The =vagus= or =pneumogastric= (N. vagus) is the longest and most widely
distributed of the cranial nerves; it is also remarkable for the
connections which it forms with adjacent nerves and with the
sympathetic. It is attached to the lateral aspect of the medulla by
several filaments which are in series with those of the ninth nerve in
front and the eleventh nerve behind (Fig. 499). The bundles converge to
form a trunk which passes outward, pierces the dura mater, and emerges
from the cranium through the foramen lacerum posterius (Fig. 515). In
the foramen the nerve bears on its lateral aspect the elongated
flattened =jugular ganglion= (G. jugulare).
The ganglion communicates with (_a_) the tympanic nerve, (_b_) the
petrous ganglion of the ninth nerve, (_c_) the spinal accessory, and
(_d_) the hypoglossal. It also gives off the =auricular branch= (R.
auricularis), which runs forward below the petrous ganglion and passes
through a small canal in the petrous temporal bone to gain the facial
canal. Here it gives filaments to the facial and emerges with that
nerve through the stylo-mastoid foramen. It ascends behind the
external auditory meatus, dips under the rotator longus muscle, and
passes through a foramen in the conchal cartilage to ramify in the
integument which lines the meatus and the adjacent part of the ear.
Beyond the ganglion the vagus runs backward and downward with the spinal
accessory in a fold of the guttural pouch (Fig. 437). Then the two
nerves separate, allowing the hypoglossal to pass between them, and the
vagus descends with the internal carotid artery and crosses the inner
face of the origin of the occipital artery. Here it is joined by the
cervical trunk of the sympathetic, and the two nerves continue along the
dorsal aspect of the common carotid artery in a common sheath (Fig.
433). At the root of the neck the vagus separates from the sympathetic,
and from this point backward the relations of the right and left vagi
differ somewhat and must be described separately.
[Illustration:
FIG. 517.—CROSS-SECTION OF NECK OF HORSE, PASSING THROUGH POSTERIOR
PART OF ATLAS.
The head and neck were extended. _1_, Dorsal arch of atlas; _2_,
odontoid process; _3_, odontoid ligament; _4_, vertebral sinuses;
_5_, dura mater; _6_, spinal cord; _7_, vertebral artery; _8_, wing
of atlas; _9_, atlanto-axial joint cavity; _10_, œsophagus; _11_,
_11_, recurrent nerves; _12_, _12_, ventral branches of spinal
accessory nerves. By an oversight the obliquus cap. post. (above
wing of atlas) is unmarked.
]
The =right vagus= (Fig. 429) enters the thorax in the angle of
divergence of the right brachial artery and the truncus bicaroticus. It
then passes backward and slightly upward, crossing obliquely the outer
surface of the brachiocephalic artery and the right face of the trachea.
Reaching the dorsal surface of the latter near the bifurcation, it
divides into dorsal and ventral branches.
The =left vagus= (Fig. 428) enters the thorax on the ventral face of the
œsophagus, crosses obliquely under the left brachial artery, and passes
back on the external surface of that vessel in company with a large
cardiac nerve.[205] Separating from the latter, the vagus continues
backward on the left face of the aorta, inclines to the upper surface of
the left bronchus, and divides into dorsal and ventral branches.
The dorsal and ventral branches unite with the corresponding branches of
the opposite nerve, thus forming =dorsal= and =ventral œsophageal
trunks= (Truncus (œsophageus dorsalis, ventralis). These run backward in
the posterior mediastinum, above and below the œsophagus respectively,
and enter the abdominal cavity through the hiatus œsophageus; they
supply branches to the œsophagus and anastomose with each other. The
dorsal trunk receives the major part of its fibers from the right vagus.
After entering the abdomen it passes to the left of the cardia, gives
branches to the visceral surface of the stomach, and ends in the cœliac
and subsidiary plexuses. The ventral trunk passes to the lesser
curvature of the stomach and ramifies on the parietal surface of the
stomach; it forms here the anterior gastric plexus, from which branches
are supplied also to the first part of the duodenum and to the liver.
The collateral branches of the vagus are as follows:
1. The =pharyngeal branch= (R. pharyngeus) is given off in relation to
the superior cervical ganglion, turns around the internal carotid
artery, and runs downward and forward on the guttural pouch to the
dorsal wall of the pharynx (Fig. 437). Here its branches concur with the
pharyngeal branch of the ninth nerve and with filaments from the spinal
accessory and the sympathetic in forming the =pharyngeal plexus=. This
supplies numerous twigs to the pharynx, and a larger branch which passes
along the side of the œsophagus and ramifies in its cervical part.
According to Ellenberger and Baum the pharyngeal plexus receives
filaments also from the digastric, superior laryngeal, hypoglossal,
and first cervical nerves. The branches of the plexus form secondary
intermuscular and submucous plexuses, in which there are numerous
minute ganglia.
2. The =superior= or =anterior laryngeal nerve= (N. laryngeus cranialis)
is larger than the preceding and arises a little behind it (Fig. 437).
