Chapter 8: The shoulder and axilla
Muscles of pectoral region
The upper limb is connected to the trunk ventrally by the pectoralis major, pectoralis minor, subclavius, and serratus anterior. The pectoralis major is inserted into the humerus, the others into the shoulder girdle. They are all supplied by branches of the brachial plexus. The origin, insertion, innervation, and action of each muscle are listed in table 8-1. In this and subsequent tables of muscles, only the chief attachments and principal actions are given.
The fascia extending between the pectoralis major and the latissimus dorsi forms the floor of the axilla. This axillary fascia is suspended from the fascia around the pectoralis minor, and traction on it produces the hollow of the armpit. It is attached above to the clavicle as the clavipectoral fascia, which is also anchored to the first rib and to the coracoid process (fig. 8-1).
The supraclavicular nerves (lateral, intermediate, and medial) arise from the cervical plexus and cross the clavicle (where they may be rolled against the bone). They supply the skin over the shoulder and, because they arise from the same roots (C3, 4) as the phrenic nerve, diaphragmatic inflammation is one cause of pain referred to the shoulder.
The pectoralis major is a large, fan-shaped muscle, the rounded, lower border of which forms the anterior axillary fold (fig. 8-2). It functions mainly as an adductor of the arm and aids in throwing, pushing, and shoveling. When the arms are fixed in climbing, it draws the body upward. The pectoralis major covers the pectoralis minor, which, in turn, covers the second part of the axillary artery.
The serratus anterior (see fig. 8-5) is a large, fan-shaped muscle that forms the medial wall of the axilla. Its digitations of origin (which interdigitate with those of the external oblique muscle of the abdomen) can be seen in a muscular person. The serratus rotates the scapula so that the inferior angle moves laterally; it is thereby important in abduction of the arm above the horizontal plane. It pulls the scapula anteriorward in throwing and pushing. Paralysis of the serratus anterior (e.g., from injury or inflammation of its nerve) is characterized by "winging" of the scapula, i.e., the medial border of the bone stands away from the chest wall.
Superficial muscles of back (see fig. 8-4)
The upper limb is connected to the vertebral column by the latissimus dorsi and trapezius and, deep to these, by the levator scapulae, rhomboid minor, and rhomboid major (table 8-2). The latissimus is inserted into the humerus, the others into the shoulder girdle. Although the muscles are located on the back, they are supplied from the ventral rami of cervical nerves. The trapezius receives its major supply from the accessory nerve (CN XI).
The trapezius (fig. 8-3) is a large, superficial, triangular muscle that is responsible for the sloping ridge of the neck. The muscles of the two sides together form a trapezoid. The region around C7 is more aponeurotic than muscular, resulting in a slightly depressed area frequently visible. The most superior part of the trapezius, together with the levator scapulae, elevates the shoulder. The muscles of the two sides together brace the shoulders by pulling the scapulae posteriorward, and their weakness results in drooping shoulders.
The latissimus dorsi is a large, mostly superficial, triangular muscle. The posterior aspect of its most superior fibers is covered by the trapezius, and its lateral part, together with the teres major, forms the posterior axillary fold. The trapezius and latissimus dorsi intersect near the medial border of the scapula, at a small area termed the triangle of auscultation (figs. 8-3 and 8-4), where the thoracic wall is minimally covered by muscles. The latissimus is a powerful adductor and extensor of the arm and is important in the downstroke in swimming as well as in rowing, climbing, and hammering, and in supporting the weight of the body on the hands.
Muscles of shoulder (table 8-3)
The deltoid, supraspinatus, infraspinatus, teres minor, teres major, and subscapularis arise from the scapula and are inserted into the humerus. They are supplied by cervical nerves 5 and 6 by way of branches of the brachial plexus.
The deltoid is responsible for the roundness of the shoulder (see fig. 8-2). It originates from the inferolateral clavicle and spine of the scapula as well as the acromion process. It is a powerful abductor of the arm in the plane of the scapula. The anterior and posterior parts of the muscle are involved, respectively, in flexion and medial rotation and in extension and lateral rotation. The posterior and middle parts together abduct the arm in a coronal plane. The deltoid acts also as a stabilizer in horizontal movements, e.g., drawing a line across a blackboard. The supraspinatus initiates abduction and aids the deltoid in that motion, which is usually incomplete in paralysis of either of these muscles (fig. 8-5). The interval between the teres minor, teres major, and the surgical neck of the humerus is divided longitudinally by the long head of the triceps into a triangular space medially and a quadrangular space (containing the axillary nerve) laterally (see fig. 9-2).
