A very rare variation of the latissimus dorsi muscle--a case with accessory insertion of the latissimus dorsi muscle into the first rib and into the pectoralis minor muscle.

Abstract

A very rare variation of the latissimus dorsi muscle was discovered on the left side of the body in a 77-year-old Japanese man in whom this muscle had accessory insertion into the first rib and into the pectoralis minor muscle in addition to its regular insertion into the crest of the lesser tubercle of the humerus.A slender fasciculus (aberrant bundle) was found which separated anteriorly from the ventral margin of the latissimus dorsi muscle and immediately became a band-like aponeurosis. This aponeursis ran upwards and medialwards between the external intercostal muscles and the pectoralis minor muscle. After giving off a digitation that attached to the pectoralis minor muscle, it inserted for the most part into the first rib.The left side of the body also had a muscular arch of the axilla which attached to the regular tendon of insertion of the latissimus dorsi muscle and to the bandlike aponeurosis.Gross and dissecting stereomicroscopic examinations were made of the nerve supply, and the aberrant bundle from the latissimus dorsi muscle was found to receive a small branch from the thoracodorsal nerve, while the muscular arch of the axilla was innervated by the pectoral nerve.The literature was reviewed for any change in shape of the latissimus dorsi muscle in the presence of a muscular arch of the axilla, and for the true nature of the aberrant fasciculus from this muscle, in particular concerning the true nature of the band-like aponeurosis with which the aberrant bundle was continuous. As a result, it is suggested that the separation of an aberrant fasciculus from the latissimus dorsi muscle is possible regardless of the presence or absence of the muscular arch of the axilla. Moreover, it is speculated that the band-like aponeurosis was a band formed from the fascia situated on the external intercostal muscles and serratus anterior muscle. Concerning the mechanism of formation of this variation, it is considered that the aberrant bundle from the latissimus dorsi muscle had become adherent to the band formed from the fascia situated on the external intercostal muscles and the serratus anterior muscle, and further that the muscular arch of the axilla had become attached to the regular tendon of insertion of the latissimus dorsi muscle and to the above-mentioned aberrant bundle.