Page 396 - Clinical Anatomy

ECA6 7/18/06 6:54 PM Page 381

The cranial nerves 381

2◊◊Asimple test for the integrity of the vagus relies on its innervation of the
muscles of the palate. In unilateral paralysis, the uvula deviates to the
normal side when the patient says ‘Ah’.
3◊◊Vagotomy—see page 74.
4◊◊Injuries to the recurrent laryngeal nerve—see page 288.

The accessory nerve (XI) (Fig. 210)
The accessory nerve is conventionally described as having a cranial and a
spinal root. According to standard descriptions, the cranial root is formed
by a series of rootlets that emerge from the medulla between the olive and
the inferior cerebellar peduncle. These rootlets are considered to join the
spinal root, travel with it briefly, then separate within the jugular foramen
and are distributed with the vagus nerve to supply the musculature of the
palate, pharynx and larynx.
Arecent, detailed dissection study has demonstrated that all the
medullary rootlets that do not join to form the glossopharyngeal nerve (IX)
join the vagus nerve at the jugular foramen. All the rootlets that form the
accessory nerve arise caudal to the olive and no connections can be demon-
strated between the accessory nerve and the vagus in the jugular foramen.
The accessory nerve thus has no cranial component and consist only of the
structure hitherto referred to as the spinal root of the accessory nerve.
This spinal root is formed by the union of fibres from an elongated
nucleus in the anterior horn of the upper five cervical segments, which
leave the cord mid-way between the anterior and posterior roots, join, then
pass upwards through the foramen magnum. The accessory nerve and the
converging rootlets of the vagus nerve then enter the jugular foramen in a
shared sheath of dura. The glossopharyngeal nerve enters the jugular
foramen anterior to the vagus through a separate dural sheath.
The nerve passes backwards over the internal jugular vein to the sternoclei-
domastoid muscle which it pierces (and supplies) and then crosses the poste-
rior triangle of the neck to enter and supply the deep surface of the trapezius.

Clinical features

Division of the accessory nerve results in paresis of the sternocleidom-
astoid and trapezius muscles. This follows, for example, most block dissec-
tions of the lymph nodes of the neck, the nerve being sacrificed in clearing
the posterior triangle.

The hypoglossal nerve (XII)

The hypoglossal nerve is entirely motor and supplies all the intrinsic and
extrinsic muscles of the tongue (with the exception of the palatoglossus).
From its nucleus, which lies in the floor of the 4th ventricle (Fig. 242), a
series of about a dozen rootlets leave the side of the medulla in the groove
between the pyramid and the olive. These rootlets unite to leave the skull