Variation archive

The articles published on the portal, most recent first.

854 published articles

Encyclopedia entry9 August 2026
Phrenic Nerve
The following variations in origin, course, and distribution of the phrenic nerve have been reported. The phrenic nerve may receive additional roots from one or more of the following nerves: nerve to subclavius; nerve to sternohyoid; second or rarely, sixth cervical nerve; descendens cervicalis; ansa cervicalis; and brachial plexus. It may receive a branch from CN XII (hypoglossal) and may communicate with CN XI (spinal accessory). The phrenic nerve may arise exclusively from the nerve to subclavius. On occasion it supplies a branch to the subclavius muscle.
- Not assessed by the portal — what this means
Encyclopedia entry9 August 2026
Lesser Occipital Nerve
The lesser occipital nerve may be very small and distributed only to the skin of the neck. In such cases, the greater occipital nerve supplies the areas usually supplied by the lesser occipital. In some cases instead of its usual dorsal and upward course beneath the deep fascia along the posterior border of the sternocleidomastoid muscle, the lesser occipital nerve passes directly backward, piercing the trapezius muscle near its upper border before reaching the scalp. The auricular branch of the lesser occipital nerve may be derived from the greater occipital.
- Not assessed by the portal — what this means
Encyclopedia entry9 August 2026
Great Auricular Nerve
The mastoid branch of the great auricular nerve may arise independently from the cervical plexus, in which case it passes upward between the lesser occipital and great auricular nerves to reach its destination. The great auricular nerve may arise solely from the third or from the third and fourth cervical nerves.
- Not assessed by the portal — what this means
Encyclopedia entry9 August 2026
Communicating Cervical Branches
An additional communicating branch from the fourth cervical nerve may join the descending cervical nerve in the ansa cervicalis.
- Not assessed by the portal — what this means
Encyclopedia entry9 August 2026
Cervical Plexus
The cervical plexus consists of three loops. A large part of the anterior primary division of the first cervical nerve is given to the hypoglossal or cervical loop. The remainder contributes to the cervical plexus thus forming the first loop. From the first loop of the plexus, two branches of the first cervical nerve usually enter the sheath of the hypoglossal nerve (or may instead enter the sheath of the vagus nerve) and descend with it to contribute to the hypoglossal or cervical loop.
- Not assessed by the portal — what this means
Encyclopedia entry9 August 2026
Cerebral Aqueduct
It varies in size from 0.40mm 2 to 9.48mm 2 (24 normal brains) according to Flyger and Hjelmquist.
- Not assessed by the portal — what this means
Encyclopedia entry9 August 2026
Cardiac Plexus
The nerves that supply the heart usually converge anteriorly toward the lower part of the trachea but may be followed separately to the heart. Cardiac ganglia, varying in size and number, may occur along the cervicothoracic nerves that supply the aorta, pulmonary trunk, and arteries as well as the heart. Image 73A Image 73B Image 73C Image 73D Image 73E Image 73F
- Not assessed by the portal — what this means
Encyclopedia entry9 August 2026
Buccal Nerve (N. to M. Buccinator)
The following variations in the course and/or branches of this nerve have been reported. Instead of its usual course on the medial side of the temporal muscle, the nerve may pierce the muscle. When absent, the nerve is replaced by the posterior superior alveolar nerves. The buccal nerve may also arise from the inferior alveolar nerve, in which case it emerges from the inferior dental canal through a small foramen in the alveloar border of the mandible, just anterior to the ramus.
- Not assessed by the portal — what this means
Encyclopedia entry9 August 2026
Brain
The brain usually weighs between 1200 and 1400 grams. It has been reported that the brain of Anatole France (French writer and Nobel Laureate) weighed 1040 gms. The brain weights (in grams) of some other notable individuals are: Ivan Turgenieff, (Russian novelist, dramatist, and short story writer) 2012; Georges Léopold Chrétien Cuvier (French zoologist and geologist) 1830; John Abercrombie, 1786; William Makepeace Thackeray (English novelist) 1658; Immanuel Kant (German metaphysician, philosopher) 1600; Spurzheim, 1559; Dirichlet, 1520; Daniel Webster, (American statesman, lawyer, and orator)…
