The cervical plexus is formed by the anterior primary divisions of the upper four cervical nerves which constitute the roots of the plexus.

It lies in the upper part of the side of the neck, under cover of the sterno-mastoid, and upon the levator scapulse and the scalenus medius. It is a looped plexus, consisting of three loops.

A large part of the anterior primary division of the first cervical nerve is given to the hypoglossal or cervical loop; the remainder passes to the cervical plexus and in doing so it runs lateralward on the posterior arch of the atlas beneath the vertebral artery, then it turns forward, between the vertebral artery and the outer side of the upper articular process of the atlas, and finally it descends, in front of the transverse process of the atlas, and unites with the upper branch of the second nerve, forming with it the first loop of the plexus. It gives branches to the rectus capitis lateralis, longus capitis (major rectus capitis anterior), and to the rectus capitis anterior (minor). The division communicates with the ganglion of the trunk of the vagus and with the superior cervical ganglion of the sympathetic system. From the first loop of the plexus, two branches of the division pass over into the sheath of the hypoglossal nerve and descend with it to contribute to the hypoglossal loop [ansa hypoglossi] or better, the cervical loop. The fibers entering the sheath of the hypoglossus, after giving a few twigs to the gpnio-hyoid and thyreo-hyoid muscles, leave the sheath as the descendens cervicalis (hypoglossi) and this latter joins the communicans cervicalis, (the portion of the loop from the second and third cervical nerves) and thus completes the cervical or hypoglossal loop.

This loop usually may be found between the sheaths of the sterno-mastoid muscle and the carotid artery, superficial to the internal jugular vein; sometimes it may lie in the carotid sheath between the carotid artery and the internal jugular vein; rarely it may lie dorsal to both the artery and vein. Sometimes it is relatively long, descending toward the sternum below the level of the thyreoid cartilage; again it is quite short and occurs near the level of the hyoid bone. The descendens cervicahs (hypoglossi) parts company with the hypoglossal nerve at the level at which the nerve curves around the occipital artery. It runs downward and shghtly medialward on the sheaths of the great vessels and occasionally within the sheath of one of them.

The second cervical nerve (anterior primary division) passes behind the upper articular process of the axis and the vertebral artery, and between the inter-transverse muscles extending from the first to the second cervical vertebrse, to the interval between the scalenus medius and the longus capitis (rectus capitis anterior major), where it divides into two parts. The upper part ascends and unites with the first nerve to form the first loop of the plexus, and the lower branch passes downward and dorsalward and joins the upper branch of the third nerve in the second loop of the plexus. This branch gives off the small occipital nerve and a filament to the sterno-mastoid, which communicates with the spinal accessory nerve in the substance of the muscle, and it gives branches which assist in forming the hypoglossal or cervical loop (ansa hypoglossi) the cervical cutaneous and the great auricular nerves.

Origin of the Cervical and Brachial Plexus. (After Toldt, "Atlas of Human Anatomy," Rebman, London and New York.)

The third and fourth cervical nerves pass behind the vertebral artery and between the intertransverse muscles to the interval between the scalenus medius and the longus capitis (rectus capitis anterior major), where the third unites with the second and fourth nerves and completes the lower two loops of the plexus. The anterior primary divisions of these nerves are about double the size of the preceding. The third gives off branches to the hypoglossal loop, to the larger part of the great auricular and cervical cutaneous nerves, a branch to the phrenic, a branch to the supra-clavicular nerves, and muscular branches to the scalenus medius, levator scapulse, longus capitis, and trapezius. The trapezius branch joins the spinal accessory nerve beneath the muscle. The fourth nerve gives a branch to the phrenic, a branch to the supra-clavicular nerves, and muscular branches to the scalenus medius, levator scapulae, longus colli, and trapezius. The branch to the trapezius unites with the one from the third nerve and joins the spinal accessory nerve beneath the muscle.

