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cervical nerves (Fig. 613). A trunk from the loop joins the hypoglossal just beyond its exit from the skull. One fine branch from this trunk passes upwards along the hypoglossal nerve to the cranium (meningeal branch). The main part of the trunk accompanies the hypoglossal and separates from it to form successively three nerves-the descendens hypoglossi, and the nerves to the thyreo-hyoid and genio-hyoid muscles. The portion of the nerve which remains accompanies the hypoglossal to the muscles of the tongue. It is probable that no part of the hypoglossal nerve itself is concerned in the formation of these three branches. The descending branch of the hypoglossal descends in front of the internal and common carotid

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DESCENDING
BRANCH

THYREO-
HYOID

ANSA

STERNO-HYOID

RNO-THYREOL

OMO-HYOID

COMMUNICATING

BRANCH

DESCENDING CERVICAL

FIG. 613.-THE MUSCLES OF THE HYOID BONE AND STYLOID PROCESS, AND THE EXTRINSIC MUSCLES OF THE TONGUE, WITH THEIR NERVES.

arteries, and is joined in the anterior triangle of the neck by the descending cervical nerve, to form the ansa hypoglossi, from which the infra-hyoid muscles are innervated. (The descending branch of the hypoglossal, in some cases, arises from the vagus nerve.)

Muscular Branches.-The muscles supplied by the medial branches of the plexus are the prevertebral muscles, the genio-hyoid and the infra-hyoid muscles, and the diaphragm.

(a) Prevertebral Muscles.-1. From the loop between the first and second cervical nerves a small branch arises, for the supply of the rectus capitis lateralis,

longus capitis, and the rectus capitis anterior. 2. From the second, third, and fourth nerves small branches supply the inter-transverse, longus colli, and longus capitis muscles. 3. From the fourth nerve a branch arises for the upper part of the scalenus anterior.

(b) Genio-hyoid and Infra-hyoid Muscles.-The descending cervical nerve is formed in front of the internal jugular vein by the union of two slender trunks from the second and third cervical nerves (communicantes hypoglossi). It forms a loop of communication in front of the carotid sheath with the descending branch of the hypoglossal nerve (derived ultimately from the first two cervical nerves). This loop of communication is called the ansa hypoglossi. It is often plexiform; and from it branches are given to the sterno-hyoid and sterno-thyreoid muscles, and both bellies of the omo-hyoid muscle. The nerve to the sterno-hyoid muscle is often continued behind the sternum, to join, in the thorax, with the phrenic nerve or the cardiac plexus.

The thyreo-hyoid and genio-hyoid muscles are supplied by branches of the hypoglossal nerve, which are also traceable back to the communication between the hypoglossal and the first two cervical nerves.

The anterior muscles in immediate relation to the median plane of the neck, between the chin and the sternum, are thus continuously supplied by the first three cervical nerves. The hypoglossal is the nerve of the muscles of the tongue, and it is not certain that it contributes any fibres to the above-named muscles.

(c) Diaphragm.-The phrenic nerve supplies the diaphragm.

NERVUS PHRENICUS.

The phrenic nerve is derived mainly from the fourth cervical nerve, reinforced by roots from the third (either directly or through the nerve to the sterno-hyoid) and fifth (either directly or through the nerve to the subclavius muscle). It runs downwards in the neck upon the scalenus anterior muscle; at the root of the neck it passes between the subclavian artery and vein, enters the thorax and traverses the mediastinum to reach the diaphragm, lying in the middle mediastinum between the pericardium and pleura, and anterior to the root of the lung. In its course it presents certain differences on the two sides. In the neck, on the left side, it crosses the first part of the subclavian artery; on the right side it crosses the second part. In the superior mediastinum, on the left side, it lies between the left subclavian and carotid arteries, and crosses the vagus nerve and the aortic arch. On the right side it accompanies the innominate vein and superior vena cava, and is entirely separate from the vagus nerve. The left nerve is longer than the right, owing to the position of the heart and the left half of the diaphragm. The right nerve sends fibres along the inferior vena cava through the foramen venæ cavæ. Reaching the diaphragm the nerve separates into numerous branches for the supply of the muscle; some enter its thoracic surface (sub-pleural branches), but most of the fibres supply it after piercing the muscle (sub-peritoneal branches).

The branches of the phrenic nerve are-1. Muscular (to the diaphragm); 2. pleural; 3. pericardiac; 4. inferior vena-caval; 5. suprarenal; and 6. hepatic.

The branches to the pleura and pericardium arise as the phrenic nerve traverses the mediastinum. The branches to the inferior vena cava, suprarenal gland, and liver arise after communication of the phrenic nerve with the diaphragmatic plexus of the sympathetic on the abdominal surface of the diaphragm.