- Not assessed by the portal — what this means
Encyclopedia entry9 August 2026
Septum Pellucidum
The septum pellucidum may be absent. Agenesis of the septum pellucidum is often associated with agenesis of the corpus callosum, but either condition may occur without the other. When agenesis of the septum pellucidum is associated with hypoplasia of the optic disk, the condition is referred to as septooptic dysplasia. The septum may also be perforated.
- Not assessed by the portal — what this means
Encyclopedia entry9 August 2026
Midline Nucleus
In 70% of brains, the massa intermedia (interthalamic adhesion) forms a bridge of gray matter, which crosses the third ventricle and constitutes part of a bilateral nuclear complex. Reference Bruce, A. (1890) On the absence of the corpus callosum in the human brain with a description of a new case. Brain 12:171-190.
- Not assessed by the portal — what this means
Encyclopedia entry9 August 2026
Insula
In a study of 200 brains, including a few mentally retarded individuals, paralytics, and a series of young fetuses, it was found that the left insula was more deeply marked by sulci than the right, and that its length, on average, was greater by 11 mm. The central sulcus of the insula is usually the first to appear developmentally, and it is more pronounced in males.
- Not assessed by the portal — what this means
Encyclopedia entry9 August 2026
Fornix
The fornix may be absent. Reference
- Not assessed by the portal — what this means
Encyclopedia entry9 August 2026
Superior Temporal Sulcus
The superior temporal sulcus is usually a continous sulcus but is occasionally separated into two or three sections, especially in the left hemisphere. The sulcus may encroach on the territory of the middle temporal sulcus. The terminal portion of the sulcus may be forked.
- Not assessed by the portal — what this means
Encyclopedia entry9 August 2026
Middle Temporal Sulcus
This sulcus may separate into two to six parts. It may interdigitate with the superior temporal sulcus.
- Not assessed by the portal — what this means
Encyclopedia entry9 August 2026
Sylvian Fissure
The caudal extremity of the sylvian (lateral) fissure may be forked (41%) forming ascending and descending branches. The ascending branch is the most constant and is regarded as the true continuation of the fissure. The ascending branch forms the axis of the supramarginal gyrus. Triradiate termination of the sylvian fissure has been reported.
- Not assessed by the portal — what this means
Encyclopedia entry9 August 2026
Postcentral Sulcus
This sulcus may be formed by up to four different segments.
- Not assessed by the portal — what this means
Encyclopedia entry9 August 2026
Precentral Sulcus
The precentral sulcus may be composed of two or three parts: precentral inferior, precentral medius, precentral superior. In about 17% of brains, the precentral sulcus does not show discontinuity into separate parts. The upper extremity of the precentral inferior sulcus may be located anterior to the lower end of the precentral superior sulcus or may unite with the latter forming a continuous sulcus. Occasionally, a horizontal ramus extends from the precentral inferior sulcus above the inferior frontal sulcus; this horizontal ramus may be confluent with the middle or superior frontal sulci.
- Not assessed by the portal — what this means
Encyclopedia entry9 August 2026
Parieto-Occipital Fissure
This fissure may take a straight course or may assume a simple or an S-shaped curve.
- Not assessed by the portal — what this means
Encyclopedia entry9 August 2026
Orbital Sulcus
In its simplest form, this sulcus is triradiate with an anterior branch (orbitalis anterior) and two posterior branches (orbitalis posterior and lateralis). The two posterior branches form a U-shaped furrow with a caudally directed concavity. A second anterior branch (orbitalis anterior lateralis) may be present lateral to the orbitalis anterior branch. The orbitalis anterior lateralis may join with the U-shaped furrow and with the orbitalis anterior forming an H-shaped configuration. The two longitudinal arms of the H-shaped pattern may separate to form two distinct sagittal furrows.
- Not assessed by the portal — what this means