The tendons of the supraspinatus, infraspinatus, teres minor, and subscapularis blend with the capsule of the shoulder joint and form a musculotendinous (or rotator) cuff, which is incomplete in its inferior aspect (fig. 8-6). The tendons of the cuff are prone to degenerative changes. Moreover, nipping of a tender structure (e.g., from a calcified deposit in the supraspinatus tendon) between the acromion and the greater tubercle of the humerus results in pain during the mid-range of abduction (painful arc syndrome).
Axilla
The pyramidal interval between the arm and the chest wall is termed the axilla. The pectoralis major and latissimus dorsi form prominent anterior and posterior axillary folds, respectively. A vertical line midway between the anterior and posterior axillary folds is referred to as the midaxillary line. The fascial base extends between these folds. The apex of the axilla is the interval between the posterior border of the clavicle, the superior border of the scapula, and the external border of the first rib. Through the apex, the axillary vessels and their accompanying nerves pass from the neck to the arm. The axilla is bounded medially by the upper ribs and their intercostal muscles and by the serratus anterior; it is limited laterally by the intertubercular groove of the humerus.
The chief contents of the axilla are the axillary artery and vein, a part of the brachial plexus and its branches, and the axillary lymph nodes.
Nerves of the upper limb
Brachial plexus (figs. 8-1, 8-7, 8-8 and 8-9)
The nerves to the upper limb arise from the brachial plexus, which is situated partly in the neck and partly in the axilla. It is formed by the union of the ventral rami of the lower four cervical and first thoracic nerves (C5,6,7,8; T1). Frequently it receives a contribution from one nerve higher or one nerve lower.
The brachial plexus descends in the posterior triangle of the neck. Here it lies superior to the clavicle and posterior to the sternomastoid muscle, where it can be palpated. In surface anatomy, the brachial plexus in the neck lies inferior to a line from the posterior margin of the sternomastoid at the level of the cricoid cartilage to the midpoint of the clavicle. Here the plexus can be injected with a local anesthetic (brachial block), the pulsations of the third part of the subclavian artery (situated inferior and anterior to the plexus) being used as a guide (fig. 8-7).
The brachial plexus descends in the concavity of the medial two thirds of the clavicle (see fig. 8-5) and accompanies the axillary artery under cover of the pectoralis major. The plexus is enclosed with the axillary vessels in the axillary sheath, which is a prolongation of the cervical fascia posterioinferior to the clavicle and into the axilla. The brachial plexus may be blocked in the axilla by injecting a local anesthetic into the axillary sheath.
A common arrangement of the brachial plexus is shown in figures 8-8 and 8-9. The first two ventral rami (C5,6) unite to form the upper trunk, the next (C7) constitutes the middle trunk, and the last two (C8; T1) join to form the lower trunk. Each trunk divides into an anterior and posterior division. The anterior divisions of the upper and middle trunks unite to form the lateral cord, that of the lower trunk constitutes the medial cord, and the three posterior divisions join to form the posterior cord. The cords are named from the positions that they occupy in relation to the second part of the axillary artery. In general, the lateral and medial cords supply the ventral aspect of the limb, whereas the posterior cord supplies the dorsal aspect. At the lateral border of the pectoralis minor, the cords divide into terminal branches, each of which contains fibers derived from several spinal nerves.
In summary, the brachial plexus is composed successively of ventral rami and trunks in the neck, divisions that are usually posterior to the clavicle, and cords and branches in the axilla. The nerve bundles that descend from the neck come to meet, and then to accompany, the more superficially and medially placed artery that has ascended from the thorax. The lower trunk lies on the first rib behind the subclavian artery. When a cervical rib is present, the lower trunk may be stretched as it crosses the rib.
Injuries to the brachial plexus are very important. "Upper type" injuries (to C5 or 6 or to the upper trunk) are produced when the arm is pulled downward and the head is drawn away from the shoulder. The upper limb tends to lie in medial rotation ("waiter's-tip hand"). Such injuries may occur during birth. "Lower type" injuries (to C8, Tl, or to the lower trunk) are produced when the arm is pulled upward and may occur during birth also. The short muscles of the hand are affected and "claw hand" results (see fig. 8-14). Involvement of the brachial plexus may also be a part of the neurovascular compression syndrome.