The fibers forming the cervical (hypoglossal) loop innervate all the muscles of the infra-hyoid group, though twigs to the genio-hyoid and thyreohyoid seemingly enter these muscles from the trunk of the hypoglossus.

Diagram of the Cervical Plexus.

The nerve to genio-hyoid is given off from the trunk under cover of the mylo-hyoid in common with the terminal branches of the hypoglossal proper going to the intrinsic muscles of the tongue. The nerve to the thyreo-hyoid muscles leaves the trunk of the hypoglossal near the tip of the great cornu of the hyoid bone, running obUquely downward and medianward to reach its muscle. A twig to the anterior belly of the omo-hyoid is given from the upper part of the descendens cervicalis and the nerves for the sterno-hyoid, the sterno-thyreoid and the posterior belly of the omo-hyoid are supplied from the turn of the loop. The nerves to the sterno-hyoid and sterno-thyreoid send twigs downward in the muscles behind the manubrium sterni and fibers from these in rare cases join the phrenic nerve in the thorax. The nerve to the posterior belly of the omo-hyoid courses as a loop in the cervical fascia below the central tendon of its muscle.

Each root of the cervical plexus receives a communicating grey ramus from the superior cervical ganglion of the sympathetic, and from the roots and loops of the plexus a number of branches arise which form two main groups, the superficial and the deep.

Superficial Branches of the Cervical Plexus

The superficial branches are described, according to the direction in which they run, as ascending, transverse, and descending branches. The ascending branches are the small occipital and the great auricular nerves. There is only one transverse branch, the cervical cutaneous (transverse cervical), and the descending branches are distinguished as the supraclavicular nerves and the cervical (hypo-glossal) loop.

The ascending branches

(1) The small occipital nerve arises from the second and third cervical nerves, or from the loop between them, and runs upward and dorsalward to the posterior border of the sterno-mastoid, where it hooks around the lower border of the spinal accessory nerve and then ascends along the posterior border of the muscle to the mastoid process. It pierces the deep cervical fascia and passes across the posterior part of the insertion of the sterno-mastoid into the superficial fascia of the scalp, in which it breaks up into auricular, mastoid, and occipital terminal branches.

 

Suprefiicial Branches of the Cervical Plexus. (After Hirschfeld and Leveillé.)

(a) The auriculax branch runs upward and slighly forward to reach the integument on the upper median part of the auricle (pinna), wliioh it supphes.

(b) The mastoid branch is distributed to the slvin covering the base of the mastoid process,

(c) The occipital branches ramify over the occipitaUs muscle and are distributed to the skin of the scalp ' they communicate with one another and with the great occipital nerve. The branches of the small occipital nerve anastomose with twigs of the posterior auricular, great auricular, and great occipital nerves.

(2) The great atiricular nerve arises from the second and third cervical nerves. It accompanies the small occipital to the posterior border of the sterno-mastoid, but at that point it diverges from the small occipital and runs upward and forward across the sterno-mastoid toward the angle of the mandible. When it is about half-way across the muscle it begins to break up into its terminal branches, which are named, according to the area of their distribution, mastoid, auricular, and facial.

As the nerve ascends obliquely across the sterno-mastoid it is embedded in the deep cervical fascia, is covered by superficial fascia and the platysma, and it lies parallel with and slightly dorsal to the external jugular vein, (a) The mastoid branch is small, and is distributed to the integument covering the mastoid process. It anastomoses with the posterior auricular and small occipital nerves. (b) The auricular branches are three or four stout twigs which interlace with the branches of the posterior auricular nerve; they cross the superficial surface of the posterior auricular branch of the facial, and are distributed to the skin on the back of the auricle with the exception of its uppermost part. One or two twigs pass through fissures in the cartilage of the auricle, and are distributed to the integument on the lateral surface of the lobule and the lateral surface of the lower part of the helix and anthelix. (c) The facial branches pass upward and forward among the superficial lobules of the parotid gland, and supply the skin over that gland and immediately in front of it, and they anastomose in the substance of the gland with the cervico-facial division of the facial nerve. In some cases fine twigs may be traced forward nearly to the angle of the mouth.

