact with the peroneus brevis, along a groove at the back of the outer mal leolus, enclosed in a synovial sheath. The tendon then passes through a deep sulcus in the cuboid bone, behind the base of the meta tarsal bone of the little toe, winding obliquely across the sole of the foot, covered by the muscles of this part, till it is inserted into the internal cuneiform bone and base of the meta tarsal bone of the great toe. In the tendon opposite the cuboid bone, is usually found a sesamoid hone. A bursa] sheath encloses it in its passage across the foot. The action of this important muscle is to assist in extending the foot upon the leg, but principally to elevate the outer side of the foot, and thus regulate the bearing of the leg so as to throw the prin cipal part of the weight upon the great toe.i.
This muscle is in contact on its outer side with the fascia of the leg. Indeed this apo neurosis almost invests it, dipping between it and the flexor behind and extensors before. The peroneus is in contact with the fibula on its inner side above, lower down it rests upon the peroneus brcvis. When passing across the foot it lies close to the bones, and conse quently is covered by all the muscles of the sole.