Relations.—Outer surface with the rectus femoris ([Fig. 92], b); borders with the vastus medialis ([Fig. 92], c) and vastus lateralis ([Fig. 90], d); inner surface with the femur.
Action.—The quadriceps extensor is an extensor of the shank. The vastus intermedius acting separately is a tensor of the capsule of the knee-joint.
3. Muscles of the Lower Leg (Crus).
A. On the Ventral Side.
—M. gastrocnemius.—This is the great muscular mass of the calf. It arises by two heads, the caput laterale or lateral head ([Fig. 90], m), and the caput mediale or medial head ([Fig. 91], k), of the gastrocnemius.
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.