Last's Anatomy: Regional and Applied

Part four. Root of the neck

The root of the neck (thoracic outlet) is bounded by the first thoracic vertebra, the first pair of ribs and their cartilages and the manubrium of the sternum. The key to the root of the neck is the scalenus anterior muscle and its relations (Figs 6.86.9 A and 6.10).

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Figure 6.9 

Root of the neck and superior mediastinum: A from the front after removal of the clavicles and manubrium; B root of the neck on the right side, from below. In both A and B, the phrenic nerve is shown crossing the internal thoracic artery on its posterior aspect; usually the nerve crosses the artery anteriorly. In A, a segment of the left vertebral artery has been removed to show the inferior cervical ganglion, which lies behind it. In B, the first costal cartilage has ossified.

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Figure 6.10 

Pyramidal space between longus colli and scalene muscles. The muscles are covered by prevertebral fascia.

Scalenus anterior

This flat muscle arises from the anterior tubercles of the four ‘typical’ cervical vertebrae (3–6) by four slender tendons of origin which lie end to end with those of longus capitis (Fig. 6.8). The muscle passes forwards, laterally and downwards to end in a narrow tendon attached to the scalene tubercle and adjacent ridge on the inner border and upper surface of the first rib (see Fig. 4.34, p. 218).

Nerve supply. By separate branches from the anterior rami of C4–6 nerves.

Action. It is more important as a landmark than an active muscle. It assists in flexion and rotation of the neck, and helps to stabilize the first rib. Even in quiet respiration it shows some electromyographic activity.

Anterior relations

The phrenic nerve passes vertically down across the obliquity of the muscle, plastered thereto by the prevertebral fascia (Fig. 6.10) and a pad of fat lies in front of the prevertebral fascia. The nerve leaves the medial border of the muscle low down and crosses in front of the subclavian artery and its internal thoracic branch, behind the subclavian vein. (Occasionally the phrenic nerve may pass in front of the subclavian vein or posterior to the internal thoracic artery.) Lying on the suprapleural membrane it passes medial to the apex of the lung, crossing in front of the vagus nerve as it enters the superior mediastinum. The ascending cervical artery, a branch of the inferior thyroid artery or the thyrocervical trunk, runs up on the prevertebral fascia medial to the phrenic nerve.

In front of the prevertebral fascia the superficial cervical and suprascapular arteries lie between the scalenus anterior and the carotid sheath (internal jugular vein). The vagus nerve in the carotid sheath passes down in front of the subclavian artery, on the right side giving off its recurrent laryngeal branch. The latter hooks under the artery and passes upwards (Fig. 6.9A). The vagus nerve inclines posteriorly and runs on the medial surface of the apex of the lung to enter the superior mediastinum. The internal jugular vein has inferior deep cervical lymph nodes closely adjacent to it.

The subclavian vein lies in a groove on the first rib and, due to the slope of the rib, lies at a lower level than the insertion of scalenus anterior (Fig. 6.9A). Running medially it joins the internal jugular vein at the medial border of scalenus anterior to form the brachiocephalic vein; the thoracic duct on the left and the right lymph duct on the right enter the angle of confluence of the two veins.

Catheterization. The right subclavian vein can be used for the placement of a central venous line, instead of the internal jugular (see p. 344); it is preferred by many operators and is more comfortable for the patient. The usual approach is infraclavicular, from a point 2cm below the midpoint of the clavicle along a line that passes behind the clavicle towards the jugular notch of the sternum. The needle pierces the clavipectoral fascia and enters the vein just behind the fascia. Pneumothorax due to puncture of the pleura and lung, and puncture of the subclavian artery are complications of this procedure. The vein is also used for the placement of wires from cardiac pacemakers, which are usually implanted in connective tissue over the upper lateral part of pectoralis major.

Medial relations

The medial edge of scalenus anterior makes a pyramidal space with the lateral border of the lower part of longus colli. The prevertebral fascia in front of these muscles is attached to bone at their opposing margins and there is no fascial roof across the pyramidal space between the muscles. The base of the space is formed by the subclavian artery, lying on the suprapleural membrane. The apex of the space is the carotid (Chassaignac's) tubercle on the transverse process of C6 vertebra (Figs 6.8 and 6.10).

