A study has been made of the long cruro-pedal flexor muscles in a number of species of Australian marsupials, which exhibit foot specializations closely paralleling those seen among the Eutheria. The phylogeny of this muscle group within the Metatheria is clearly apparent and elucidates the probable history of parallel eutherian specializations. It is suggested that M. flexor perforatus is, in its most primitive form, a crural muscle which has descended to the foot in a number of mammalian species, often becoming incorporated with the plantar aponeurosis as a fibrous or fleshy deep stratum. M. soleus has been derived by a splitting off of part of the substance of the lateral head of gastrocnemius. The large M. flexor fibularis was, apparently, the primary deep flexor of all the digits, and the smaller M. flexor tibialis waa probably at first partly attached to the plantar aponeurosis and medial cuneiform bone and partly to the M. flexor fibularis tendon. Loss of the latter attachment has produced the common marsupial arrangement, and loss of the former has produced the typical eutherian condition with fusion of the flexor tibialis and flexor fibularis tendons in the sole. A variable shredding of this common tendon has produced the different primate arrangements.
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