Levator claviculae muscle
|Levator claviculae muscle|
|Latin||musculus levator claviculae|
|Origin||Anterior portion of transverse processes of C1 – C4 vertebrae|
|Insertion||Lateral half of the clavicle|
|Artery||Ascending cervical artery|
In human anatomy, the levator claviculae is a very rare accessory and vestigial skeletal muscle in the posterior triangle of the neck. It originates on the transverse processes of the upper cervical vertebrae and is inserted in the lateral half of the clavicle. Though a supernumerary muscle present in only 2–3% of all people, it is not an abnormality but a variant of normal human anatomy with an atavistic character.
It has also been referred to as the omocervicalis, cleidocervicalis, and tracheloacromial muscle, with the variation in names indicating different sites of origin and insertion.
Origin and insertion
Its origin is on at least the third uppermost vertebrae, from where it courses inferiorly and laterally, lateral to the scalene and levator scapulae muscles but medial to the sternocleidomastoid muscle. Passing posteriorly to the latter muscle, it is inserted either to the middle of the clavicle or, more frequently, to the lateral third of the clavicle. It might also blend with the trapezius before its insertion. 
It has been reported to originate on the sixth cervical vertebra.
The levator claviculae is an infrequently recognized anatomical variant in humans, distinguished from, for example, cervical adenopathy or a thrombosed vein, but a normal muscle in lower mammals and anthropoids. In humans, when present, it often appears unilaterally, most commonly on the left side, or bilaterally. 
The embryologic origin of the muscle is controversial and subject to numerous hypotheses. It has been proposed to originate from several neighbouring muscles, including the sternocleidomastoid, the trapezius, the scalenus anterior, and the longus colli, but is possibly derived from an additional segmentation of the ventrolateral muscle primordia of the neck. 
Uncommon in human anatomy, the levator claviculae is nearly always present in most other mammals — including gibbons, orangutans, and chimpanzees. There are yet no phylogenetic explanations for its absence in humans. 
History of research
The first documented observation of the levator claviculae occur in an anatomical drawing by Leonardo da Vinci, but the first documented case was not reported until 1813. Darwin briefly described the muscle as an example of many muscles "found in all kinds of apes" but rare in humans. About the same time, Huxley and Wood reported the muscle has a predominantly unilateral occurrence. Since then only a dozen cases have been reported and the traditional 2–3% ratio, attributed to Wood, is possibly an overestimation not clearly corroborated by recent research. 
- Darwin, Charles (1871). The descent of man.
- Loukas, M.; Sullivan, A.; Tubbs, R.S.; Shoja, M.M. (2008). "Levator claviculae: a case report and review of the literature". Folia Morphol. 67 (4): 307–310.
- Natsis, Konstantinos; Apostolidis, Stylianos; Nikolaidou, Elisavet; Noussios, Georgios; Totlis, Trifon; Lazaridis, Nikolaos (2009). "Levator claviculae muscle: a case report". Cases Journal 2 (2): 6712. doi:10.1186/1757-1626-2-6712. PMC 2740057. PMID 19829849.
- Rosenheimer, JL; Loewy, J; Lozanoff, S (2000). "Levator claviculae muscle discovered during physical examination for cervical lymphadenopathy". Clin Anat. 4 (13): 298–301. doi:10.1002/1098-2353(2000)13:4<298::AID-CA11>3.0.CO;2-Y. PMID 10873223.
- Rubinstein, David; Escott, Edward J.; Hendrick, Laura L. (April 1999). "The prevalence and CT appearance of the levator claviculae muscle: a normal variant not to be mistaken for an abnormality". AJNR Am J Neuroradiol (American Society of Neuroradiology) 20 (4): 583–6. PMID 10319965.
- Wiedersheim, Robert (1895). The structure of man an index to his past history. Macmillan and co. p. 207. (26 MB)
- Wood, John (January 1, 1869). "On a group of varieties of the muscles of the human, neck, shoulder, and chest with their transitional forms and homologies in the mammalia" (PDF). Proceedings of the Royal Society of London (18): 114–122. (TOC)