The action of the two last described muscles is to elevate the os calcis, and thereby to lift up the whole body. When this is done on one foot in the act of progression, the other is capable of being carried forward unimpeded by the irregularities of the surface. When the foot is the fixed point, the soleus by acting on the tibia and fibula fixes the leg, while the gastrocnemius fixes the femur, or by acting further, draws it backward so as to bend the knee and lower the body.
3. Plantaris.—This little muscle is entirely covered by the outer head of the gastro cnemius. It arises from the upper part of the external condyle of the femur, and from the posterior ligament of the knee-joint. Its mus cular structure is only about two inches in length, and it sends its long slender tendon downwards and inwards, between the two great muscles of the calf, emerging from between them just where their two tendons unite ; it then passes down in contact with the edge of the tendo Achillis, to be inserted into the heel at the inner side of that tendon.
The action of the plantaris is to assist thE great extensors of the foot, and to draw upon the capsule of the knee-joint, so as to prevent any ill effects upon that ligament from thE motions of the knee-joint. It is occasionally deficient.
The deep layer of muscles consists of four : I. popliteus ; 2. flexor longus digitorum ; 3. flexor longus pollicis ; 4. tibialis posticus. They lie in close contact with the bones, and the last three of them are covered by the deep fascia of the leg.
This membrane is a thin expansion, dense in structure, connected on each side with the borders of the bones, and towards the ankles with the sheaths of the tendons ; and if traced along the interval between the inner ankle and the heel, it will be found to cover the vessels and to terminate at the internal annular liga ment. Immediately underneath it we find the deep layer of muscles now under consi deration.
I. Popliteus is situated below and behind the knee-joint, is flat and somewhat triangular, being broader below than above. Arises within the capsular ligament of the knee-joint, by a round tendon, from the under and back part of the outer condyle of the femur ; ad heres to the posterior and outer surface of the external semilunar cartilage; perforates the back part of the capsular ligament, and forms a fleshy belly which runs obliquely downwards and inwards. It is covered by a thin tendi nous fascia from the tendon of the semi-mernbra nosus ; inserted broad, thin, and fleshy into an oblique ridge on the posterior surface of the tibia, a little below its head, and into the trian gular space above that ridge. Action, to bend
the knee-joint, and when bent, to roll it so as to turn the toes inwards.
2. Flexor longus digitoruin is thin and pointed at its commencement, but gradually increases, and then diminishes again as its fibres end in a tendon. Arises fleshy from the posterior flattened surface of the tibia, be tween its internal and external angles, be low the attachment of the soleus, and con tinues to arise from the bone to within two or three inches of the ankle. The fibres run obliquely into a tendon, which is situated on the posterior edge of the muscle. This tendon runs in a groove of the tibia, behind the inner ankle, and then passing obliquely forwards into the sole of the foot, receives in its passage a strong slip from the tendon of the flexor longus pollicis. It then divides into four tendons, which pass through the slits in the tendons of the flexor brevis digitorum, and as they run along the under surface of the toes they are bound down by strong fibrous sheaths, within which there are also little accessory ligaments assist ing in fixing them. They are inserted into the bases of the extreme phalanges of the four lesser toes. The action of this muscle is to flex all the four smaller toes, and to assist in elevating the foot upon the toes.
' Previously to its division, the tendon of the . flexor longus gives insertion to an accessory muscle of considerable power (flexor acces- sorius), which connects it to the calcaneum, L and materially modifies the direction of its action upon the toes. Close to the point of ?. division, the tendons give origin to four small muscles (lumbricales), which may also be con sidered as accessories to the flexor longus.
When passing behind the inner malleolus, this tendon is in contact with that of the tibialis posticus, which lies close to the bone. They are inclosed in separate sheaths of synovial mem brane. In the leg this muscle is bound down by the deep fascia, and covered partly by the posterior tibial vessels which separate it from the soleus; its anterior surface rests against the tibia, and overlaps the tibialis posticus muscle; in the foot, its tendon lies between those of the flexor longus pollicis which are above it., and the flexor brevis digitorum which lies beneath it.