Part 51

=Caput laterale= (Fig. 90, m, m′, m″).--The origin of the lateral head is in four portions:

(1) From the tendon of origin of the plantaris which comes from the middle of the lateral border of the patella. The tendon ends in a small conical mass of fibres (m′) which joins the ventral part of the common head.

(2) By a slender flat tendon, distad of the first, from the superficial fascia of the shank (m″). The tendon ends in a conical mass which joins the dorsal border of the common head.

(3) By a strong tendon and by muscle-fibres from the distal border of the lateral sesamoid bone (Fig. 61, 3) of the femur. These fibres (m) form the middle and outer part of the common head.

(4) By muscle-fibres from the middle of the outer surface of the aponeurosis covering the plantaris (Fig. 90, l) along rather more than the proximal half of the tibia. The lateral head thus formed is flat and fusiform in section.

=Caput mediale= (Fig. 91, k; Fig. 92, j).--The medial head resembles the lateral head closely.

Origin by a strong tendon from the medial sesamoid bone of the femur (Fig. 61, 4) and by muscle-fibres from the surface of the shaft for a short distance proximad of this.

The two heads unite by their contiguous borders at the junction of the first and second thirds of the tibia. The common muscle narrows rapidly and ends in a flat tendon which joins the tendon of the soleus to form the so-called tendon of Achilles (tendo calcaneus) (Fig. 90, t). This has its

Insertion into the proximal end of the calcaneus (Fig. 90, 4) near its ventral border. This tendon together with that of the soleus (Fig. 90, o) and the fascia of the shank forms a tubular sheath for the tendon of the plantaris.

Relations.--(1) Of the lateral head. Lateral (outer) surface with the biceps femoris (Fig. 68, t) and the superficial fascia. Medial (inner) surface with the popliteus (Fig. 92, k), the plantaris (Fig. 90, l) (to which it is closely united), the medial head of the gastrocnemius (Fig. 91, k), the soleus (Fig. 90, o), and the peroneus longus (Fig. 90, q). (2) Of the medial head. Medial (outer) surface with the superficial fascia and the tendons of the gracilis (Fig. 91, b), semimembranosus (Fig. 92, h′), and semitendinosus (Fig. 92, i). Lateral (inner) surface at the origin with the adductor femoris (Fig. 92, g), then with the plantaris (Fig. 91, l), the popliteus (Fig. 92, k), flexor longus digitorum (Fig. 91, n), and the lateral head of the gastrocnemius (Fig. 90, m).

Action.--Extensor of the foot. (It is to be noted that what is called =extension= in the foot is analogous to what is called =flexion= in the hand, so that the extensors of the foot are most readily to be compared with the flexors of the hand.)

=M. plantaris= (Fig. 90, l; Fig. 91, l).--A fusiform muscle covered distally by the gastrocnemius, but appearing between the two heads of the gastrocnemius at their proximal ends (Fig. 90, l).

Origin by a strong tendon from the middle of the lateral border of the patella and by fleshy fibres from the ventral border of the lateral sesamoid.

The patellar tendon ends in a flattened head which passes ventrad over the lateral epicondyle of the femur and then curves distad; it is closely united with the lateral head of the gastrocnemius. The muscle ends in a thick tendon which passes through a sheath formed by the tendons of the gastrocnemius and soleus, and the fascia of the shank. The tendon passes over the grooved proximal end of the calcaneus onto its ventral surface. It is held in place against the groove by two sheets of aponeurosis, which leave the tendon on either side for a distance of more than a centimeter and are attached to the lateral surfaces of the calcaneus at its proximal end. Lateral motion is thus hindered, while longitudinal motion is permitted.

Between this tendon and the tendon of Achilles is a synovial bursa which aids the gliding movement of the tendon. In the sole of the foot opposite the distal end of the calcaneus the tendon broadens and ends in the flexor brevis digitorum (pedis perforatus) muscle, which might also be considered as a second part of the plantaris. In man the plantaris is inserted into the calcaneus along with the gastrocnemius.

Relations.--The muscle is covered throughout by the two heads of the gastrocnemius (Fig. 90, m; Fig. 91, k), except at the proximal end (Fig. 90, l), where it is in contact with the integument. Inner surface in relation with the popliteus (Fig. 92, k), the soleus (Fig. 90, o), the flexor longus digitorum (Fig. 91, n), and flexor longus hallucis (Fig. 91, m).

=M. soleus= (Fig. 90, o).--A flat fusiform muscle lying beneath the plantaris.

Origin by muscle-fibres from the lateral surface of the head of the fibula, and by tendon-fibres from the proximal two-fifths of its ventral border.

Insertion.--The muscle ends in a slender tendon which joins the lateral border of the gastrocnemius tendon to form the tendon of Achilles (Fig. 90, t), which forms the sheath of the plantaris tendon.

Relations.--Outer surface with the gastrocnemius (Fig. 90, m) and plantaris (Fig. 90, l). Inner surface with the peroneus longus (Fig. 90, q), peroneus tertius (Fig. 90, r), and flexor hallucis (Fig. 91, m).

Action.--Assists the gastrocnemius to extend the foot.

=M. triceps suræ.=--The gastrocnemius and the soleus are sometimes considered as forming a single muscle, with three heads, under the name triceps suræ.

=M. popliteus= (Fig. 92, k).--A triangular muscle passing from the femur obliquely toward the medial side over the ventral surface of the proximal end of the tibia.

Origin by a strong tendon from the popliteal groove on the surface of the lateral epicondyle of the femur. In the tendon is a sesamoid bone, the popliteal bone (Fig. 61, 5, page 89), which glides over the ventral part of the lateral articular facet on the proximal end of the tibia. As the muscle passes over the joint it is closely attached to the joint capsule. From the popliteal bone, which with the tendon lies within the capsule of the knee-joint, the muscle-fibres diverge to their

Insertion into the proximal end of the ventral surface of the tibial shaft on the medial side of the medial oblique ridge. The area of insertion extends slightly onto the medial surface of the bone.

Relations.--Outer surface with the gastrocnemius (Fig. 92, j) and plantaris (Fig. 91, l), and with the tendon of the semitendinosus (Fig. 92, i). Distal border with the origins of the soleus and flexor longus digitorum. Inner surface with the capsule of the knee-joint and the tibia.

Action.--Rotates the thigh so as to turn the toes inward.

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