Communications of the Phrenic Nerve.-1. The phrenic nerve may communicate with the nerve to the subclavius muscle. 2. It may communicate with the ansa hypoglossi, or a branch from it (the nerve to the sterno-hyoid). 3. It frequently communicates with the cervical part of the sympathetic. 4. It communicates with the coeliac plexus by a junction upon the abdominal surface of the diaphragm with the diaphragmatic plexus on the inferior phrenic artery, in which a small diaphragmatic ganglion is found on the right side. From this junction branches are given off to the inferior vena cava, suprarenal gland, and hepatic plexus.

MORPHOLOGY OF THE CERVICAL PLEXUS.

The characteristic feature of the cervical plexus is the combination of parts of adjacent nerves into compound nerve-trunks by the formation of series of loops. The result of the formation of these loops is that parts (particularly cutaneous areas) are supplied by branches of more than one spinal nerve.

A. Cutaneous Distribution.-By the combinations of the nerves into loops the discrimination of the elements in the upper cervical nerves, corresponding to the lateral and anterior rami of a typical thoracic nerve, is made a matter of some difficulty. The second, third, and fourth nerves, through the cervical plexus, supply an area of skin extending, laterally, from the side of the head to the shoulder; anteriorly, from the face to the level of the third rib. The higher nerves supply the upper region (second and third); the lower nerves supply the lower region (third and fourth). It is not possible to compare the individual nerves strictly with the lateral and anterior rami of a thoracic nerve. A line drawn from the ear to the middle of the clavicle separates, however, a lateral from an anterior cutaneous area; and certain of the cutaneous nerves fall naturally into one of these two categories. The nerves homologous with anterior rami of intercostal nerves are the n. cutaneus colli and the anterior branches of the supra-clavicular series; those homologous with lateral branches are the smaller occipital and posterior supra-clavicular branches. The great auricular and middle supra-clavicular branches are mixed nerves, comprising elements belonging to both sets.

B. Muscular Distribution.-The nerves from the cervical plexus supplying muscles are simpler in their arrangement. They are not generally in the form of loops, and they are easily separated into lateral and anterior series. The lateral nerves comprise the branches to the rectus capitis lateralis, sterno-mastoid, trapezius, levator scapula. The nerves in the anterior series are those to the longus capitis, rectus capitis anterior, the hyoid muscles, and the diaphragm.

It is noteworthy that the last-named muscles-genio-hyoid, thyreo-hyoid, sterno-hyoid, omohyoid, sterno-thyreoid, and diaphragm-are continuously supplied by branches from the first five cervical nerves: the higher muscles by the higher nerves; the lower muscles by the lower nerves.

PLEXUS BRACHIALIS.

The Brachial Plexus is formed by the anterior rami of the fifth, sixth, seventh, and eighth cervical nerves, along with the greater part of the first thoracic nerve. In some cases a slender branch of the fourth cervical nerve is also engaged; and the second thoracic nerve also, in all cases, contributes to the innervation of the arm, through the intercosto-brachial (O.T. intercosto-humeral) nerve. In many cases it contributes also directly to the plexus, by an intra-thoracic communication with the first thoracic nerve.

Position of the Plexus.-The nerves forming the brachial plexus appear in the posterior triangle of the neck between the scalenus anterior and scalenus medius muscles; the plexus is formed in close relation to the subclavian and axillary arteries; the nerves emanating from it accompany the artery to the axilla, whence they are distributed to the shoulder and upper limb.

Communications with the Sympathetic.—The lower four cervical nerves communicate with the cervical portion of the sympathetic by means of gray rami communicantes. Two branches arise from the middle cervical ganglion, and join the anterior rami of the fifth and sixth nerves. Two arising from the inferior cervical ganglion join the seventh and eighth nerves. They reach the nerves either by piercing the prevertebral muscles or by passing round the border of the scalenus anterior muscle.

Composition of the Brachial Plexus.-In an analysis of the brachial plexus four stages may be always seen :

(1) The undivided nerves.

(2) The separation of the nerves into anterior and posterior trunks; and the formation of three primary cords.

(3) The formation of three secondary cords-lateral, medial, and posterior. (4) The origin of the nerves of distribution.

(1) The undivided nerves have only a very short independent course at the side of the neck, after passing between the scalene muscles.

(2) Three primary cords are formed almost immediately after the undivided nerves enter the posterior triangle: the first cord is formed by the union of the fifth and sixth nerves; the second, by the seventh nerve alone; and the third, by the union of the eighth cervical and first thoracic nerves. While these cords are being formed, a division occurs in each of the nerves, into anterior and posterior trunks. The anterior and posterior trunks of the fifth, sixth, and seventh

nerves are nearly equal in size. The posterior trunk of the eighth cervical nerve is much smaller. The posterior trunk of the first thoracic nerve is very minute, and may not be present at all.

(3) The secondary cords of the plexus are formed by combinations of these anterior and posterior trunks, in relation to the axillary artery. They are three in number. The lateral cord is formed by a combination of the anterior trunks of the fifth, sixth, and seventh nerves, and lies on the lateral side of the axillary artery. The medial cord is formed by a combination of the anterior trunk of the eighth cervical with the part of the first thoracic nerve engaged in the formation of the plexus; it lies on the medial side of the axillary artery. The posterior cord is made up of all the posterior trunks from the fifth, sixth, seventh, and eighth cervical and first thoracic nerves, and lies behind the axillary artery.

