The brachial plexus
DISSECTION 5.5 Supraclavicular part of the brachial plexus Objective
I. To expose the roots and trunks of the brachial plexus.
Instructions
The brachial plexus should be dissected in conjunction with the dissection of the axilla. To expose the brachial plexus in its full extent, the clavicle should be divided between drill holes placed in its intermediate third.
This allows the shoulder to fall back and opens the cer- vico-axillary canal. Subsequently, the clavicle should be wired together through the drill holes to replace the parts in their normal relationships.
1. Clean the fascia overlying the scalenus anterior and medius, and find the C. 5, C. 6, C. 7, C. 8, and T. 1 roots of the brachial plexus.
2. Trace C. 5 and C. 6 laterally to the formation of the upper trunk.
3. Note that C. 7 continues as the middle trunk.
4. Trace C. 8 and T. 1 laterally to the formation of the lower trunk.
5. Find the long thoracic nerve on the medial wall of the axilla, and trace it back to its origin from the roots.
6. Find and trace the nerve to the subclavius and the suprascapular nerve to the upper trunk.
7. Trace the phrenic nerve from where it has been identified on the scalenus anterior to the roots.
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The posterior triangle of the neck
Fig. 3.21, Vol. 1; see Fig. 10.1], and the phrenic nerve.
The dorsal scapular nerve passes backwards from the fifth cervical ventral ramus through the scalenus medius. It runs inferolaterally, anterior to the levator scapulae and the two rhomboids, sup- plying all three.
The thin nerve to the subclavius arises where the fifth and sixth cervical ventral rami unite. It de- scends across the brachial plexus and subclavian ves- sels, and enters the posterior surface of the subclavi- us. It often sends a branch to the phrenic nerve which may replace the phrenic contribution of C. 5.
The suprascapular nerve arises from the junc- tion of the fifth and sixth cervical ventral rami. It runs postero-inferiorly on the scalenus medius, lateral to the plexus. It descends with the supras- capular vessels over the scapula and supplies the su- praspinatus, infraspinatus, and the shoulder joint.
The long thoracic nerve arises by a series of branches from the upper three roots of the plexus.
It descends on the surface of the scalenus medius and enters the axilla over the serratus anterior on the first rib.
See Clinical Applications 5.1 and 5.2 for the prac- tical implications of the anatomy discussed in this chapter.
Divisions of the brachial plexus
Each trunk splits into an anterior and a posterior division. The three posterior divisions unite to form the posterior cord. The anterior divisions of the upper and middle trunks unite to form the lateral cord. The anterior division of the lower trunk forms the medial cord.
Relations of the brachial plexus
The supraclavicular part of the brachial plexus lies on the scalenus medius. The lower trunk lies on the superior surface of the first rib, posterior to the subclavian artery [Fig. 5.6]. The roots of the long thoracic nerve are posterior to the plexus. The ex- ternal jugular vein, the inferior belly of the omohy- oid, and the suprascapular and superficial cervical vessels are anterior to it.
Brachial plexus branches in the neck
All the roots of the brachial plexus receive grey rami communicantes from the sympathetic trunk. The upper two roots receive from the middle cervical ganglion, and the others from the cervico- thoracic ganglion.
Most of the branches in the neck are muscu- lar and pass to the upper limb. Small nerves also pass to the scalene muscles, the longus colli [see
CLINICAL APPLICATION 5.1 External jugular vein laceration A young man sustained a cut on the right side of his neck in
a street fight and was found bleeding heavily on the road.
He was rushed to a hospital. On arrival in the emergency unit, he was bleeding heavily from the site of injury, and was cyanotic and dyspnoeic. On examination, it was found that his external jugular vein had been lacerated.
Study question 1: how and where is the external jugular vein formed? And how and where does it terminate? (An
swer: the external jugular vein begins by the union of the posterior branch of the retromandibular and posterior auric
ular veins on the surface of the sternocleidomastoid. It ends by draining into the subclavian vein in the posterior triangle.)
Study question 2: name the layer of the deep cervical fascia pierced by the vein before its termination. (Answer:
investing layer of the deep fascia.)
The tight attachment of the fascia to the wall of the vein causes the wound to be pulled open. The negative intratho
racic pressure causes air to enter into the vein, leading to an air embolus.
Study question 3: from your knowledge of anatomy, in which chamber of the heart would the air embolus in the external jugular vein enter? (Answer: the embolus will go through the external jugular vein, subclavian vein, right brachiocephalic vein, and superior vena cava into the right atrium. The air embolus in the right atrium accounts for the cyanosis and dyspnoea.)
