The true nature of the adductor brevis dually innervated by the anterior and posterior branches of the obturator nerve in humans

A morphological study on the human obturator externus muscle with reference to anomalous muscle and anomalous fasciculus originating from the obturator externus muscle

To elucidate the essential nature of the human obturator externus muscle (OE), the gross anatomy of the anomalous muscle and anomalous fasciculus originating from this muscle as well as the posterior division of the obturator nerve (P) were examined in 73 thighs of 45 Japanese adult cadavers. The investigation of anomalous muscle and anomalous fasciculus was effected by observation of the supernumerary muscular branch and supernumerary muscular twig from the P. The anomalous muscle occurring between the adductor brevis muscle (AB) and adductor minimus muscle (AMi) (37/73 thighs; 50.7%) and the anomalous fasciculus fused to the posterior surface of the AMi (18/73 thighs; 24.7%) were detected. It was demonstrated by the course of the P and the suppling nerve, that the OE was divided into its superior fasciculus (or fasciculi) (SF) and main belly by the P, moreover the anomalous muscle originated from the former and the anomalous fasciculus from the latter. Presumably, in the process of ontogeny, the SF of the original OE retained its original morphology (23/73 thighs; 31.5%), a part of the SF was converted to anomalous muscle (27/73 thighs; 37.0%), the entire SF was converted to anomalous muscle (10/73 thighs; 13.7%), and the entire SF underwent retrogression and disappeared (13/73 thighs; 17.8%). The presence (50/73 thighs; 68.5%) or absence (23/73 thighs; 31.5%) of piercing of the OE by the P seemed to depend upon the circumstances surrounding the morphological change in the SF. The original OE was considered to occur as an OE pierced by the P not possessing any supernumerary muscular branch or twig. Actually, the original OE was confirmed to occur with a low frequency (15/73 thighs; 20.5%). It was inferred that the original OE readily underwent variation in the process of ontogeny due to the influence of the obturator nerve. The segmental composition of the obturator nerve was considered not to be involved in the morphological change in the OE.