Illustrated Encyclopedia of Human Anatomic Variation: Opus III: Nervous System: Cranial Nerves and Ganglia
Ronald A. Bergman, PhD
Adel K. Afifi, MD, MS
Ryosuke Miyauchi, MD
Peer Review Status: Internally Peer Reviewed
The hypoglossal nerve has been reported to exit from the posterior surface of the medulla oblongata instead of its usual exit anteriorly.
In some cases, instead of coursing dorsal to the vertebral artery, the rootlets of the nerve, upon leaving the medulla oblongata, lie on either side of the artery (artery passing between rootlets), lie anterior to the artery, or pass through an annulus of the vertebral artery.
The right and left nerves may be connected by crossing fibers situated between the genioglossus and the geniohyoid muscles or in the substance of the geniohyoid (ansa suprahyoidea hypoglossi of Hyrtl). The reported frequency is 8-10% of individuals.
The hypoglossal nerve has been reported to carry sensory fibers which supply the tip of the tongue.
The hypoglossal may send branches to one or more of the following muscles: mylohyoid, diagastric, and stylohyoid.
In some cases, the descendens hypoglossi (C1 and C2) is carried partly or totally by the vagus nerve. The origin of fibers, in either case, remains the first and second cervical nerve. The descendens hypoglossi may send branches to the thorax, where it joins either the vagus nerve or the sympathetics. The origin of these branches is probably either vagal or sympathetic fibers that have joined the descendens hypoglossi. The descendens hypoglossi occasionally sends a branch to the sternal head of the sternocleidomastoid muscle.
Branches from the hypoglossal have been interpreted as consisting of aberrant vagal nerve fibers.
The ansa hypoglossis may be absent.
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References
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Chiarugi, G. (1889) Sur l'existence d'une racine dorsale rudimentaire avec ganglion pour le nerf hypoglosse chez l'homme. Arch. Ital. Biol. 11:421-422.
Fitzgerald, M.J.Y. and M.E. Law. (1958) The peripheral connexions between the lingual and hypoglossal nerves. J. Anat. 92:178-188.
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Wimmer, A. (1958) Einige darunter seltene Nervenvariationen des Halsgebietes A. Vier Variationen der Ansa nervi hypoglossi. B. Verlauf des Nervus accessorius durch ein "Knopfloch" der Vena jugularis interna sinistra. Anat. Anz. 105:391-399.
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