The common carotid artery, medial to the internal jugular vein, lies deep to sternocleidomastoid immediately in front of the pyramidal space. Behind the artery and the carotid sheath, the space contains the inferior cervical sympathetic (or stellate) ganglion, with the vertebral artery and vein(s) in front of it. The inferior thyroid artery arches medially in a bold curve whose upper convexity lies in front of the apex of the pyramidal space (C6 level), with the sympathetic chain, usually the middle ganglion, in front of the artery. At a lower level, and further forward, the thoracic duct (or right lymphatic duct) makes a similar convexity behind the carotid sheath as it arches over the lung apex and subclavian artery to enter the confluence of the subclavian and internal jugular veins (Fig. 6.10).

The relationship of the scalenus anterior to the subclavian artery is used to descriptively divide the subclavian artery into three parts. The first part of the subclavian artery is medial to scalenus anterior. It arches over the suprapleural membrane and impresses a groove upon the apex of the lung. It has three branches. The vertebral artery is the first; this arises from the upper convexity of the subclavian and passes up to disappear, at the apex of the pyramidal space, into the foramen of the transverse process of C6 vertebra. The accompanying sympathetic nerve runs up behind the artery. Rarely this first part of the vertebral artery may initially enter the foramen of the transverse process of a higher vertebra than C6. A connecting loop between middle and inferior cervical ganglia passes in front of the subclavian artery and turns up behind it, forming the ansa subclavia. The recurrent laryngeal nerve recurves under the right subclavian artery, while the thoracic duct loops over the left artery. The thyrocervical trunk arises lateral to the vertebral artery from the upper surface of the subclavian. It divides immediately into superficial cervical, suprascapular and inferior thyroid arteries, which have already been noted. The proximal part of the superficial cervical is named transverse cervical artery when it gives off the dorsal scapular artery as a deep branch. The internal thoracic artery arises from the lower surface of the subclavian and passes downwards over the lung apex, crossed usually anteriorly by the phrenic nerve.

The vertebral vein emerges from the foramen in the transverse process of C6 vertebra and runs forward in front of the vertebral and subclavian arteries to empty into the brachiocephalic vein. It may be accompanied by a companion vein that passes through the foramen of the transverse process of C7 vertebra and passes behind the subclavian artery to the same destination.

Posterior relations

Scalenus anterior is separated from scalenus medius by the subclavian artery and the anterior rami of the lower cervical and first thoracic nerves. The second part of the subclavian artery lies behind scalenus anterior. Its only branch is the costocervical trunk. It passes back across the suprapleural membrane towards the neck of the first rib and there divides into a descending branch, the superior intercostal artery, which enters the thorax across the neck of the first rib, and an ascending branch, the deep cervical artery, which passes backwards between the transverse process of C7 vertebra and the neck of the first rib to run upwards behind the cervical transverse processes.

Lateral relations

The trunks of the brachial plexus and the third part of the subclavian artery emerge from the lateral border of scalenus anterior. They lie behind the prevertebral fascia on the floor of the posterior triangle (Fig. 6.10). The dorsal scapular usually arises from the third part. It runs laterally through the brachial plexus in front of scalenus medius and then deep to levator scapulae to take part in the scapular anastomosis (see p. 46). It is frequently replaced by the deep branch of the transverse cervical artery, and this branch then takes the name of dorsal scapular.

The surface marking of the subclavian artery in the neck is along a line arching upwards from the sternoclavicular joint to the middle of the clavicle and about 2cm above it.

Surgical approach. The artery can be exposed by dividing the clavicular head of sternocleidomastoid from the clavicle and then detaching scalenus anterior from the first rib, taking particular care not to damage the phrenic nerve.

Pressure on the subclavian artery and lowest root (T1) of the brachial plexus as they cross over a cervical rib or fibrous band, when present at the root of the neck, is described on page 422. Elevation of the first rib by scalenus anterior may also cause or aggravate such a thoracic outlet syndrome, and the muscle is usually divided close to its insertion when the syndrome is treated surgically.

Scalenus medius and scalenus posterior

Scalenus medius arises from the lateral ends of the transverse processes of atlas and axis and from the posterior tubercles of all the other cervical vertebrae and is inserted into the quadrangular area between the neck and subclavian groove of the first rib (see Fig. 4.34, p. 218).

Scalenus posterior is a small unimportant muscle that arises from the posterior tubercles of the lower cervical vertebrae, passes across the outer border of the first rib deep to the upper digitation of serratus anterior, and is inserted into the second rib.

Nerve supplies. Both muscles are supplied segmentally by the anterior rami of cervical nerves, scalenus medius by C3–8.

Actions. Scalenus medius, mainly a lateral flexor of the neck, can elevate the first rib as an accessory muscle of respiration.



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