Intramuscular distribution of nerves supplying the soleus muscle in the chimpanzee

The human soleus muscle is considered a specialized muscle in terms of its origin, insertion and muscle fibre architecture (especially with regard to the existence of the bipenniform part). Its peculiarities have been understood as results of erect posture and bipedal walking (Frey, 1913). Sekiya (1991) pointed out that another feature of the human soleus muscle the nerve supply, i.e. the muscle received two kinds of nerves, the anterior branch (R. anterior) and the posterior branch (R. posterior); the former supplied the bipenniform part at the anterior surface of the muscle and communicated with the R. posterior within the muscle. In nonhuman primates, the soleus muscle has no bipenniform part and the nerve, identical with the R. anterior to the human soleus muscle, is unknown. The purpose of the present study is to clarify the pattern of the nerve supply to the soleus muscle in the chimpanzee, with special reference to the intramuscular distribution of the nerves and to discuss the origin of the R. anterior to the human soleus muscle from a comparative anatomical point of view. Six soleus muscles from three chimpanzees (Pan troglodytes) were examined under a stereomicroscope to clarify the intramuscular distribution of nerves supplying these muscles. The nerves supplying the soleus muscle were classified into three types according to the sites of their entry into the muscle. The first group nerve was the thickest of all nerves innervating the muscle, entered the muscle at the posterior surface of the proximal third and was considered as homologous with the R. posterior in the human.(ABSTRACT TRUNCATED AT 250 WORDS)