Lateral Crural Muscles
3. THE LATERAL LEG (Crural) MUSCLES (fig. 659)
The peroneus longus (figs. 654, 657, 659). which is the more superficial of the two muscles; is situated at the upper part of the lateral side of the leg. It arises from the head and upper two-thirds of the lateral surface of the shaft of the fibula, from the deep surface of the fascia cruris, and from the anterior and posterior crural intermuscular septa: occasionally also by a few fibers from the lateral condyle of the tibia. Between its attachments to the head and body of the fibula, there is a gap through which the lateral popliteal (common peroneal) nerve passes to the front of the leg. It ends in a long tendon, which runs behind the lateral malleolus, in a, groove common to it and the tendon of the Peroneus brevis, behind which it lies; the groove is converted into a canal by the superior peroneal retinaculum, and the tendons in it are contained in a common synovial sheath. The tendon then runs obliquely forwards across the lateral side of the calcaneum, below the peroneal tubercle and the tendon of the Peroneus brevis, and under cover of the inferior peroneal retinaculum (fig. 659); it crosses the lateral side of the cuboid bone, and then runs on the under surface of that bone in a groove which is converted into a canal by the long plantar ligament (fig. 668)- It crosses the sole of the foot obliquely, and is inserted by two slips into (a) the lateral side of the base of the first metatarsal bone and (b) the lateral side of the medial cuneiform bone; occasionally a third slip is attached to the base of the second metatarsal bone. The tendon changes its direction at two points: (a) below the lateral malleolus, (b) on the cuboid bone; in both of these situations it is thickened, and, in the latter, a sesamoid fibrocartilage (sometimes a bone) is usually developed in its substance.
Nerve-supply.-The Peroneus longus is supplied by the musculocutaneous (superficial peroneal) nerve (L. 4 and 5 and S. 1).
Actions.-The Peroneus longus plantar-flexes the ankle-joint and everts the foot. As a consequence of the oblique direction of its tendon across the sole it is an important agent in maintaining the transverse and lateral longitudinal arches of the foot. Taking its fixed point below, the Peroneus longus serves to steady the leg on the foot; this is especially the case in standing on one leg, when the tendency of the superincumbent weight is to throw the leg medially; the Peroneus longus overcomes this tendency by drawing on the lateral side of the leg.
The Peroneus brevis (figs. 659, 660) arises from the lower two-thirds of the lateral surface of the shaft of the fibula, in front of the Peroneus longus; and from the anterior and posterior crural intermuscular septa. The fibers pass vertically downwards, and end in a tendon which runs behind the lateral malleolus along with, but in front of, that of the Peroneus longus, the two tendons being enclosed in the same compartment and lubricated by a common synovial sheath. It then runs forwards on the lateral side of the calcaneum above the peroneal tubercle (trochlear process) and the tendon of the Peroneus longus, and is inserted into the tuberosity at the base of the fifth metatarsal bone, on its lateral side.
On the lateral surface of the calcaneum the tendons of the Peronei longus et brevis occupy separate, osseo-aponeurotic canals formed by the calcaneum and the inferior peroneal retinaculum; each tendon is enveloped by a forward prolongation of the common synovial sheath.
Nerves.-The Peroneus brevis is supplied by the musculocutaneous (superficial peroneal) nerve (L. 4 and 5 and S. 1).
Action. - The Peroneus brevis plantar-flexes the foot upon the leg, and assists the Peroneus longus to evert the foot.
Applied Anatomy.-Rigidity and contraction of the tendons of the various muscles of the leg give rise to one or other of the kinds of deformity known as club foot. The most simple and common deformity, and one that is rarely, if ever, congenital, is talipes equinus, the heel being raised by the rigidity and contraction of the Gastrocnemius so that the patient walks upon the ball of the foot. In talipes varus the foot is forcibly adducted and the medial side of the sole raised, sometimes to a right angle with the ground, by the action of the Tibialis anterior et posterior. In talipes valgus, the lateral edge of the foot is raised by the Peronei, and the patient walks on the medial side of the foot. In talipes calcaneus the toes are raised by the extensor muscles, the heel is depressed and the patient walks upon it. Other varieties of deformity are met with, as talipes equinovarus, equinovalgus and calcaneovalgus, names which sufficiently indicate their nature. Of these, talipes equinovarus is the most common congenital form; the heel is raised by the tendo calcaneus, the medial border of the foot drawn upwards by the Tibialis anterior; the anterior two-thirds twisted medially by the Tibialis posterior, and the arch increased by the contraction of the plantar aponeurosis, so that the patient walks on the middle of the lateral border of the foot.
Rupture of a few fibers of the Gastrocnemius, or rupture of the Plantaris tendon, not uncommonly occurs, especially in men somewhat advanced in life, from some sudden exertion, and frequently occurs during the game of lawn-tennis, and is hence known as ` lawn-tennis leg.' The accident is accompanied by a sudden pain, and produces a sensation as if the individual bad been struck a violent blow on the part. The tendo calcaneus is also sometimes ruptured. It is stated that John Hunter ruptured his tendo calcaneus while dancing, at the age of forty.