The anatomy, fiber architecture, and innervation patterns of cat lateral gastrocnemius (LG) and plantaris (P) muscles are described. The plantaris is a simple unipennate muscle arising from an aponeurosis in common with LG and inserting primarily into the tendon of m. flexor digitorum brevis, but with ligamentous connections to the calcaneus. The lateral gastrocnemius is more complex and contains three distinctly identifiable heads, each of which is a unipennate band of fibers coursing between a proximally attached aponeurosis of origin and a distal aponeurosis of insertion that gives rise to the tendocalcaneus. Following microdissection of the LG and P nerves, and using glycogen depletion of the primary muscle nerve branches, discrete motor subvolumes are demonstrated in both muscles. Despite large specific differences in fiber architecture between the LG and P muscles, their organization into compartments about primary muscle nerve branches is fundamentally similar. This principle of organization may be a basis for the observed functional and structural properties of other vertebrate muscles. It may thus constitute a unifying concept in the organization of motor control mechanisms.