An anomaly of the right pectoralis major muscle is reported. Two muscle bundles are demarcated from the pars abdominalis. Some fibres of these bundles radiate into the distal part of the normal insertion on the humerus; most fibres, however, insert into an accessory tendon which attaches to the medial humeral epicondyle. The medial intermuscular septum is fixed to this tendon. The septum is traversed by filaments of the triceps muscle. Fibres of the triceps muscle also surround the ulnar nerve and the ulnar collateral blood vessels before they pass through the medial part of the septum. These findings are discussed under functional aspects. Catamnestically a tuberculosis cutis luposa exists on the right hand. A patho-causal connection between the anomaly and the tuberculosis of the skin cannot be excluded, because this may arise easier in an area of disturbed blood supply.