It crosses the deep face of the origin of the external carotid artery,
runs downward and forward over the lateral wall of the pharynx behind
the hypoglossal nerve, and passes through the foramen below the anterior
cornu of the thyroid cartilage. Its terminal branches ramify in the
mucous membrane of the larynx, the floor of the pharynx, and the
entrance to the œsophagus; they anastomose with those of the recurrent.
At its origin the nerve gives off its small =external branch= (R.
externus); this descends to the crico-thyroid muscle, which it supplies,
and sends filaments to the crico-pharyngeus also. It may arise from the
trunk of the vagus or from the pharyngeal branch.
At the point of origin of the superior laryngeal nerve there is a
plexiform widening which is regarded by some authors as the homologue
of the ganglion nodosum of man. From it a filament arises which, after
a short course, rejoins the vagus or enters the sympathetic trunk.
Stimulation of its central end causes a reduction of the
blood-pressure, and it is therefore termed the depressor nerve (N.
depressor).
3. The =recurrent nerve= (N. recurrens), also termed the inferior or
posterior laryngeal nerve, differs on the two sides in its point of
origin and in the first part of its course. The =right nerve= (Fig. 429)
is given off opposite the second rib, turns around the dorso-cervical
artery from without inward, runs forward on the lower part of the
lateral surface of the trachea, and ascends in the neck on the ventral
face of the common carotid artery. The =left nerve= (Fig. 428) arises
from the vagus where the latter begins to cross the aortic arch. It
passes back over the ligamentum arteriosum, winds around the concavity
of the aortic arch from without inward, runs forward on the lower part
of the left face of the trachea, and continues in the neck in a similar
position to the right nerve.
It is worthy of note that the left nerve passes beneath the bronchial
lymph glands as it winds around the aorta; also that in the next part
of its course it lies between the left surface of the trachea and the
deep face of the aorta, and is then related to lymph glands which lie
along the ventral aspect of the trachea. The left recurrent is often
incorporated in part of its course in the anterior mediastinum with a
deep cardiac nerve. Further, the left nerve lies at first ventral to,
and then upon, the œsophagus in the neck. The right recurrent is given
off from or in common with a considerable trunk which connects the
vagus with the first thoracic ganglion of the sympathetic. The
arrangement here is commonly more or less plexiform, and from it one
or two cardiac nerves arise.
The terminal part of each nerve (Fig. 517) lies on the dorsal surface of
the trachea, in relation to the œsophagus internally and the carotid
artery above.[206] It passes between the crico-arytenoideus posterior
and the crico-pharyngeus; the =terminal branches= supply all the muscles
of the larynx except the crico-thyroid, and communicate with branches of
the superior laryngeal nerve. =Collateral branches= are given off to the
cardiac plexus (Rr. cardiaci), to the trachea (Rr. tracheales), to the
œsophagus (Rr. œsophagei), and to the inferior cervical ganglion of the
sympathetic.
4. =Cardiac branches= (Rr. cardiaci), usually two or three in number,
are given off from each vagus within the thorax (Figs. 428, 429). These
concur with the cardiac branches of the sympathetic and recurrent nerves
to form the cardiac plexus, which innervates the heart and great
vessels.
5. Small =tracheal= and =œsophageal branches= (Rr. tracheales et
œsophagei) are given off from both vagi in the thorax. These concur with
branches from the recurrent nerves and the inferior cervical and
anterior thoracic ganglia of the sympathetic in forming the =posterior
tracheal= and =œsophageal plexuses=, from which twigs go to the trachea,
œsophagus, heart, and large vessels.
6. =Bronchial branches= (Rr. bronchiales) are detached at the roots of
the lungs and unite with sympathetic filaments in forming the =pulmonary
plexuses=. From the latter numerous branches proceed in a plexiform
manner along the bronchi and vessels into the substance of the lungs.
The vagus and glosso-pharyngeal nerves are so closely associated in
origin and central connections that they may be described together in
this respect.
The =sensory fibers= arise from the petrous and jugular ganglia, and
their central parts enter the lateral aspect of the medulla and divide
into anterior and posterior branches like the fibers of the dorsal
roots of the spinal nerves. Most of the fibers end in arborizations
about the cells of the vago-glosso-pharyngeal nucleus of termination,
which consists of two parts. Of these the =dorsal sensory nucleus=
(Nucleus alæ cinereæ) is situated in the posterior part of the floor
of the fourth ventricle and in the adjacent part of the closed portion
of the medulla near the median plane. The other part is termed the
=nucleus of the solitary tract=, and is so named because its cells are
grouped about the bundle (Tractus solitarius) formed by the posterior
divisions of the afferent nerve-fibers. It ends about the level of the
pyramidal decussation. The secondary central connections are similar
to those of the sensory part of the trigeminus.
The =motor fibers= (and those of the medullary part of the accessory)
arise from the =dorsal motor nucleus= and the =ventral motor nucleus=.
The cells of the former lie in groups along the ventro-medial side of
the dorsal sensory nucleus. The latter, also termed the nucleus
ambiguus, is situated more deeply in the lateral part of the formatio
reticularis.