The lower leg of humans can be divided into a posterior compartment whose muscles include the muscles which allow plantar flexion of the foot (the gastrocnemius, soleus) and toes and an anterior compartment with muscles which allow the dorsiflexion of the foot (tibialis anterior) and toes.  The peroneus muscles allow eversion of the foot.  These muscles are not unique to humans.  The primitive condition of the gastrocnemius seems to be 2 separate muscles.  (Hartman, 1933).  The reptilian gastrocnemius internus forms gastrocnemius medialis and flexor hallucis longus in mammals (Kardong, p. 388) and the reptilian gastrocnemius externus forms gastrocnemius lateralis, soleus, and plantaris in mammals (Kardong, p. 388)

FROG

TURTLE

ALLIGATOR

CHICKEN

PIGEON

OPOSSUM

CAT

GOAT

SHEEP

PIG

MONKEY

Although the peroneus tertius has frequently been described as a muscle which only exists in humans as an adaptation to bipedal locomotion, it occasionally occurs in gorillas and rarely in chimps (Hartman, 1933, p. 166).

     The flexor digitorum brevis in apes is intermediate between humans and lower primates (Hartman, 1933,p 172).  In gorillas, chimps, and humans, the peroneus brevis may insert onto digit V proximal and middle phalanges and the origin of soleus frequently on tibia (Gibbs, 2002).  In chimps and humans, the origin of extensor digitorum is on crural fascia, the insertion of the abductor hallucis may include the medial cuneiform, and the origin of flexor digitorum brevis is on plantar aponeurosis (Gibbs, 2002).

HUMAN MODEL