An unusual elbow muscle in the red howler monkey: does it deserve invention of a new name musculus contrahens cubiti?

Abstract

During dissection of a juvenile specimen of howler monkey (Alouatta seniculus), we discovered a supernumerary muscle. This muscle originates from the lateral epicondyle of the humerus, distal to the m. brahioradialis and beneath the m. extensor carpi radialis longus, and runs deeply to insert on the proximal part of the radius adjacent to the m. supinator. To determine homology of the unusual muscle, we compared it with the known abnormal extra muscles in the group of preaxial forearm extensors in humans and other primates, as well as other mammals. The only similar muscle is the so-called m. brachioradialis accessorius, which is very rarely found in humans medial to the n. radialis r. superficialis. Both in howlers and humans, its unique topological interrelations with the n. radialis suggest that this muscle is fundamentally different from all surrounding forearm extensors including the proper m. brachioradialis. At the same time, its innervation by the n. radialis confirms that it is a true extensor, contrary to the reptilian m. tractor radii. The general problem of identifying homology of anomalies and novelties is considered. As the enigmatic muscle departs from rules of myological architecture of the tetrapod forelimb, we failed to establish its general homology and, instead, suggest naming it as m. contrahens cubiti. This means that the muscle acts as the elbow flexor although it intimately belongs to extensors.