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The first four cervical nerves, by means of the cervical plexus, innervate the neck;
VENTRAL AXIAL LINE
FIG. 609.-THE DISTRIBUTION OF CUTANEOUS NERVES ON THE FRONT OF THE TRUNK. On one side the distribution of the several nerves is represented, the letters indicating their nomenclature. G.A, Great auricular nerve; S.C, N. cutaneus colli; S.CL, Supra-clavicular nerves; ACR, Posterior; CL, Middle;
ST, Anterior; T.2-12, Lateral and anterior branches of thoracic nerves; I.H, Ilio-hypogastric nerve; I.I, Ilio-inguinal nerve; CIRC, Cutaneous branch of axillary nerve; L.I.C, Medial cutaneous nerve of the arm (lesser internal cutaneous nerve); I.H, Intercosto-brachial; I.C, Medial cutaneous nerve of the forearm (internal cutaneous); M.S, Cutaneous branch of radial nerve; E.C, Lateral cutaneous nerves; G.C, Lumbo-inguinal nerve; M.C1 2, Intermediate cutaneous nerves; I.C', Branch of medial cutaneous nerve; P, Branches of pudendal nerve; S.Sc, Branches of posterior cutaneous nerve of the thigh.
On the other side a schematic representation is given of the areas supplied by the above nerves, the numerals indicating the spinal origin of the branches of distribution to each area.
the last four cervical nerves, together with a large part of the first thoracic nerve,
through the brachial plexus, supply the upper limb. The second thoracic nerve may contribute a trunk to this plexus, and always assists in the innervation of the arm.
The anterior rami of the first four cervical nerves are concerned in forming the cervical plexus. Each nerve emerges from the vertebral canal posterior to the
FIG. 610.-THE LEFT CERVICAL PLEXUS.
vertebral artery. Each is joined on its emergence from the intervertebral foramen, at the side of the vertebral column, by a gray ramus communicans from the superior cervical ganglion of the sympathetic. In the neck the cervical nerves are concealed by the sterno-mastoid muscle; in front lies the longus capitis muscle, and behind are the scalenus medius, and (behind the first or sub-occipital nerve) the rectus capitis lateralis. The cervical plexus is constituted by the combination of the four nerves in an irregular series of loops under cover of the sterno-mastoid muscle, and overlapped, in part, by the internal jugular vein.
From the loops of the plexus the branches of distribution arise, as (a) cutaneous branches to the head, neck, and shoulder; (b) muscular branches to muscles of the
neck and to the diaphragm; and (c) communicating branches to the vagus, accessory, hypoglossal, and sympathetic nerves.
For convenience of description, the nerves derived from the plexus may be classified as follows:
The second, third, and fourth cervical nerves are the chief nerves engaged in forming the plexus. The first cervical nerve only enters into the formation of a small part-the medial portion of the deep part of the plexus.
Superficial Cutaneous Branches.-These nerves, six in number, are entirely cutaneous. They radiate from the plexus, and appear in the posterior triangle of the neck at the posterior border of the sterno-mastoid muscle. They are divisible into two series-the one ascending: lesser occipital, great auricular, and nervus cutaneus colli; the other descending (supra-clavicular): posterior, middle, and anterior.
Ascending Branches.-The lesser occipital nerve is variable in size and is sometimes double. Its origin is from the second and third cervical nerves (more rarely from the second only). It extends backwards under cover of the sternomastoid, and then upwards along its posterior border. Piercing the deep fascia near the apex of the posterior triangle, it divides into auricular, mastoid, and occipital branches, and supplies small cervical branches to the upper part of the neck. The auricular branch supplies the skin of the cranial surface of the auricle; the mastoid and occipital branches supply the scalp. The nerve communicates on the scalp with the greater occipital and great auricular nerves, and with the posterior auricular branch of the facial nerve.
