Voce enciclopedica
Vestibulocochlear
See: Facial Nerve
Archivio delle variazioni
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See: Facial Nerve
The right and left vagus may differ significantly in size. The nerves may lie in front of both vessels (internal carotid and internal jugular) in the carotid sheath. The left vagus has been found passing anterior to the common carotid artery and the thyroid gland. Descendens hypoglossi (C1 and C2) may be carried partly or completely by the vagus nerve instead of the hypoglossal nerve. The distribution of the right and left vagus nerves around the esophageal hiatus is variable. The vagus nerve may contain accessory parathyroid tissue. See also Recurrent Laryngeal Nerve.
Variations in the branches and orbital course of the trochlear nerve have been described. The nerve has been reported to send a branch to M. orbicularis palpebrarum (orbicularis oculi) and to join sensory nerves (frontal, suprtrochlear, infratrochlear, and nasal). These branches probably represent an aberrant course of sensory fibers from the trigeminal nerve. One variation in the orbital course consists of the trochlear piercing (instead of coursing over) M. levator palpebrae superioris on its way to the superior oblique muscle.
The following variations have been reported in this nerve or its two branches (the temporal or facial or malar). The nerve may pass through the zygomatic bone before it divides into two branches, or the two branches may pass separately through foramina in the zygomatic bone instead of passing through a common foramen (sphenozygomatic foramen). The temporal branch in some cases passes through the sphenomaxillary fissure into the temporal fossa.
Mandibular Division (V3)
In 50% of cases studied, the twelfth thoracic participates in the lumbosacral plexus.
Defects in roots of thoracic nerves have been reported. In one study of 16 spinal cords, only three had the full complement of thoracic roots. In three cases, one thoracic spinal nerve (dorsal and ventral roots) was absent; in three, a dorsal root was absent; and in seven, a ventral root was absent. The first thoracic may fail to contribute to the formation of the posterior cord of the brachial plexus or it may be the sole source of the medial cord of the plexus. The first and second thoracic nerves are occasionally joined by a connecting branch.
The levator ani may be supplied by either the third or the fourth sacral nerves. In one study of 30 subjects, the levator muscle was supplied by the third sacral in 17 and by the fourth sacral in 13 subjects References Cruveilhier, J Traité d'Anatomie Descriptive, 3rd ed., G. Doin & Cie., Paris. Henle, J. (1868) Handbuch der Systematischen Anatomie des Menschen. von Friedrich Veweg und Sohn, Baunschweig. Jackson, C.M., Ed. (1933) Morris' Human Anatomy, 9th ed., P. Blakiston's Son & Co., Inc., Philadelphia. Kopsch, F. (1908) Rauber's Lehrbuch der Anatomie des Menschen. Georg Thieme, Leipzig.
See also: Facial Nerve
See also: Sacral Ganglia
See also: Cardiac Plexus
See also: Tibial and Sural Nerves
Wrisberg observed a supreme splanchnic nerve eight times in a "large number" of bodies. It was described as being formed by branches from cardiac nerves, and from lower cervical and upper thoracic ganglia.
Instead of its usual course on the medial side of the internal carotid artery, this nerve may pass lateral to the artery. The external laryngeal branch often arises independently from the main trunk. The superior laryngeal nerve may anastomose with the recurrent laryngeal nerve.
The superior cervical ganglion may be doubled, with one part superficial to and the other posterior to the carotid sheath. A variable number of fibers from the superior ganglion join the inferior ganglion of the vagus, hypoglossal, and descendens cervicalis nerves. Ganglion cells, intermingled with nerve fibers from the sympathetic trunk, have been reported within the cranium.
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.
See also: Cervical Plexus Brachial Plexus Lumbosacral Plexus
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.
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.
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.