Variation archive

854 published articles

• Encyclopedia entry9 August 2026

  Optic (CN II) Nerve

  In one study of 50 brains, the intracranial length of the nerve varied between 4.8 and 15.1 mm; the majority were 8-10mm. The optic chiasm may be absent. The optic nerves and tracts may be unequal on either side, or the nerve may be absent on either side. The optic nerves are occasionally doubled and "accessory" optic nerves have been reported. There are reports of the optic tract being directly joined to the peduncles cerebri and to the brain stem. The entire visual system has been reported absent in an adult.

  • Not assessed by the portal — what this means
• Encyclopedia entry9 August 2026

  Olfactory (CN1) Nerve

  The following parts of the olfactory nerve have been reported absent: olfactory nerve, bulb, tract and lobes.

  • Not assessed by the portal — what this means
• Encyclopedia entry9 August 2026

  Oculomotor (CN III) Nerve

  Reported variations in the oculomotor nerve involve the occurrence of unusual branches or deviations in the course of its branches. The oculomotor may give rise to a branch that supplies the lateral rectus muscle, either in addition to or replacing its usual supply from the abducens nerve, which may be absent. It may provide a branch to the superior oblique muscle. The oculomotor nerve has been reported to communicate with the sixth nerve (abducens) as they pass through the cavernous sinus. The upper division of the oculomotor nerve may communicate with the nasociliary nerve.

  • Not assessed by the portal — what this means
• Encyclopedia entry9 August 2026

  Non-Recurrent Recurrent Laryngeal Nerve

  See also: Vagus Nerve Image 27B Image 27C Image 27D Image 27F

  • Not assessed by the portal — what this means
• Encyclopedia entry9 August 2026

  Nerve to M. Quadratus Femoris

  The nerve to the quadratus femoris has been reported to supply (in addition to the quadratus femoris, the inferior gemellus, and the hip joint) branches to the upper part of the adductor magnus and superior gemellus. This branch to the superior gemellus may be this muscle's only nerve supply.

  • Not assessed by the portal — what this means
• Encyclopedia entry9 August 2026

  Nerve to External M. Sphincter Ani

  The nerve to the external sphincter ani (muscular branch of the pudendal plexus) may pass between mm. coccygeus and levator ani instead of piercing the coccygeus.

  • Not assessed by the portal — what this means
• Encyclopedia entry9 August 2026

  Nasal (Nasociliary) Nerve

  Several variations in the branches of this nerve have been reported. It may send branches to the superior rectus, medial rectus, and superior levator palpebral muscles. Branches emaninating from a small ganglion connected to the nasal nerve have been followed to the oculomotor and abducens nerves. The infratrochlear branch of the nasal nerve may be missing, in which case the areas normally supplied by this branch (skin of the upper eyelid, root of nose, conjunctiva, lacrimal caruncle and sac) receive their supply from the supratrochlear branch of the frontal nerve.

  • Not assessed by the portal — what this means
• Encyclopedia entry9 August 2026

  Middle Cervical Ganglion

  The sympathetic plexus in the region of the middle cervical ganglion may be posterior to the inferior thyroid artery or may be bifurcated, with the artery lying between the two trunks. The middle cervical ganglion may be absent, in which case the middle cardiac nerves are carried in the interganglionic chain.

  • Not assessed by the portal — what this means
• Encyclopedia entry9 August 2026

  Median-Ulnar Anastomoses

  See also: Median Nerve Ulnar Nerve

  • Not assessed by the portal — what this means
• Encyclopedia entry9 August 2026

  Mandibular Nerve

  Some of the branches of the smaller, superior division of this nerve may arise directly from the main trunk before it divides in two. In some cases, the two divisions of the nerve are separated by a small fibrous band (pterygosphenoid ligament), which extends from the root of the external (lateral) pterygoid plate to a point on the greater wing of the sphenoid in front of the foramen spinosum. This ligament is sometimes replaced by bone, and the smaller division of the nerve passes through a special foramen beneath the greater wing of the sphenoid, just lateral to the foramen ovale.