The great auricular nerve is the largest of the cutaneous branches. It arises from the second and third cervical nerves (or, more rarely, from the third alone). Winding round the posterior border of the sterno-mastoid muscle, it courses vertically upwards towards the ear. In this course it crosses the sterno-mastoid muscle obliquely and is covered by the platysma muscle. Before arriving at the ear it subdivides into mastoid, auricular, and facial branches. The mastoid branches ascend over the mastoid process and supply the skin of the scalp behind the ear, communicating with the lesser occipital and posterior auricular nerves. The auricular branches ascend to the ear and supply the lower part of the auricle on both aspects; they communicate with the same nerves. The facial branches, passing over the angle of the mandible and through the substance of the parotid
gland, supply the skin of the cheek over the inferior part of the masseter muscle and the parotid gland. They communicate with branches of the facial nerve in the parotid gland.
The nervus cutaneus colli arises from the second and third cervical nerves. It winds round the posterior border of the sterno-mastoid muscle, and crosses the muscle to reach the anterior triangle, under cover of the platysma muscle and the external jugular vein. It divides near the anterior edge of the sterno-mastoid muscle into superior and inferior branches, which are distributed through the platysma to the skin covering the anterior triangle of the neck. The upper branches communicate freely beneath the platysma with the cervical branch of the facial nerve.
Descending (supra-clavicular) Branches.-By the union of two roots derived
from the third and fourth cervical nerves a considerable trunk is formed, which emerges from under cover of the sternomastoid muscle and extends obliquely downwards through the inferior part of the posterior triangle of the neck. It subdivides into radiating branches -anterior, middle, and posterior -which pierce the deep fascia of the neck above the clavicle, and are distributed to the skin of the inferior part of the side of the neck, to the front of the chest, and the shoulder. The anterior (O.T. supra- sternal) branches are the smallest. Passing over the medial end of the clavicle, they supply the skin of the neck and chest as far down as the synchondrosis sternalis. The middle (O.T. supra-clavicular) branches pass over the intermediate third of the clavicle, beneath the platysma, and can be traced as low as the third rib. The posterior (O.T. supra-acromial) branches pass over or through the insertion of the trapezius
FIG. 611.—DISTRIBUTION OF CUTANEOUS NERVES TO THE HEAD muscle, and over the lateral third of the clavicle, to the shoulder, where they supply the skin as far down as the distal third of the deltoid muscle.
Deep Branches.-The deep branches of the cervical plexus are separated into a lateral and a medial set by their relation to the sterno-mastoid muscle. Beneath the muscle, the lateral branches are directed laterally towards the posterior triangle, and the medial branches pass medially towards the anterior triangle.
The lateral branches consist of muscular and communicating nerves, which for the most part occupy the posterior triangle.
The muscular branches are the following: (1) To the sterno-mastoid, from the second cervical nerve. This enters the muscle on its deep surface and communicates with the accessory nerve. (2) To the trapezius, from the third and fourth cervical nerves. These nerves cross the posterior triangle and end in the trapezius, after having communicated with the accessory nerve, both in the posterior triangle, and under cover of the muscle. (3) To the levator scapulae, from the third and fourth
cervical nerves. Two independent branches enter the lateral surface of the muscle in the posterior triangle. (4) To the scaleni (medius and posterior), from the third and fourth cervical nerves.
The communicating branches are three in number. They join the accessory nerve in three situations:-(a) A branch from the second cervical nerve to the sterno-mastoid joins the accessory nerve under cover of that muscle. (b) Branches to the trapezius from the third and fourth nerves are connected with the accessory nerve in the posterior triangle. (c) Branches from the same nerves join the nerve under cover of the trapezius muscle.
The medial branches of the plexus also comprise muscular and communicating branches. The first cervical nerve assists in the formation of this series of nerves, forming a slender loop with part of the second nerve in front of the transverse process of the atlas.
Communicating Branches.-(a) With the sympathetic.-Gray rami communicantes pass to each of the first four cervical nerves, near their origins, from the superior cervical ganglion or from the trunk below the ganglion. (b) With the vagus nerve. The ganglion nodosum of the vagus nerve may be connected by a slender nerve with the loop between the first two cervical nerves. This communication is not constant. (c) With the hypoglossal.-An important communication occurs between the hypoglossal nerve and the loop between the first and second.