The =spinal accessory nerve= (N. accessorius) is purely motor. It
consists of two parts which differ in origin and function.
The =medullary part= arises from the lateral aspect of the medulla by
several rootlets which are behind and in series with those of the vagus
(Fig. 499). The =spinal part= arises from the cervical part of the
spinal cord by a series of fasciculi which emerge between the dorsal and
ventral roots. The bundles unite to form a trunk which is very small at
its origin at the fifth segment of the cord, but increases in size when
traced toward the brain, since it continually receives accessions of
fibers. It passes through the foramen magnum and joins the medullary
part. The trunk thus formed sends its medullary fibers to the tenth and
ninth nerves and emerges through the foramen lacerum posterius. It then
runs backward and downward with the vagus in a fold of the guttural
pouch, separates from that nerve, crosses the deep face of the
submaxillary gland and the occipital artery, and divides in the recessus
atlantis into dorsal and ventral branches.
[Illustration:
FIG. 518.—NECK OF HORSE, AFTER REMOVAL OF PART OF PANNICULUS AND
TRAPEZIUS.
_a_, _a′_, Mastoido-humeralis; _b_, anterior superficial pectoral
muscle; _c_, cervical panniculus; _d_, sterno-cephalicus; _e_,
omo-hyoideus; _f_, sterno-thyro-hyoideus; _g_, trachea; _h_,
_h′_, _h″_, tendons of splenius, mastoido-humeralis, and
trachelo-mastoideus; _i_, trapezius cervicalis; _k_,
supraspinatus; _l_, anterior deep pectoral muscle; _m_,
rhomboideus cervicalis; _n_, serratus cervicis; _o_, splenius,
upper and lower borders of which are indicated by dotted lines;
_p_, parotid gland; _q_, parotido-auricularis muscle; _r_, wing
of atlas; _s_, spine of scapula; _1_, external maxillary vein;
_2_, _3_, jugular vein; _4_, carotid artery; _5_, descending
branch of inferior cervical artery; _6_, cephalic vein; _7–11_,
ventral branches of second to seventh cervical nerves; _12_,
cutaneous branch of second cervical nerve; _13_, cervical branch
of facial nerve; _14_, terminal branches of dorsal divisions of
cervical nerves; _15_, dorsal branch of spinal accessory nerve.
(After Ellenberger-Baum, Top. Anat. d. Pferdes.)
]
It is connected by anastomotic branches with the vagus and hypoglossal
nerves and the superior cervical ganglion of the sympathetic, and
contributes a branch to the pharyngeal plexus.
The =dorsal branch= (R. dorsalis) (Figs. 433, 517, 518) receives a twig
from the second and third cervical nerves and turns around the atlantal
tendon of the splenius under cover of the mastoido-humeralis. It then
passes obliquely through the latter muscle and continues backward on the
splenius and the cervical part of the serratus, inclines upward across
the anterior deep pectoral and the supraspinatus, and enters the deep
face of the trapezius, in which it ramifies.
The =ventral branch= (R. ventralis) (Fig. 436) is smaller. It enters the
sterno-cephalicus muscle behind the cervical angle of the parotid gland.
The fibers of the spinal part of the accessory arise from the
ventro-lateral cells of the ventral gray column of the cord as far
back as the fifth cervical segment. The fibers of the medullary part
come chiefly from the nucleus ambiguus in common with the motor fibers
of the vagus.