Variation archive

854 published articles

• Encyclopedia entry9 August 2026

  Superior Cardiac Nerve

  The superior cardiac nerve (a branch from the superior cervical ganglion) may be absent, particularly on the right side. In such cases, it is replaced by a branch from the vagus or from the external laryngeal nerve. The nerve may not have a separate course, but may be a part of another cardiac nerve from one of the other cervical ganglia. Variations in the course of the other cervical nerves are common. A pretracheal branch arising from the communication between the superior cardiac nerve and the recurrent laryngeal nerve has been reported.

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• Encyclopedia entry9 August 2026

  Spinal Cutaneous Nerves

  See also: Cervical Plexus Brachial Plexus Lumbosacral Plexus

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• Encyclopedia entry9 August 2026

  Spinal Nerves and Ganglia

  There are a number of variations in the posterior (dorsal or sensory) and anterior (ventral or motor) roots and spinal ganglia. The first cervical nerve may either have no dorsal root or share a dorsal root with, or derived a dorsal root from the spinal accessory (CN XI) nerve. The dorsal root and ganglion of the first cervical nerve may be rudimentary or entirely absent. With this singular exception, the sensory or dorsal root of spinal nerves is always larger than the motor or ventral root. The anterior or posterior root (or both) are sometimes absent in one or more thoracic spinal nerves.

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• Encyclopedia entry9 August 2026

  Spinal Cord

  Diastematomyelia is a condition in which a portion of or the entire spinal cord is divided into two lateral halves. Each half of the cord is enveloped in its own membranes and give rise to its own spinal roots. Hydromyelia or hydrorrhachis interna is a defective closure or arrangement of the divisions of the primary fetal central canal. Its presence may be indicated by subsequent development of syringomyelia or by its association with spina bifida. Hydrorrhachis externa is the abnormal congenital accumulation of fluid between the meninges of the cord causing a diminution of cord volume.

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• Encyclopedia entry9 August 2026

  Spinal Accessory (CN XI) Nerve

  The following variations in the origin, course, and branches of this nerve have been reported. The lower limit of origin from the anterior horn of the spinal cord may be as high as the third cervical segment (instead of the fifth or sixth segment) or as low as the first thoracic segment. The nerve may exit from the subdural space below the first cervical nerve and reenter the space at a higher level. It may pass beneath (but never pierce) the sternocleidomastoid muscle.

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• 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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