Branches of the brachial plexus
Several branches arise superior to the clavicle (fig. 8-9). The ventral rami give rise to (1) the dorsal scapular nerve (chiefly C5) to the rhomboids and (2) the long thoracic nerve (C5-7) to the serratus anterior. The upper trunk gives off (1) the nerve to the subclavius (chiefly C5), which frequently contributes to the phrenic nerve, and (2) the suprascapular nerve (C5,6) to the supraspinatus and infraspinatus.
The terminal branches of the cords arise inferior to the clavicle. Several of them send articular twigs (e.g., to the shoulder joint), although these are not listed separately here.
The lateral cord gives origin to (1) lateral pectoral nerves (C5-7) to the pectoralis major and minor, (2) the musculocutaneous nerve, (3) the lateral head of the median, and (4) possibly a contribution to the ulnar (fig. 8-9).
The medial cord (fig. 8-9) provides (1) medial pectoral nerves (C8; T1) to the pectoralis major and minor, (2) the medial brachial cutaneous nerve (T1) to the medial side of the arm, (3) the medial antebrachial cutaneous nerve (C8; T1) to the medial side of the forearm, (4) the ulnar, and (5) the medial head of the median.
The posterior cord gives off (1) the upper subscapular nerve(s) (C5) to the subscapularis, (2) the thoracodorsal nerve (C7,8) to the latissimus dorsi, (3) the lower subscapular nerve(s) (C5,6) to the subscapularis and teres major, (4) the axillary, and (5) the radial.
A simple scheme (fig. 8-10) shows the basic arrangement of the segmental innervation (dermatomes) of the skin of the upper limb, which is supplied successively down the preaxial border and then up the postaxial border. Table 8-4 shows the segmental innervation of the muscles of the upper limb.
The five most important branches of the brachial plexus are the musculocutaneous, median, ulnar, axillary, and radial nerves.
The musculocutaneous nerve (C5-7) (fig. 8-11)
The musculocutaneous nerve, very variable, arises from the lateral cord and usually pierces the coracobrachialis. It may carry a part or all of the lateral head of the median nerve and send these fibers to the medial head in the arm. The musculocutaneous nerve supplies the flexor muscles on the anterior aspect of the arm and the skin on the lateral side of the forearm.
The median nerve (C[5,] 6-8; T1) (fig. 8-11)
The median nerve arises by lateral and medial heads from the lateral and medial cords, respectively. The median nerve supplies most of the flexor muscles on the front of the forearm, most of the short muscles of the thumb, and the skin on the lateral part of the front of the hand.
In injury to the median nerve (see fig. 8-14), anesthesia and proprioceptive loss in the digits impose a severe handicap on the proper use of the hand. In section superior to the elbow, pronation is lost, flexion and abduction of the hand are impaired, interphalangeal flexion is lost in the lateral two fingers, and thumb movements, especially opposition, are severely impaired. In section at the wrist, a serious condition, anesthesia and impaired thumb movements result. Median nerve lesions are commonly followed by painful disorders (e.g., causalgia).
The ulnar nerve (C7, 8; T1) (fig. 8-12)
The ulnar nerve arises from the medial cord, occasionally with a small contribution from the lateral cord. The ulnar nerve supplies some of the flexor muscles of the anterior forearm, many of the short muscles of the hand, and the skin on the medial part of the palmar and dorsal aspect of the hand.
In injury to the ulnar nerve, anesthesia and proprioceptive loss occur in the ulnar portion of the hand, including the little and ring fingers. In section superior to the elbow, adduction of the hand is impaired, the fingers cannot be adducted or abducted, the proximal phalanges cannot be flexed, and the middle and distal phalanges cannot be extended. The result is a "claw hand" (main en griffe) (see fig. 8-14), which is the opposite of the Z-position (see fig. 11-8). In section at the wrist, clawing is more marked. Recovery after ulnar nerve lesions is seldom complete.
The axillary nerve (C5, 6) (fig. 8-13)
The axillary nerve is a branch of the posterior cord. The site of the axillary nerve may be represented by a horizontal line through the middle of the deltoid. At the lower border of the subscapularis, it turns posteriorward through the quadrangular space (with the posterior circumflex humeral artery), between the long and lateral heads of the triceps (see fig. 9-2A). It divides into anterior and posterior branches. The former winds around the surgical neck of the humerus and supplies the deltoid and gives some cutaneous twigs. The posterior branch supplies the teres minor and deltoid and becomes the upper lateral brachial cutaneous nerve.
Section of the axillary nerve results in paralysis of the deltoid (and incomplete abduction of the arm by the supraspinatus) and loss of sensation in a small patch of skin over the deltoid.
