By the end of the third week after fertilization the paraxial mesoderm begins to divide into mesodermal somites which are easily recognizable during the fourth and fifth weeks (Fig. 2.1A). Eventually some 44 pairs of somites develop, although not all are present at the same time; however, the paraxial mesoderm at the cranial end of the embryo remains unsegmented. There are 4 occipital somites, followed by 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 8–10 coccygeal somites. The growth and migration of these somitic cells are responsible for the thickening of the body wall, as well as the development of bone and muscle. The deeper layers of the skin are also of somitic origin. Somite-derived tissue spreads medially to form the vertebrae, dorsally to form the musculature of the back, and ventrally into the body wall to form the ribs, and the intercostal and abdominal muscles.

Figure 2.1 (A) Somites in the developing embryo, (B) the development of the upper limb showing limb buds and the direction they will rotate.

Soon after its formation each somite becomes differentiated into three parts. The ventromedial part forms the sclerotome, which migrates medially towards the notochord and neural tube to take part in the formation of the vertebrae and ribs (Fig. 2.1A). The remainder of the somite is known as the dermomyotome. The cells of the dorsal and ventral edges proliferate and move medially to form the myotome, whose cells migrate widely and become differentiated into myoblasts (primitive muscle cells). The remaining thin layer of cells forms the dermatome, which spreads out to form the dermis of the skin.

The myotome of each somite receives a single spinal nerve which innervates all the muscle derived from that myotome, no matter how far it eventually migrates. The dorsal aortae lie adjacent to the somites and give off a series of intersegmental arteries which lie between them.

Development of the limbs

The limbs initially appear as flipper-like projections (the limb buds), with the forelimbs appearing first, between 24 and 26 days, each bud consisting of a mass of mesenchyme covered by ectoderm with a thickened ectodermal ridge at the tip; the ectodermal ridge controls normal development of the limb. Consequently, damage to it will result in some trauma to the limb. At the beginning of the second month the elbow and knee prominences project laterally and backwards. At about the same time, the hand and foot plates appear as flattened expansions at the end of the limb bud. Between 36 and 38 days, five radiating thickenings forming the fingers and toes can be distinguished, the webs between the thickenings disappear freeing the digits. Appropriate spinal nerves grow into the limbs in association with migration of the myotomes: C5, 6, 7, 8 and T1 for the upper limb, and L4, 5, S1, 2 and 3 for the lower limb. The limb bones differentiate from the mesenchyme of the bud. The limbs grow in such a way that they rotate in opposite directions, the upper limb laterally and lower limb medially (Fig. 2.1B). Consequently, the thumb becomes the lateral digit of the hand, while the great toe is the medial digit of the foot.

During development the upper limb bud appears as a swelling from the body wall (Fig. 2.2(i)) at the level of the lower cervical and first thoracic segments. At first the limb buds project at right angles to the surface of the body, having ventral and dorsal surfaces and cephalic (preaxial) and caudal (postaxial) borders (Fig. 2.2(ii)). As the limb increases in length it becomes differentiated, during which time it is folded ventrally so that the ventral surface becomes medial (Fig. 2.2(iii)), with the convexity of the elbow directed laterally (Fig. 2.2(iv)). At a later stage the upper and lower limbs rotate in opposite directions so that the convexity of the elbow is directed towards the caudal end of the body (Fig. 2.2(v)). As the limb bud develops, the primitive muscle mass becomes compartmentalized, foreshadowing the adult pattern. Intermuscular septa, extending outwards from the periosteum of the humerus, divide the arm into anterior and posterior compartments. As in the lower limb, some of the anterior compartment musculature has become separated into an adductor group. However, in the upper limb this muscle mass has degenerated phylogenetically so that all that remains is coracobrachialis. Adduction of the upper limb is a powerful action in humans, being served by great sheets of muscle that have migrated into it; these are latissimus dorsi posteriorly and pectoralis major anteriorly. In the forearm, the radius (preaxial bone) and the ulna (postaxial bone) are connected by an interosseous membrane, and to the investing fascia by intermuscular septa. The compartments so formed enclose muscles of similar or related functions.

