The nerves to the plantaris muscle and a bipennate part of the soleus muscle.

We performed nerve fiber analysis of the nerve to the plantaris muscle in ten cases. Macroscopically, the nerve to the plantaris muscle has a tendency to branch off from the tibial nerve itself independent of the nerves to the gastrocnemius and soleus muscles (the triceps surae muscle). After removing the epineurium of the tibial nerve, it was revealed that, in all ten cases, the nerve to the plantaris muscle formed a common funicular trunk with the nerve to a bipennate part of the soleus. This trunk is akin to the nerves to the deep muscles of the calf. In addition, in five of the ten cases, the nerve to the plantaris muscle had another component, which arose from the branch to the popliteus muscle. By removing the perineurium of the nerves concerned, it became clear that the elements of the nerve to the plantaris muscle, and of the nerve to the bipennate part of the soleus, had an intimate relation (inseparable). On the other hand, the elements of the nerve to the plantaris muscle and those to the popliteus were separable and they showed different routes proximally. Based on the present findings derived from nerve fiber analysis, we postulate that the plantaris muscle and bipennate part of the soleus muscle were probably derived from the deep muscle anlage of the calf in spite of their topographical closeness to the superficial muscles of the calf.