The radial nerve (C[5,] 6-8; [T1]) (fig. 8-13)
The radial nerve may be regarded as the continuation of the posterior cord. It spirals around the posterior aspect of the humerus under cover of the lateral head of the triceps (see figs. 6-12 and 9-2) and thereby reaches the lateral aspect of the limb (hence the name radial). The radial nerve supplies the extensor muscles of the posterior arm and forearm and the skin on the posterior arm, forearm, and hand.
The radial nerve may be injured in the axilla by pressure of a crutch or by hanging the arm over the back of a chair (" Saturday night palsy"). Section of the radial nerve in the axilla causes extensive muscular paralysis, including loss of extension of the forearm, wrist, and proximal phalanges (fig. 8-14). In a lesion at the elbow, extension of the forearm and wrist are preserved; supination mayor may not be weakened. Thumb movements are impaired. Anesthesia is slight, owing to overlapping of adjacent nerves. Radial nerve fibers regenerate well.
Blood vessels
Subclavian and axillary artery
The main artery carrying blood to the upper limb provides branches to the cervical, scapular and pectoral regions on its course. It also gives rise to the vertebral artey, that is critical for the supply of vital centers in the medulla oblongata. The vessel is named successively subclavian, axillary, and brachial (see figs. 7-3 and 8-7). The subclavian artery ascends into the neck to form an arch, which lies on the first rib and extends superior to the clavicle. In severe arterial bleeding from the upper limb, the main vessel can be compressed against the first rib in the angle between the clavicle and the posterior margin of the sternomastoid.
At the apex of the axilla, where the subclavian artery reaches the lateral border of the first rib (see fig. 8-7), its name is changed to axillary. The pectoralis minor crosses the axillary artery, and it is customary to consider the vessel in three parts: (1) medial, (2) posterior, and (3) lateral to that muscle. The first part is enclosed in fascia (axillary sheath) in common with the axillary vein and the brachial plexus (see fig. 8-1). The second part, being covered by the pectoralis minor, is inferior and medial to the coracoid process. The cords of the plexus are named from the positions that they occupy in relation to the second part of the artery. The third part is fairly superficial and is the one used for compression and ligation.
Branches of the axillary artery (see fig. 7-3)
The axillary artery provides half a dozen named but quite variable branches. The branches vary in their level of origin, but it is frequently stated that the first part gives one branch, the second part two, and the third part three. The significant point is the existence of an extensive arterial anastomosis around the scapula and on its costal and dorsal surfaces. This anastomosis usually enables a collateral circulation to become established after ligature of the third part of the subclavian or the first part of the axillary artery.
- The highest, or superior thoracic, artery supplies adjacent muscles.
- The thoracoacromial artery divides into branches that ramify on the thoracic wall (supplying the pectoral muscles) and the acromion (hence the name).
- The lateral thoracic artery descends along the lateral border of the pectoralis minor and gives off mammary branches.
- The subscapular artery descends along the lateral border of the subscapularis, gives off the circumflex scapular artery (which passes posteriorward through the triangular space), and then accompanies the thoracodorsal nerve as the thoracodorsal artery.
- The anterior circumflex humeral artery is inconstant.
- The posterior circumflex humeral artery passes posteriorward through the quadrangular space with the axillary nerve.
Axillary vein (see fig. 8-7)
The basilic and brachial veins unite to form the axillary, which accompanies the artery on its medial side. It receives the cephalic vein and commonly the thoraco-epigastric veins (which provide a collateral route in obstruction of the inferior vena cava). At the lateral border of the first rib, the axillary becomes the subclavian vein, which does not rise superior to the clavicle. It unites with the internal jugular to form the brachiocephalic vein.
Joints of shoulder
Shoulder joint (see figs. 6-3, 6-4, and 8-6)
The shoulder, or glenohumeral, joint is a large, freely movable, ball-and-socket articulation between the glenoid cavity of the scapula and the head of the humerus. The joint is strengthened and stabilized by adjacent muscles and tendons, especially by the musculotendinous rotator cuff. The coracoid process, the coraco-acromial ligament, and the acromion form a protective arch above the supraspinatus tendon and the head of the humerus. The arch and the cuff are separated by a large, common subacromial and subdeltoid bursa. The shallow glenoid cavity is slightly deepened by a fibrous or fibrocartilaginous lip, the glenoid labrum, attached to its margin. The joint capsule, which is fused to the tendons of the cuff, is attached to the margin of the glenoid cavity and to the anatomical neck of the humerus, except medially, where it extends inferiorly onto the shaft, thereby allowing abduction although permitting downward dislocation. The upper epiphysial line of the humerus is extracapsular, except medially. The capsule is thickened by various ligaments between the two bones. The joint is supplied by adjacent nerves, e.g., from the posterior cord.