Figure 2.2 Rotation of the limbs during development.

With the upper limb in the anatomical position the anterior preaxial compartments are in a continuous plane with the muscles being supplied by branches from the lateral and medial cords of the brachial plexus, which are all derived from the anterior divisions of the nerve trunks. Similarly, the posterior postaxial compartment muscles are all supplied by branches of the posterior cord, derived from the posterior divisions of the nerve trunks.

The median, musculocutaneous and ulnar nerves are responsible for preaxial innervation, whereas the radial nerve supplies all of the postaxial musculature of the upper limb below the shoulder. Within the pectoral girdle the clavicle is the anterior preaxial bone and the scapula, with the exception of the coracoid process (which is also an anterior bone), is the posterior postaxial bone. The distinction with respect to the coracoid process is that phylogenetically it is a separate bone; its fusion with the scapula is secondary. Consequently, muscles arising from the clavicle or coracoid process belong to the preaxial group, and are therefore supplied by preaxial branches of the brachial plexus. Similarly, muscles arising from the remainder of the scapula are part of the postaxial group and are innervated by postaxial branches of the plexus.

Furthermore, there is a serial arrangement of the nerves in the brachial plexus with respect to both their motor innervation and their sensory supply. The order is retained from the primitive serial morphology of the embryo. Remembering that the skin has essentially been stretched over the developing limb, the fifth cervical nerve in the adult is sensory to the cranial part of the limb, and the first thoracic to its caudal part, with the seventh cervical nerve lying in the middle of the limb. The pattern of motor innervation, in simple terms, progresses from C5 for shoulder movements to T1 for intrinsic hand movements, with the elbow being served by C5 and 6, the forearm by C6, the wrist by C6 and 7, and the fingers and thumb by C7 and 8.

As in the lower limb, many muscles cross two or more joints, and can therefore act on all of them. Consequently, a complex system of synergists and fixators is required in order to prevent or restrict unwanted movements. Procedures for testing for loss of muscle action, as in paralysis, can thus be quite complicated.

The upper limit of the upper limb is not so easily defined as in the lower limb. In spite of muscular attachments to the head, neck and thorax, the upper limits can be conveniently considered as the superior surface of the clavicle anteriorly and the superior border of the scapula posteriorly. The free upper limb is divided into the arm between the shoulder and elbow, the forearm between the elbow and the wrist, and the hand beyond the wrist. The hand has an anterior or palmar surface, and a posterior or dorsal surface (Fig. 2.3).

Figure 2.3 The regions, bones and joints of the upper limb.

The bones of the upper limb are the clavicle and scapula of the pectoral girdle, the humerus in the arm, the lateral radius and medial ulna in the forearm, the eight carpal bones of the wrist, the five metacarpals of the hand and the phalanges of the digits – two in the thumb and three in each finger (Fig. 2.3).

Bones

The scapula

This is a large, flat, triangular plate of bone on the posterolateral aspect of the thorax, overlying the second to the seventh ribs. Suspended in muscles, the scapula is held in position by the strut-like clavicle, but retains great mobility relative to the thorax. Being a triangular bone it has three angles, three borders and two surfaces which support three bony processes (Figs. 2.4 and 2.5A).

Figure 2.4 The right scapula: (A) posterior view, (B) anterior view.

Figure 2.5 (A) Left scapula, lateral view, (B) right clavicle, superior view, (C) right clavicle, inferior view.

The costal surface which faces the ribs is slightly hollowed and ridged with a smooth, narrow strip along its entire medial border. It is also known as the subscapular fossa.

