Axillary arch muscle and pectoralis quartus: an unusual combination of two variant supernumerary muscles in the axillary region - a case report.

Variant anatomy in the axillary region is of great clinical significance. It is one of the most frequently accessed regions for radical dissection surgery. During routine dissection of embalmed cadavers, we found a rare case of two accessory muscular slips emerging from the lateral border of latissimus dorsi (LD) and the inferolateral border of pectoralis major (PM), crossing the neurovascular structures in the axilla and merging distally together to the brachial fascia at the upper end of humerus below the bicipital groove. The accessory slip from LD is commonly referred to as the "axillary arch" in literature. We identified the accessory slip from the PM crossing over the axilla as pectoralis quartus. These aberrant slips can cause neurovascular compression in the axilla and can have clinical implications. Prior knowledge of the variant anatomy is the key to successful surgery in the axilla, thereby avoiding inadvertent injuries and post-operative complications.