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FIG. 614.-THE NERVES OF THE BRACHIAL PLEXUS.

The first thoracic nerve may not contribute to the posterior cord, and the branch, when present, is a very small nerve.

(4) The nerves of distribution for the shoulder and arm are derived from these secondary cords, and receive in this way various contributions from the constituent spinal nerves. From the lateral cord arise the lateral anterior thoracic and musculocutaneous nerves, and the lateral head of the median nerve. From the medial cord arise the medial head of the median nerve, the ulnar nerve, medial cutaneous nerve of the forearm, medial cutaneous nerve of the arm, and the medial anterior thoracic nerve. From the posterior cord arise the axillary nerve, the two subscapular nerves, the thoraco-dorsal nerve, and the radial nerve.

It is to be remembered that, although derived from a secondary cord formed by a certain set of spinal nerves, any given nerve does not necessarily contain fibres from all the constituent nerves; e.g., both the musculo-cutaneous and axillary nerves, from the lateral and posterior cords respectively, are ultimately derived only from the fifth and sixth cervical nerves. In other words, the secondary cords are merely collections of nerves of distribution bound together in a common sheath in their passage through the axilla.

THE BRANCHES OF THE BRACHIAL PLEXUS.

It is customary to separate artificially the nerves of distribution of the brachial plexus into two sets: (1) supra-clavicular and (2) infra-clavicular. Clinically it is

important to realise the position of origin of certain nerves. The nerves to the prevertebral muscles, the communication with the phrenic, the dorsal scapular, and long thoracic nerves, arise from the anterior rami of the nerves involved in the plexus. The supra-scapular and the nerve to the subclavius arise at the level of formation of the secondary cords; and the anterior thoracic, subscapular, and thoraco-dorsal nerves arise from the secondary cords, prior to their ultimate subdivision into the nerves of distribution for the upper limb.

Pars Supraclavicularis.-The nerves derived from the plexus above the level of the clavicle are, like the main trunks, divisible into two series: anterior branches, arising from the front of the plexus; posterior branches, arising from the back of the plexus (Fig. 614, p. 701).

Anterior Branches.

1. Nerves to scalenus anterior and
longus colli.

2. Communicating nerve to join

the phrenic nerve.

3. Nerve to the subclavius muscle.

The muscular twigs to the anterior scalene and longus colli muscles arise from the lower four cervical nerves, as they emerge from the intervertebral foramina.

CL.

The communicating branch to the phrenic nerve arises usually from the fifth cervical nerve at the lateral border of the anterior scalene muscle. It is sometimes absent, and occasionally an additional root is present from the sixth cervical nerve. In some instances the nerve is replaced by a branch which springs from the nerve to the subclavius, and passes medially behind the sterno-mastoid muscle to join the phrenic at the inlet of the thorax.

N. Subclavius.-The nerve to the subclavius is a slender nerve, which arises from the front of the cord formed by the fifth and sixth cervical nerves. It descends in the posterior triangle of the neck over the third part of the subclavian artery. It often communicates with the phrenic

nerve.

OUTER

CORD

ART.

OC.8 INNER
OTI CORD

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The branches to the scalenus medius and scalenus posterior, are small trunks which arise from the lower four cervical nerves as they emerge from the intervertebral foramina.

1

N. Dorsalis Scapulæ. The dorsal scapular nerve (O.T. posterior scapular or nerve to the rhomboids) arises from the back of the fifth cervical nerve, as it emerges from the intervertebral foramen. It appears in the posterior C.5, 6, 7, C.8, T.1, Nerves of the brachial triangle of the neck, after piercing the scalenus plexus; ART, Axillary artery; CL, medius muscle. It is directed downwards, Clavicle; SCL, Subclavius muscle; under cover of the levator scapula and rhomboid P. MI, Pectoralis minor, joined to sub

FIG. 615.-DIAGRAM OF THE ORIGIN AND
DISTRIBUTION OF THE NERVES TO THE

PECTORAL MUSCLES.

L.A.T, Lateral anterior thoracic nerve;

I.A.T, Medial anterior thoracic nerve;

clavius by costo coracoid membrane;

P.MA, Pectoralis major.

muscles, and along the vertebral margin of the scapula, to be distributed to the levator scapulæ, rhomboideus minor, and rhomboideus major

muscles. It occasionally pierces the levator scapula.

N. Thoracalis Longus. The long thoracic nerve (O.T. posterior thoracic or external respiratory nerve of Bell) arises by three roots, of which the middle one is usually the largest, from the back of the fifth, sixth, and seventh nerves, as they emerge from the intervertebral foramina. The nerve pierces the scalenus medius as two trunks, of which the lower represents the contribution from the seventh cervical nerve, and, descending along the side of the neck behind the cords of the brachial plexus, it enters the axilla between the superior edge of the serratus

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