The visceral space is a neck area spanning from the hyoid bone level to the mediastinum,
within the middle layer of the deep cervical fascia,
and containing the thyroid and parathyroid glands,
hypopharynx,
larynx,
trachea,
oesophagus and paraesophageal lymph nodes ( Fig. 2,Fig. 3).
Extrinsic larynx (strap) muscles are innervated by the ansa cervicalis of the cervical plexus,
which results from the anastomosis between the ventral branches of the first three cervical nerves,
gathered together after they exit through the intervertebral foramen,
between the origins of the anterior and middle scalene,
and the hypoglossal nerve ( Fig. 4).
The descendent branch of the hypoglossal nerve (anterior or superior branch of the ansa cervicalis) runs down anterior to the great vessels,
along the anterior diehedral angle formed by the common carotid artery and the internal jugular vein.
When arriving to the level of the intermediate tendon of the omohyoid muscle,
it meets with the descendent branch of the cervical plexus (posterior or inferior branch of the ansa cervicalis) in front of the internal jugular vein (Fig. 5,
Fig. 6,Fig. 7,
Fig. 8,
Fig. 9).
They create a U-shaped handle leaving the nerves for the sternohyoid and sternothyroid muscles and the upper belly of the omohyoid from the anterior branch,
and the nerves for the lower belly of the omohyoid and the sternohyoid muscle from the convexity.
The nerve for the thyrohyoid muscle comes directly from the hypoglossal (Fig. 10).
Taking into account these characteristics,
the anatomic reference to identify the anterior branch of the ansa cervicalis is the anterior diehedral angle between the internal jugular vein and the common carotid artery,
posterior to the sternocleidomastoid muscle in superior segments,
and posterior to the omohyoid in the inferior segments.
The vagus nerve goes through the neck to the abdomen.
Even though it doesn´t run through the visceral space,
it is an important nerve as it innervates some related muscles,
such as the inferior constrictor muscle of the pharynx,
the thyroid and parathyroid glands,
the trachea and the oesophagus.
The nerve’s origin is located in the nucleus ambiguous,
emerges through the posterolateral sulcus of the bulb to,
later on,
leave out the skull through the jugular foramen.
Once out the skull,
the vagus descends within the carotid space (Fig. 2,Fig. 3) through the posterior diehedral angle formed by the internal jugular vein and the internal carotid artery (superiorly) and the common carotid artery (inferiorly).
The vagus gives multiple branches,
even though we will just name those relevant in the visceral space.
Within the cervical branches,
we will emphasize the pharyngeal branches and the superior laryngeal nerve.
The first branches run ahead the common carotid artery and finish in the anterior wall of the pharynx,
where they innervate its muscles,
as the inferior constrictor of the pharynx.
On the other hand,
the superior laryngeal nerve,
which innervates the cricothyroid muscle,
begins in the inferior part of the inferior vagus node,
and runs inferiorly,
anterior and medially to the pharynx wall,
posterior and medial to the internal carotid artery.
Later on,
the nerve descends over the lateral wall of the pharynx,
crosses the medial wall of the external carotid artery,
under the lingual artery,
to be divided between the origin of this artery and the greater horn of the hyoid into two terminal branches: superior (internal) and inferior (external).
The superior branch runs inferior to the lesser horn of the hyoid and runs over the thyrohyoid membrane,
which crosses through the same foramen than the superior laryngeal artery.
Finally,
it is divided into terminal branches for the pharyngeal and laryngeal mucosa,
among which stands out the communicating branch with the recurrent laryngeal nerve,
making up the ansa of Galen.
The inferior branch,
however,
descends anterior to the inferior pharyngeal constrictor muscle to innervate the cricothyroid muscle and,
finally,
the laryngeal mucosa,
passing through the cricothyroid membrane (Fig. 11).
Regarding the thoracic branches of the vagus nerve,
the recurrent laryngeal nerve is relevant since it innervates the intrinsic muscles of the larynx,
except for the cricothyroid.
It is important to outline that the origin of both recurrent laryngeal nerves is different.
On the right side,
the recurrent laryngeal nerve leaves the vagus anterior to the subclavian artery,
surrounds the vessel and ascend to the larynx through the sulcus between the oesophagus and the trachea.
On the left side,
the nerve leaves the vagus at the level of the aortic arc.
Then surrounds the vessel and reaches the larynx lying on the left anterolateral wall of the oesophagus (Fig. 12,
Fig. 13,
Fig. 14).
When reaching the superior portion of the trachea,
both nerves are located deep to the inferior pharyngeal constrictor,
and go down the mucosa of the pyriform sinus (Fig. 15).
They finally give branches to all the intrinsic muscles of the larynx (except to the cricothyroid) and a communicating branch that forms the ansa of Galen.
For the cervical assessment of the vagus,
the anatomic reference that must be considered is the posterior dihedral angle between the common carotid artery and the internal jugular vein,
anterior to the scalene muscles.
Regarding the recurrent laryngeal nerve,
it can be located lateral to the wall of the oesophagus,
posterior and medial to the lateral lobes of the thyroid gland, while at superior levels,
it is located anterior to the pyriform sinus.