Variation archive

754 published articles

• Encyclopedia entry9 August 2026

  Tibial and Sural Nerves

  Several variations have been reported in the formation and distribution of branches of these nerves. The point of union of the two branches of the sural nerve (tibial), which connects the medial sural cutaneous with the peroneal communicating branch (arising from the common peroneal nerve), is subject to wide variation. It may be high in the popliteal space or there may be no union at all. The union may occur 3 cm below the origin of the peroneal communicating nerve.

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• Encyclopedia entry9 August 2026

  Sciatic Nerve

  A number of variations in the course and distribution of the nerve have been reported. Bifurcation into its two major divisions (common peroneal and tibial) may occur anywhere between the sacral plexus and the lower part of the thigh. The two terminal branches of the sciatic may arise directly from the sacral plexus. The division of the nerve into its terminal branches has been reported to occur below the popliteal space. The nerve to the short head of the biceps femoris sometimes arises directly from the sacral plexus.

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• Encyclopedia entry9 August 2026

  Superficial Peroneal Nerve

  The medial branch may be absent and may be replaced by the deep peroneal (anterior tibial) nerve. Deficiences in the lateral branch are satisfied by the sural nerve. The nerve has been described as arising from the nerve to peroneus brevis, and extending downward to supply a rare peroneus quartus (extensor digiti quinti brevis), after which it continued on the dorsum of the foot to furnish a nerve to extensor digitorum brevis (Bryce).

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• Encyclopedia entry9 August 2026

  Superior Gluteal Nerve

  This nerve may receive fibers from the second sacral nerve. A branch to m. piriformis may arise from the lowest root of this nerve.

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• Encyclopedia entry9 August 2026

  Rolando Fissure

  See also: Central Sulcus (Fissure)

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• Encyclopedia entry9 August 2026

  Recurrent Laryngeal Nerve

  There are several reported variations in the course and branches of this nerve. Stedman (1823) reported his discovery of the right recurrent laryngeal nerve as follows: "While employed in dissecting the neck, in the Dissecting-room of the Royal Academy at Copenhagen, in the beginning of May 1823, I discovered a singular distribution of the nerves and arteries on the right side, which may not be uninteresting to the medical public. The subject was a thin aged female.

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• Encyclopedia entry9 August 2026

  Pudendal Nerve

  Several variations have been reported in the formation of the nerve and course of its branches. This nerve may have roots arising from both the fifth lumbar and the first sacral nerves. The inferior rectal branch may also pierce the sacrospinous ligament on its course to the perineum. The lateral superficial perineal branch of the pudendal may pierce the sacrospinous ligament.

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• Encyclopedia entry9 August 2026

  Pterygopalatine (Sphenopalatine) Ganglion

  Several variations in the branches of this ganglion have been reported. The lesser palatine nerve, which is ordinarily one of the descending (palatine) branches of the ganglion and supplies the mucous membrane of the soft palate and tonsils, may be absent. There may be a reversal in the course of the left and right nasopalatine nerves through the foramina of Scarpa (greater and lesser palatine): the left nerve passes through the posterior (instead of the anterior) foramen and the right nerve passing through the anterior (instead the posterior) foramen.

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• Encyclopedia entry9 August 2026

  Presacral Sympathetic Nerves

  See also: Sacral Ganglia

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• Encyclopedia entry9 August 2026

  Posterior Femoral Cutaneous Nerve

  In cases of separate origin of the peroneal and tibial nerves, the posterior femoral cutaneous nerve arises from the sacral plexus in two parts. The ventral part descends with the tibial (division) below the piriformis and gives rise to the inferior pudendal and medial femoral branches. The dorsal part passes through the piriformis with the common perineal nerve (division), and supplies the guteal and lateral femoral branches. A perineal branch may pierce the sacroiliac ligament.

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• Encyclopedia entry9 August 2026

  Sacral Plexus

  The origins of nerves of the sacral plexus are variable. The range of these variations, primarily from the work of Eisler, is tabulated below. Range of Variation of Sacral Plexus (table from Bardeen) Nerve High form of plexus Usual form of plexus Low form of plexus Composition Lumbosacral Trunk L4 L4 L4 N. to Quad. Femoris L4, L5 L4, L5, S1 L5, S1 N. to Obturator Internus L4, L5, S1, S2 L5, S1, S2 S1, S2, S3 Tibial L3, L4, L5, S1, S2 L4, L5, S1, S2, S3 L5, S1, S2, S3, S4 Superior Gluteal L4, L5, S1 L4, L5, S1, S2 L5, S1, S2 Inferior Gluteal L4, L5, S1 L5, S1, S2 L5, S1, S2 N.

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• Encyclopedia entry9 August 2026

  Pudendal Plexus

  The pudendal plexus, like the parts of the lumbosacral plexus described previously, varies in its formation. The following table shows the extreme range of variation and the usual form of the largest nerve (pudendal or pudic) of this plexus. Range of Variation of Pudendal Plexus (from Bardeen) Nerve High form of plexus Usual form of plexus Low form of plexus Composition Pudendal S2, S3 S2, S3, S4 S3, S4 Extreme Range of Variation Pudendal L5, S1, S2, S3, S4, S5 S1, S2, S3, S4 S2, S3, S4, S5

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• Encyclopedia entry9 August 2026

  Pharyngeal Plexus

  The pharyngeal plexus, formed by the pharyngeal branches of the vagus and glossopharyngeal nerves and branches from the superior cervical ganglion, has been reported to give a slender branch, the middle laryngeal nerve. It supplies the cricothyroid muscle, then pierces the cricothyroid membrane to supply the mucous membrane of the lower part of the larynx.

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• Encyclopedia entry9 August 2026

  Perforating Cutaneous Nerve

  This nerve, when present (64% of cases), arise from the second and third sacral nerves. Instead of piercing the sacrosciatic ligament, the nerve may accompany the pudendal nerve or pass between the ligament and gluteus maximus. It may be replaced by a branch of the posterior femoral cutaneous nerve or by the perforans coccygeus major, which arises from the third and fourth or the fourth and fifth sacral nerves. It is sometimes joined at its origin with the pudendal nerve.

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• Encyclopedia entry9 August 2026

  Optic Chiasma

  See also: Optic (CN II) Nerve

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• 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.

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• Encyclopedia entry9 August 2026

  Olfactory (CN1) Nerve

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

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• 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.

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• Encyclopedia entry9 August 2026

  Non-Recurrent Recurrent Laryngeal Nerve

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

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• 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.

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