Journal of Morphological Sciences Vol. 38/2021 347 Journal of Morphological Sciences J Morphol Sci Brazilian Society of Anatomy Case Report J Morphol Sci 2021;38:347-350 Accessory Head of the Flexor Pollicis Longus Muscle (Gantzer Muscle) in a Human Fetus: Case Report and Literature Review José Aderval Aragão, 1 Lucimario de Carvalho Barros, 2 Isaias Felipe dos Santos, 2 Iapunira Catarina Sant’Anna Aragão, 3 Felipe Matheus Sant’Anna Aragão, 3 Wallace Brener Santana Campos, 2 Myrla Lorenna Andrade Cruz, 2 Asaf Ramos dos Santos, 2 Francisco Prado Reis 4 1 Associate Professor of Clinical Anatomy, Department of Morphology, Federal University of Sergipe (UFS), Aracaju, SE, Brazil 2 Medical student at the Federal University of Sergipe (UFS), Aracaju, SE, Brazil 3 Medical Student, University Center of Volta Redonda (UNIFOA), Volta Redonda, RJ, Brazil 4 Titular Professor, Medical School of Tiradentes University (UNIT), Aracaju, SE, Brazil Disclose and conflicts of interest: none to be declared by all authors ABSTRACT Introduction: the accessory head of the flexor pollicis longus muscle (Gantzer muscle) is an additional muscle of the anterior compartment of the forearm that has a very variable incidence in the population. Case report: During a routine dissection, we found a fusiform muscle mass with tenuous tendons in the deep anterior portion of the right forearm of a fetus. Conclusion: Due to its location and relationship with the surrounding nervous structures, the presence of the Gantzer muscle may be related to the triggering of clinical syndromes, such as the anterior interosseous nerve and pronator syndrome. Keywords: Gantzer muscle, forearm, variation, dissection DOI: 10.51929/jms.38.60.2021 ISSN 2177-0298 Introducton Several cases of anatomical variations of the forearm musculature are described in the literature. In 1813, Gantzer described an accessory muscle of the forearm that joins the fexor pollicis longus as an anatomical variant that bears his name, and it has been widely debated. 1-7 The Gantzer muscle is an additional muscle of the deep anterior compartment that can be found bilaterally or unilaterally, which has the function of assisting in the fexion of the forearm. 8 Its origin, despite being quite variable, can occur in the following places: deep surface of the superfcial fexor muscle of the fngers, coronoid process of the ulna, medial epicondyle and/or as an aggregation of two or all of the anteriors. 1,3-9 Its insertion, on the other hand, occurs more frequently on the medial margin of the tendon part of the fexor digitorum longus muscle. 2,5,8,10-12 Regarding its topography with neurovascular structures, the accessory head of the fexor pollicis longus can be found anteriorly, 9,11,13,14 as well as posteriorly to the anterior interosseous nerve, 10,14 and rarely by the median nerve. 2-4,7,9 Its vascularization has been scarcely described, but for Jones et al., 2 this irrigation originates from several vessels: direct branch of the ulnar artery (55.9%), anterior recurrent ulnar artery (17.6%), anterior interosseous artery (11.8%), and median artery (2.8%). Also in relation to its morphology, the accessory head of the fexor pollicis longus muscle can be classifed into several types: thin (similar to a strip), fusiform, triangular, papillary, girdle and bulky. 2-4 However, its morphometry is quite variable, in adults the muscle length is on average 78.86 ± 10.94 mm, its tendon 8.53 ± 9.02, the muscle belly 72.71 ± 12.43 and its width 4.15 ± 1.71. 2-4,6 Anterior interosseous nerve syndrome is believed to be caused by trauma and structural abnormalities in the forearm, 8 but the presence of the accessory head of the fexor pollicis longus muscle (the Gantzer muscle) can lead to compression of the anterior interosseous nerve leading to paralysis, also called Kiloh-Nevin Syndrome, and should be suspected in patients with carpal tunnel syndrome who do not respond to conservative or surgical therapy, 8,15 as well as in surgical procedures from decompressive fasciotomies for compartment syndrome of the forearm. 8 Therefore, our objective was to report and describe the existence of the accessory head of the fexor pollicis longus muscle in a male fetus. Case Report During a routine dissection of a 33-week-old human fetus cadaver, male, in the human anatomy laboratory of the Department of Morphology of the Federal University of Sergipe, a muscle was found on the anterior surface of the right forearm that was found deep in the fexor carpi radialis, pronator teres and fexor digitorum superfcialis muscles. Its muscular belly was found medial and deep to the median nerve, where it received innervation from