2. Peroneus Envois is situated at the outer side of the leg, but lower down as to its at tachments than the preceding muscle. It arises fleshy from the lower half of the outer side of the fibula to near the outer malleolus. It sends offa roundish strong tendon, which passes in the same groove behind the outer nialleo has, and in the same synovial sheath as the pre ceding muscle, but after passing the malleolits it has a sheath proper to itself. It is inserted into the base of the metatarsal bone of the little toe. Connected on its outer side to the peroneus longus, on the inner side to the fibula, anteriorly to the common extensor and pero neus tertius, and posteriorly to the flexor longus pollicis.
The action of these two muscles is peculiar. By the change in their direction, after turning behind the outer ankle, they are enabled to draw the foot back, and so extend it on the leg.
The penoneus tertius is on the contrary a flexor ; it lies before the fibula, and combines in this action with the tibialis anticus to assist the flexor. When, however, the three peronei act together, and without the other flexors, their combined action is to evert the sole of the foot, and thus counterbalance the effect of the feeble ness of the outer side of the foot by trans ferring the superincumbent weight to the inner side. This action is particularly exemplified in skaiting, but it is essential to every move ment of ordinary progression. (See article Poor, MUSCLES or.) When the foot is the fixed point, the peronei act by keeping the fibula and the whole leg steady, and thus, as in the act of standing on one foot, counter acting the tendency of the body to fall in wards.
The posterior region of the leg comprises seven muscles, six of which are acting on the foot and toes, and one is proper to the knee joint. We shall examine them as they are met with in dissection, and shall therefore describe them as forming two layers, superficial and deep. The first contains three muscles : 1. gas• trocnernius ; 2. soleus ; 3. plantaris.
1. Gasirocnesnius.—This is situated imme diately under the aponeurosis, and is a power ful muscle, broad and flat anteriorly, and con vex posteriorly, and forming the greater part of what is called the calf. It arises by two distinct heads from the back and upper part of the two condyles of the femur, of which the inner is the longer, and somewhat larger.
These heads have between them a broad sulcus, which forms the lower part of the pop liteal space. They unite a little below the knee joint, in a middle tendinous line, and below the middle of the tibia send off a flat tendon which unites with the tendon of the soleus, a little above the ankle.
The posterior surface is covered by the fascia of the leg ; anteriorly it rests upon the popli teus, soleus, and plantaris, and popliteal vessels. When its heads pass over the con dylesof the femur, they are guarded by synovial b ti rste.
2. Sokns.—This is the second portion of that great muscle of the leg which has been named by Meckel the triceps sure. It is seen immediately on raising the last muscle. It arises from two distinct situations ; first, from the upper and back part of the head of the fibula, and from the posterior surface and outer edge of that bone for some way down. Se cond, from the oblique ridge on the posterior surface of the tibia, just below the popliteus, and from the inner edge of that bone during the middle third of its length. From these two attachments the muscle almost imme diately forms a thick fleshy belly, which de scends lower than the gastrocnemius before it sends off its tendon. This, which is flat and strong, soon unites to the tendon of the gastro cnemius to form the tendo Achillis, and is then passing to he inserted into the upper and back part of the projecting portion of the os calcis. At its insertion there is a small bursa between the upper part of the bone and the tendon.
The soleus is in contact with the gastro cnemius posteriorly ; below its fleshy fibres ap pear on each side of the tendon of that muscle. Between its two origins the posterior tibial vessels and nerve are passing, defended from pressure by the tendinous expansion which is on the under side of the muscle, and which spreads across from tibia to fibula. This muscle is also in contact with the plantaris, the tendon of which crosses it obliquely from without to within. In front it rests upon the deep layer of muscles and upon the posterior tibial vessels.
The tendo Achillis is the thickest and strongest tendon in the body ; it tapers down wards nearly to the heel, and before its attachment expands again a little. It lies immediately under the skin, and between it and the bones is a considerable layer of cel lular tissue containing fat.