Transverse branch of the plexus. - The superficial cervical cutaneous nerve

(transverse cervical) arises from the second and third cervical nerves, and appears at the posterior border of the sterno-mastoid, a little below the great auricular nerve. It passes transversely across the sterno-mastoid under cover of the integument, platysma, and external jugular vein, and divides into a number of twigs which spread out after the manner of a fan, and, as they approach the middle line, extend from the chin to the sternum.

The upper two or three of these twigs unite, beneath the platysma, with the cervical (infra-mandibular) branch of the facial and thus form loops. From the terminal branches of the nerve numerous twigs arise which pierce the platysma and end in the skin of the front part of the neck.

The descending or supra-clavicular branches

These are derived from the third and fourth cervical nerves, and arise under cover of the sterno-mastoid. At their commencements they are usually united with the muscular branches destined for the trapezius. They become superficial at the middle of the posterior border of the sterno-mastoid, and as they pass downward they pierce the deep cervical fascia. They include the following:

1. The anterior supra-clavicular (suprasternal) branches are small, and cross over the clavicular attachment of the sterno-mastoid to reach the integument over the upper part of the manulDrium sterni. They also supply the sterno-clavicular joint.
2. The middle supra-clavicular (supra-clavicular) nerves are of considerable size. They cross in front of the middle third of the clavicle under cover of the platysma, and are distributed to the skin covering the upper part of the pectoralis major as low as the third rib.
3. The posterior supra-clavicular (supra-acromial) branches cross the clavicular insertion of the trapezius and the acromion process. They are distributed to the skin which covers the upper two-thirds of the deltoid muscle and they supply the acromio-clavicular joint.

Deep Branches of the Cervical Plexus

The deep branches of the plexus pass lateralward and dorsalward, or ventralward and medialward; therefore they form two series, the lateral and the medial.

The lateral branches of the deep series include communicating branches from the second, third, and fourth cervical nerves to the spinal accessory nerve, and muscular branches to the sterno-mastoid and the scalenus medius, levator scapulae, and trapezius.

The communicating branches

The communicating branch from the second cervical nerve is ultimately distributed to the sterno-mastoid, and those from the third and fourth nerves end in the trapezius.

1. The nerve to the sterno-mastoid arises from the second cervical nerve. It pierces the deep surface of the sterno-mastoid, and coinmunicates within the muscle with the spinal accessory nerve.
2. The nerves to the scalenus medius are derived from the third or fourth to the eighth cervical nerves close to their exit from the intervertebral foramina.
3. The nerves to the levator scapulae are derived from the third and fourth cervical nerves, and occasionally from the second or fifth. They pierce the superficial surface of the levator scapute, and supply the upper three divisions of that muscle.
4. The branches to the trapezius are usually in the form of two stout twigs which are given off by the third and fourth cervical nerves. They emerge from under cover of the sterno-mastoid at its posterior border and cross the posterior superior triangle of the neck at a lower level than the spinal accessory nerve. They pass under cover of the trapezius in company with the last-named nerve, and communicate with it to form the subtrapezial plexus, from which the trapezius is supplied.

The medial branches of the deep series also comprise communicating and muscular branches.

The communicating branches  include

1. branches which connect each of the first four cervical nerves with the superior cervical ganghon of the sympathetic;
2. a branch to the vagus;
3. a branch to the hypoglossal; and
4. branches which pass from the second and third cervical nerves to the descendens cervicalis (hypoglossi).

The ultimate distribution of the twigs connected with the sympathetic and the vagus nerves is not known, but the fibers which pass to the hypoglossal nerve pass from it to the thyreo-hyoideus muscle, and to the descendens cervicalis and the latter joins with the branches from the second and third cervical nerves, forming with them the cervical or hypoglossal loop [ansa hypoglossi] which lies on the carotid sheath. From this loop the two beUies of the omo-hyoid muscle and the sterno-hyoid and sterno-thyreoid muscles are supphed as described above.

