A Cadaveric Analysis of Morphological Variations of Pectoral Girdle Muscles: Axillary Arch and Coexisting Pectoralis Muscle Variations

Abstract

Muscles that demarcate the axilla function as critical landmarks of the region, therefore awareness of anatomical variations of these muscles allows more effective decision making in clinical procedures performed at a location within the axilla. One of the well‐known anatomical variations of this category is the axillary arch. The axillary arch, also referred to as Langer’s axillary arch and Achselbogen (among many other terms used in literature), is a muscular‐tendinous slip crossing the axilla anteriorly. Typically, the axillary arch originates from the anterior border of the latissimus dorsi near the base of the axilla and inserts onto the deep tendon of the pectoralis major, as well as to the coracoid process and the muscular and tendinous structures found in the upper part of the humerus. In itself the axillary arch is considered a common variant, as indicated by a recent meta‐analysis report describing its prevalence as 5.3%; however, occurrence of bilateral axillary arches as well as additional thoracic muscle variations accompanying the axillary arch are less common.In the current study we report the identification of axillary arch variations in 4 out of 48 donors (8.3%) in the ethnically diverse whole‐body donation population in Northern California in 2020 and 2021. Two cases were accompanied with additional pectoral muscle variations. In one, bilateral axillary arches and a left pectoralis quartus were present. In this individual the proximal ends of the left axillary arch and pectoralis quartus formed a common aponeurosis and inserted onto the deep tendon of the pectoralis major. In the other, two supernumerary fascicles deep to the pectoralis major on both sides and a right axillary arch were present. The pectoral fascicles originated from the surface of the ribs and joined to the deep surface of the pectoralis major. The right axillary arch separately inserted onto the deep tendon of the pectoralis major.Embryologically, the two pectoral girdle muscles associated with the variations described above (pectoralis major and latissimus dorsi) originate from the ventrolateral lip of the dermomyotome and develop through the “In‐Out” mechanism; that is, their myogenic precursors first migrate into the forelimb bud, but then return to the thorax to form these muscles. This complex migration pattern of the precursors would increase the likelihood of accessory muscles arising in this region. Since the axillary arch and pectoralis quartus in humans are considered as vestigial remnants of the panniculus carnosus (cutaneous skeletal muscle sheets found in many mammals and some primates), the migrating precursors may occasionally reactivate a limited fraction of the panniculus carnosus developmental program, leading to the development of the accessory muscles.