The synovial membrane that lines the capsule provides a sheath for the biceps tendon as the latter traverses the joint and descends in the intertubercular groove of the humerus. The joint cavity usually communicates with the subcoracoid bursa and the subscapular bursa (beneath the subscapularis tendon). Several other bursae (e.g., the subacromial bursa and its continuation, the subdeltoid bursa) occur around the shoulder but usually do not communicate with the joint.
Sternoclavicular joint
The sternum and first costal cartilage articulate with the medial end of the clavicle to form, on the basis of the movements that occur, a ball-and-socket joint. The capsule is strengthened by ligaments such as the costoclavicular. The articular surfaces are largely fibrocartilaginous and are separated by a fibrous or fibrocartilaginous articular disc.
Acromioclavicular joint
The medial border of the acromion forms a plane joint with the lateral end of the clavicle (see fig. 6-3). The articular surfaces are largely fibrocartilaginous, and the joint may be divided. The coracoclavicular ligament (which consists of conoid and trapezoid parts) extends from the coracoid process to the conoid tubercle and trapezoid line on the inferior surface of the clavicle. This ligament reinforces the acromioclavicular joint.
Scapular ligaments
The coraco-acromial ligament has already been mentioned. The (superior) transverse scapular ligament, which may be ossified, bridges the scapular notch. The suprascapular artery and nerve pass over and under the ligament, respectively. ("Army over and navy under the bridge.")
Movements of shoulder
Movements at shoulder joint (figs. 6-4 and 8-15).
The movements at a ball-and-socket joint are abduction and adduction, flexion and extension, and rotation and circumduction. The shoulder has the greatest freedom and range of movement of any joint, owing in large part to the addition of scapular movement. The glenoid cavity and the head of the humerus do not fit snugly, but the ever-present tendency for dislocation during movement is resisted by the muscles of the musculotendinous cuff, which hold the head of the humerus in place.
The movements described here are those related to the plane of the body of the scapula. Hence abduction is movement lateral and slightly anterior, away from the trunk; adduction is the converse. Similarly, in flexion the arm is carried anterior and medial across the front of the chest, and in extension it moves posterior and lateral away from the chest. Lateral movement in a strictly coronal plane includes extension and lateral rotation as well as abduction. If the arm is fixed, as in climbing, movements of the scapula on the humerus occur.
Abduction is performed by the deltoid and supraspinatus, and, at least in many patients, this movement is impaired if either muscle is paralyzed. The chief adductors against resistance are the pectoralis major, latissimus dorsi, and teres major. The main flexors are the pectoralis major, anterior part of the deltoid, and biceps. The extensors are the latissimus dorsi and posterior part of deltoid. The subscapularis rotates the humerus medially, and the infraspinatus and teres minor rotate it laterally. Abduction and lateral rotation are mainly under the control of the C5 segment of the spinal cord, and adduction and medial rotation are controlled by C6-8 segments.
Movements of shoulder girdle.
Little if any motion occurs at the shoulder joint without accompanying movement or displacement of the rest of the shoulder girdle.
Displacements of the scapula include (1) elevation and depression, (2) rotation, (3) lateral or anterior movement (protraction), and (4) medial or posterior movement (retraction). During movement, the acromion is kept away from the chest wall by the clavicle, which acts as a strut. The lateral end of the clavicle travels in an arc around the sternoclavicular joint, which acts as a pivot. The medial border of the scapula is held against the chest and travels in the arc of the chest wall. Movement at the acromioclavicular joint is continuous, and the clavicle must be free to rotate around its longitudinal axis.
Elevation of the scapula, as in shrugging the shoulders, is produced by the upper part of the trapezius and the levator scapulae. Depression is usually left to gravity. Anteriorward movement (protraction) of the scapula along the chest wall, as in pushing, is produced by the serratus anterior. Posteriorward movement (retraction), as in bracing the shoulders, is caused by the trapezius muscles and the rhomboids. The serratus anterior rotates the scapula so that its inferior angle moves laterally, and this movement generally accompanies elevation of the arm. The opposite rotation can be carried out against resistance by the levator scapulae and rhomboids.