  • Not assessed by the portal — what this means
• Encyclopedia entry9 August 2026

  Lumbosacral Trunk

  The fourth lumbar (or furcal) nerve usually contributes to both the lumbar plexus and (through the lumbosacral trunk) the sacral plexus. Contribution to the lumbosacral trunk may come from the third or the fifth lumbar nerve. Sometimes the branch from the fourth lumbar nerve to the lumbosacral plexus is absent. In such cases, the fifth lumbar nerve supplies branches to both the lumbar and sacral plexuses. Thus, contributions to both plexuses may come from the fourth lumbar nerve, the third and fourth, the fourth and fifth or the fifth lumbar nerve alone.

  • Not assessed by the portal — what this means
• Encyclopedia entry9 August 2026

  Lumbosacral Plexus

  The lumbosacral plexus is formed by the union of the anterior primary divisions of the lumbar, sacral, and coccygeal nerves. In about 50% of cases it receives a branch from the twelfth thoracic nerve. Its components are distributed to the lower extremity in a manner homologous and similar to the distribution of the parts of the brachial plexus to the upper limb.

  • Not assessed by the portal — what this means
• Encyclopedia entry9 August 2026

  Lumbar Sympathetic Trunks

  See also: Sacral Ganglia

  • Not assessed by the portal — what this means
• Encyclopedia entry9 August 2026

  Lumbar and Sacral Spinal Nerves

  The ganglia of lumbar and upper sacral nerves may appear to be doubled, with two distinct swellings on the dorsal roots.

  • Not assessed by the portal — what this means
• Encyclopedia entry9 August 2026

  Obturator Nerve

  There are reports of several variations in the formation, course, and distribution of this nerve. The contribution to the nerve from the second lumbar nerve may be absent. Branches from the obturator to the following structures have been reported: mm. obturator internus, and pectineus, the obturator artery, the periosteum of the pelvic surface of os pubis. Both the anterior and posterior branches of the nerve may pass posterior to adductor brevis. The branch from the main trunk of the nerve to obturator externus may pass to the lateral (instead of medial) side of the obturator nerve.

  • Not assessed by the portal — what this means
• Encyclopedia entry9 August 2026

  Lumbar Plexus

  In one study of 64 lumbar plexuses, 47 were formed by four spinal nerves, 14 by five, and three plexuses by six spinal nerves. In its usual form, the first three lumber nerves are entirely distributed to the lumbar plexus. The fourth lumbar is divided between the lumbar and sacral plexuses, hence it is called the furcal (meaning forked) nerve. The name furcal is also applied to any nerve or nerves that enter into the formation of both the lumbar and sacral plexuses. The proportion of the fourth lumbar contributing to the sacral plexus, however, is variable.

  • Not assessed by the portal — what this means
• Encyclopedia entry9 August 2026

  Lumbar Ganglia

  Although there are usually four lumbar ganglia, the number may vary from three to eight. When there are fewer than four ganglia they tend to be larger than "usual."

  • Not assessed by the portal — what this means
• Encyclopedia entry9 August 2026

  Lingual Nerve

  A minute sublingual ganglion has been described arising from the lingual nerve or submandibular ganglion (a ganglion of the facial nerve carried by the lingual nerve) supplying the sublingual gland. This nerve may pierce the lateral pterygoid rather than passing between the two pterygoid muscles. It occasionally provides motor branches to the medial and lateral pterygoids and to the palatoglossus muscles.

  • Not assessed by the portal — what this means
• Encyclopedia entry9 August 2026

  Lesser Splanchnic Nerve

  The nerve may be absent (7% on the right side, 2% on the left, and 2% bilaterally). It is usually formed from the tenth and eleventh thoracic ganglia. The nerve arises symmetrically in 35% of bodies. "Normal" (usual) nerves (T10-T11) occurred in 14% of subjects; only 1% of these were symmetric bilaterally. The greater and lesser nerves may be carried in the same trunk.

  • Not assessed by the portal — what this means
• Encyclopedia entry9 August 2026

  Least (Lowest) Splanchnic Nerve

  This nerve may be absent (11% on the right side and 4% on the left side). The splanchnicus imus usually arises from the twelfth thoracic ganglion. It may be absent bilaterally in 2% of bodies. When absent, the lesser splanchnic supplies the renal plexus. The nerve arises symmetrically in 79% of bodies. "Normal" nerves were found in 92% of bodies and were symmetric in 77%.

  • Not assessed by the portal — what this means