Study question 4: speculate on what steps could be taken to prevent a fatal air embolism in this case. (Answer: pressure on the vein will help control the bleeding and prevent air from entering into it.)
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The posterior triangle
CLINICAL APPLICATION 5.2 Scalenus anterior syndrome A 43yearold painter presented with a history of pain and tin
gling sensation in the medial aspect of his forearm and hand, especially while lifting his arm above his shoulder to paint a wall.
Study question 1: what is the likely diagnosis? (Answer:
compression of the lower part of the brachial plexus. The scalene triangle is formed by the scalenus anterior anteriorly, the scalenus medius posteriorly, and the first rib inferiorly, and contains the brachial plexus and subclavian artery. Com
pression of the brachial plexus in the scalene triangle is a probable diagnosis.)
Study question 2: what factors often lead to compression of the contents? (Answer: any clinical conditions which de
crease the space within the scalene triangle can cause com
pression. Common conditions include hypertrophied scale
nus anterior muscle, cervical rib, and fibromuscular bands between the two scalene muscles.)
Study question 3: what are the symptoms that arise due to compression? (Answer: compression of the lower trunk of the brachial plexus causes pain, numbness, or tingling sensation of the medial aspect of the forearm and arm.
Compression of the subclavian artery can cause ischaemia, leading to pain and blanching of the hand.)
Study question 4: how is the diagnosis of the scalenus anterior syndrome confirmed? (Answer: several manoeuvres can be done to precipitate symptoms and confirm the di
agnosis of the scalenus anterior syndrome. The Wright’s test stimulates change in the radial pulse with hyperabduction of the arm. A fall in the radial pulse on hyperabduction of the arm indicates compression of the subclavian artery above the shoulder joint. More conclusive results are obtained by Xray, magnetic resonance imaging, and subclavian Doppler sonography).
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chapter 6
The anterior triangle of the neck
Introduction
The anterior triangle of the neck is the area bound by the midline, the mandible, and the sternocleid- omastoid muscle [Figs. 6.1, 6.2].
Surface anatomy
Draw a finger down the anterior median line of your neck from the chin to the sternum, and iden- tify, in sequence: (1) the body of the hyoid bone
Fig. 6.1 Dissection of the front of the neck. The right sternocleidomastoid has been retracted.
Facial V.
Submental triangle
Facial A.
Submandibular gland Parotid gland External carotid A.
Common carotid A.
Levator glandulae thyroideae Cricothyroid
Sternohyoid
Isthmus, thyroid gland
Jugular venous arch Superior thyroid A.
Platysma, turned up
Mylohyoid Common facial V.
Internal jugular V.
External jugular V.
Anterior jugular V.
Inferior thyroid Vv.
Omohyoid
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The anterior triangle of the neck
approximately 1 cm below, and 6 cm behind, the chin; (2) the laryngeal prominence, the sharp protuberance of the anterior border of the thy- roid cartilage; (3) the rounded arch of the cricoid
cartilage; and (4) the rings of the trachea which are partly masked by the isthmus of the thyroid gland.
Grasp the front of the U-shaped hyoid bone be- tween the finger and thumb. Trace it backwards to the greater horns. The tips of the greater horn may be overlapped by the sternocleidomastoid muscles.
Trace the superior border of the thyroid cartilage posteriorly from its midline notch. Note that it ends in a projection—the superior horn—immediately an- terior to the sternocleidomastoid. The isthmus of the thyroid gland [Fig. 6.1] forms a soft mass on the second, third, and fourth tracheal rings and slips upwards past the palpating finger when you swallow.
Press the tip of your fingers into the side of your neck from the mastoid process downwards.
The deep bony resistances felt are the transverse processes of the cervical vertebrae. Only the transverse process of the first cervical vertebra can be felt clearly, immediately antero-inferior to the tip of the mastoid process. The fourth is at the level of the upper border of the thyroid cartilage. The sixth is at the level of the cricoid cartilage.
Using the instructions given in Dissection 6.1, open the anterior triangle and study its superficial content.
Digastric Digastric
Omohyoid
1 2 5 3
4
Trapezius Sternocleido- mastoid Stylohyoid
Fig. 6.2 The triangles of the neck. Anterior triangle: 1. Digastric triangle; 2. Carotid triangle; 3. Muscular triangle. Posterior triangle:
4. Subclavian triangle; 5. Occipital triangle.
DISSECTION 6.1 Skin reflection of the anterior triangle