[Variable extensor apparatus of the small toe. Opposing muscle groups in competition for extensor function of the...

In 200 human feet 37 differently formed extensor apparatus of the fifth toe were found. This great variability is obviously caused by several independent phylogenetic processes related with the acquisition of the upright gait. In primitive tetrapods three extensor muscles exist for the toes, whereas in mammals two of these muscles are in a rivalling but not vicarious relationship: While the long extensor muscles (M. extensor digitorum longus) remain in all trinomial toes, the fibular extensor muscles (Mm. peronaei digitorum) are laterally displaced by the short extensor muscles (M. extensor digitorum brevis). Although in man the fibular extensor muscles are largely suppressed, the short extensor muscles hardly often reach the fifth toe because this involuting toe is only of limited motor importance with regard to the entire function of the human foot. The ontogenetic involution of the fibular extensor muscles - which in the human embryo are regularly developed as M. peronaeus digiti IV and M. peronaeus digiti V - usually remains at an intermediate stage, i.e. a tendon of the M. peronaeus brevis branches off towards the dorsum pedis. In the anthropoids this involution is associated with the evolution and acquisition of the M. peronaeus III. To obtain strong effects for pronation and dorsiflexion necessary for the upright gait the M. peronaeus III inserts at the stable metatarsus instead of the mobile fifth toe by which an earlier phylogenetic stage is achieved. In the metatarsus the peronaeus tertius muscle inserts in different positions with regard to the remnants of the fibular extensor muscles. In this paper the two processes - both involution of the peronaei digitorum muscles and evolution of the peronaeus tertius muscle - are illustrated by several intermediate links found in dissected feet.