A Historical Review on the Muscle of Cruveilhier

Despite the fact that the human thumb has been investigated intensely with reference to its functional morphology, controversies remain; for example, regarding the muscle of Cruveilhier (deep head of the flexor pollicis brevis). Originally described in 1749, the human flexor pollicis brevis (FPB) only received special attention since Cruveilhier described it in 1834 as having a superficial and a deep head. Since then the existence of Cruveilhier’s deep head has been debated. By 1920 five views existed: 1. The deep head is not part of FPB because of its nerve supply; 2. It became extinct and was replaced by a slip from the oblique adductor pollicis; 3. It is a part of the “composite” FPB and is synonymous with Henle’s "interosseous volaris primus”; 4. The deep head received ontogenetically myofibrils from the primordial flexores breves medius and from the adductor (contrahentes) muscle plates; and 5. The deep head, “Henle’s muscle”, and oblique adductor pollicis are distinct muscles. In the 1960s Day and Napier revealed variations of the insertion and innervation of the deep head, but did not delineate their deep head from the "Henle’s muscle" or the adductor pollicis. They hypothesized, that the shift of the deep head’s insertion from ulnar to radial facilitated “true opposability” in anthropoids. We revealed that there are still new aspects to the story of this muscle, including new interpretations / conclusions regarding its development and evolution. The history of the investigations around this muscle is a fascinating story showing the next generation of anatomists the importance of detailed observation combined with the knowledge of several fields (anthropology, developmental biology, functional morphology, etc.). We discuss the muscles’ functional significance for the evolution of the precision grip and why – with respect to surgery (replantation) - it is important to know the anatomical details of this muscle.