A movement such as elevation of the arm is highly complicated and requires precise integration. The deltoid and supraspinatus begin abduction, then the scapula commences to rotate. The inferior angle of the scapula travels laterally (serratus anterior), and the lateral angle proceeds superiorly and medially (trapezius). The clavicle moves also. The humerus can be elevated on the scapula to about 120 degrees, and the scapula on the chest about 60 degrees, so that the combined movements allow elevation of the arm to a fully vertical position.
Injury to the accessory nerve (CN XI) interferes with shrugging of the shoulder. Damage to the long thoracic nerve (which innervates the serratus anterior) results in "winging" (posterior displacement of the medial border) of the scapula during movements of the shoulder girdle.
Questions
8-1 Name important muscles involved in (a) throwing and pushing, (b) climbing and hammering, (c) drawing a line across a blackboard.
8-2 What is "winging of the scapula" and how is it produced?
8-3 What is the chief effect of paralysis of the trapezius?
8-4 What gives roundness to the shoulder?
8-5 Which structures form the musculotendinous cuff and what is its clinical importance?
8-6 On what does the lower trunk of the brachial plexus lie?
8-7 Why is the upper limb in medial rotation ("waiter's-tip hand") after an "upper type" (Erb-Duchenne) injury to the brachial plexus?
8-8 Why may "claw hand" follow a "lower type" (Klumpke-Dejerine) injury to the brachial plexus?
8-9 What are the results of deltoid paralysis?
8-10 Which bursae usually communicate with the cavity of the shoulder joint?
8-11 What normally resists the tendency of the shoulder joint to become dislocated?
Figure legends
Figure 8-1 Diagram of the clavipectoral fascia and its relation to the axillary sheath. Sagittal section.
Figure 8-2 Surface landmarks of the upper limb. (From Royce, J., Surface Anatomy, Davis, Philadelphia, 1965.)
Figure 8-3 Surface landmarks of the back and upper limbs. (From Royce, J., Surface Anatomy, Davis, Philadelphia, 1965.)
Figure 8-4 The superficial muscles of the back. Note how the latissimus dorsi covers the inferior angle of the scapula. The left trapezius is removed to show the rhomboids and levator scapulae. (Based on Mollier.)
Figure 8-5 The shoulder from above. Note the relation of the neurovascular bundle (subclavian vessels and brachial plexus) to the clavicle. Note also that the insertion of the trapezius in the concavity formed by the spine, acromion, and clavicle is embraced by the origin of the deltoid muscle from the corresponding convexity.
Figure 8-6 The musculotendinous (or rotator) cuff and the capsule of the shoulder joint, shown after cutting the cuff and removing the humerus. The tendons of the supraspinatus, infraspinatus, teres minor, and subscapularis blend with the capsule and form the cuff, which is incomplete below. The subscapular bursa communicates with the joint cavity (between the superior and middle glenohumeral ligaments).
Figure 8-7 The relationships of the brachial plexus and the axillary vessels. Note that the subclavian artery and vein are separated by the scalenus anterior.
Figure 8-8 Diagram of the anterior aspect of the brachial plexus. A, anterior divisions of trunks; P, posterior divisions of trunks. The muscles to which the rami are ultimately distributed are listed in Table 8-1.
Figure 8-9 Simplified scheme of cervical and brachial plexuses, showing the distribution of nerve fibers in the roots of origin. T.D., thoracodorsal. (Brachial plexus based partly on Seddon.)
Figure 8-10 A, The basic arrangement of the segmental innervation (dermatomes) of the skin of the upper limb, which is supplied successively down the pre-axial border and then up the postaxial border. B, An embryo of 7 weeks (measuring 17 mm between small arrows), showing the longitudinal axes (A) of the upper and lower limbs. The parts of the limbs rostral to the axes are called pre-axial, those caudal to the axes are termed postaxial. The sequence of dermatomes from pre-axial to postaxial is shown on each limb. Cf. fig. 15-11.
Figure 8-11 Diagram of the muscular and cutaneous branches of the musculocutaneous and median nerves. The smaller drawing shows the approximate areas of cutaneous innervation.
Figure 8-12 Diagram of the muscular and cutaneous branches of the ulnar nerve. The smaller drawing shows the approximate areas of cutaneous innervation. Two common varieties of distribution are presented for the back of the hand.
Figure 8-13 Diagram of the muscular and cutaneous branches of the axillary and radial nerves. The adjacent drawing shows the approximate areas of cutaneous innervation. Two common varieties of distribution are presented for the back of the hand.
Figure 8-14 Drawings showing the main motor defects after section or other damage to the ulnar, radial, and median nerves.
Figure 8-15 Movements at the shoulder.