The muscular branches supply the rectus capitis lateralis, the longus captis (rectus capitis anterior major), the rectus capitis anterior (minor), the scalenus anterior, and the diaphragm. The nerve to the latter muscle is the phrenic.

1. The branch to the rectus capitis lateralis is furnished to that muscle by the first cervical nerve as it crosses the deep surface of the muscle.
2. The nerve to the rectus capitis anterior (minor) is given off by the first nerve at the upper part of the loop in front of the transverse process of the atlas.
3. The longus capitis (rectus capitis anterior major) receives twigs from the upper four cervical nerves.
4. The longus colli receives branches from the second, third, and fourth cervical nerves, and additional branches also from the fifth and sixth nerves.
5. The phrenic nerve springs chiefly from the fourth cervical nerve, but it usually receives a twig from the third and another from the fifth cervical nerve, a small communicating branch from the sympathetic, and, rarely, a branch from the vagus.

The twig from the fifth cervical nerve is frequently connected with the nerve to the subclavius. After the union of its roots the phrenic nerve passes downward and medialward on the scalenus anterior. In this part of its course it is crossed by the tendon of the omo-hyoid and by the transverse cervical and transverse scapular (suprascapular) arteries. It is overlapped by the internal jugular vein, and it is covered by the sterno-mastoid muscle. At the root of the neck the left phrenic nerve lies behind the terminal portion of the thoracic duct, and each nerve passes off the anterior border of the scalenus anterior and descends in front of the first part of the subclavian artery and the pleura immediately below that artery; each nerve passes dorsal to the terminus of the subclavian vein, crosses either in front of or dorsal to the internal mammary artery and gains the medial surface of the pleural sac. From the root of the neck the relations of the phrenic nerves differ. The right phrenic nerve descends along the medial surface of the right pleural sac and crosses in front of the root of the lung. It is accompanied by the pericardiaco-phrenic artery (comes nervi phrenici), and it is in relation medially, and from above downward, with the right innominate vein, the superior vena cava, and the pericardium, the latter membrane separating it from the wall of the right atrium (auricle) . The left phrenic nerve descends along the medial surface of the left pleural sac accompanied by the pericardiaco-phrenic (comes nervi phrenici) artery. In the superior mediastinum it lies between the left common carotid and the left subclavian arteries, and it crosses in front of the left vagus, the left superior intercostal vein, and the arch of the aorta. Below the arch of the aorta it crosses in front of the root of the left lung, and then hes along the left lateral surface of the pericardium, which separates it from the wall of the left ventricle.

Branches of the phrenic

Both phrenic nerves distribute branches to the pericardium and to the pleura. The right nerve gives off a branch, pericardiac, which accompanies the superior vena cava and supplies the pericardium. Each phrenic nerve divides into numerous terminal phrenico-abdominal branches. As a rule, the right phrenic nerve divides into two main terminal branches, an anterior and a posterior. The anterior branch runs forward and one of its terminal filaments anastomoses with the phrenic of the opposite side in front of the pericardium; others descend between the sternal and costal attachments of the diaphragm into the abdomen, where some of them supply the diaphragm and others descend in the falciform Ligament to the peritoneum on the upper surface of the liver. The posterior branch passes through the vena caval opening and ramifies upon the lower surface of the diaphragm, anastomosing with the diaphragmatic plexus of the sympathetic, and its terminal branches supply the muscular fibi-es of the right half of the diaphragm, the inferior vena cava, and the right suprarenal gland.

The left phrenic nerve divides into several branches. One of the most anterior branches anastomoses with the right phrenic nerve; the others pierce the diaphragm and ramify on its under surface, where they anastomose with filaments of the left diaphragmatic plexus of the sympathetic and supply the left half of the diaphragm and the left suprarenal gland. The left phrenic nerve is considerably longer than the right nerve, partly on account of the lower level of the diaphragm on the left side, and partly on account of the greater convexity of the left side of the pericardium.