204 THE MUSCLES. 
superficial fascia of the shank (#’’). 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 (7) 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, 7) 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, 4; Fig. 92, 7).—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 ten- 
don of Achilles (tendo calcaneus) (Fig. 90, 7). 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, 0) 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, 7) and the superficial fascia. 
Medial (inner) surface with the popliteus (Fig. 92, #), the 
plantaris (Fig. 90, 7) (to which it is closely united), the medial 
head of the gastrocnemius (Fig. 91, 2), the soleus (Fig. go, 0), 
and the peroneus longus (Fig. 90, g). (2) Of the medial head. 
Medial (outer) surface with the superficial fascia and the tendons 
of the gracilis (Fig. 91, 4), semimembranosus (Fig. 92, 4’), 
and semitendinosus (Fig. 92, 7). Lateral (inner) surface at 
the origin with the adductor femoris (Fig. 92, g), then with the 
plantaris (Fig. 91, 7), the popliteus (Fig. 92, £), flexor longus 
digitorum (Fig. 91, 2), and the lateral head of the gastroc- 
nemius (Fig. 90, 7). 
Action.—Extensor of the foot. (It is to be noted that what 
