In 8 out of 30 cases we found the fibrous arch of the Adductor pollicis muscle. In 4 cases, the resistant arch is situated between the 2 fasciculi of the Adductor Pollicis and inserted on the upper third of the third metacarpian on its anterior face. Il resists a traction of 500 g.; in 4 other cases, it is less resisting. The connections with the deep cubital vasculo-nervous fasciculus are important, especially the deep branch of the cubital nerve which can be compressed at this level.