The dorsal surface faces posterolaterally and is divided by the spine of the scapula into a smaller supraspinous fossa above and a larger infraspinous fossa below. The supraspinous and infraspinous fossae communicate via the spinoglenoid notch between the lateral end of the spine and the neck of the scapula. The spine of the scapula has upper and lower free borders which diverge laterally enclosing the acromion.

The thin medial border lies between the inferior and superior angles, being slightly angled at the medial end of the spine. The lateral border is thicker, being deeply invested in muscles, and runs down from the infraglenoid tubercle below the glenoid fossa to meet the medial border at the inferior angle. The superior border, which is thin and sharp, is the shortest, and has the suprascapular notch at the junction with the root of the coracoid process.

Inferiorly the thick inferior angle lies over the seventh rib and is easily palpated. The superior angle lies at the junction of the medial and superior borders, whilst the lateral angle is truncated and broadened to support the head and glenoid fossa of the scapula.

The head of the scapula is an expanded part of the bone joined to a flat blade by a short inconspicuous neck. The glenoid fossa (or cavity) is found on the head as a shallow, pear-shaped concavity, facing anterolaterally. The glenoid fossa is broader below and articulates with the head of the humerus forming the shoulder (glenohumeral) joint. Immediately above the glenoid fossa is the supraglenoid tubercle.

The acromion, which is the expanded lateral end of the spine, is large and quadrilateral, projecting forwards at right angles to the spine. The lower border of the crest of the spine continues as the lateral border of the acromion; the junction of these two borders forms the palpable acromial angle. The upper border of the crest becomes continuous with the medial border of the acromion and presents an oval facet for articulation with the clavicle at the acromioclavicular joint. The superior surface of the acromion is flattened and subcutaneous.

The coracoid process is a hook-like projection with a broad base directed upwards and forwards from the upper part of the head, and a narrow more horizontal part which passes anterolaterally from the upper edge of the base. The tip lies below the junction of the middle and lateral thirds of the clavicle.

Palpation

Starting at the lowest point, the inferior angle can readily be gripped between thumb and index finger and, if the subject relaxes sufficiently, can be lifted away from the thorax. The medial border can be followed along its whole length from inferior to superior angles. The spine of the scapula can be palpated as a small triangular area medially increasing in size as the fingers move laterally along it. The flat crest with its upper and lower borders can be identified. Continuing along the lower border of the crest to its most lateral point, the sharp 90° acromial angle can be felt; this continues as the palpable lateral border of the acromion. Running onto this lateral border, the flat upper surface of the acromion can be felt above the shoulder joint. The coracoid process can be palpated as an anterior projection below the junction of the middle and lateral thirds of the clavicle, and therefore is a useful reference point for surface marking the shoulder joint as it lies just medial to the joint line.

Ossification

The scapula ossifies from a number of centres. The primary ossification centre appears in the region of the neck by the eighth week in utero, so that at birth the coracoid process, acromion, glenoid cavity, medial border and inferior angle are still cartilaginous. Secondary centres appear in each of these regions, except the coracoid, between the ages of 12 and 14 years, fusing with the body between 20 and 25 years. The secondary centre for the coracoid process, however, appears during the first year and fuses with the body between 12 and 14 years.

The clavicle

This is a subcutaneous bone running horizontally from the sternum to the acromion (Fig. 2.5B,C). It acts as a strut holding the scapula laterally, thus enabling the arm to be clear of the trunk – an essential feature in primates. The scapula and clavicle together form the pectoral (shoulder) girdle, transmitting the weight of the upper limb to the axial skeleton and facilitating a wide range of movement of the upper limb.

The medial two-thirds of the clavicle is convex forwards and is roughly triangular in cross-section. The lateral third is concave forwards and flattened from above downwards. The medial convexity conforms to the curvature of the superior thoracic aperture, the lateral concavity to the shape of the shoulder.

The lateral (acromial) end of the clavicle is the most flattened part and has a small deltoid tubercle on its anterior border. Inferiorly the rounded conoid tubercle is present at the posterior edge, with the rough trapezoid line (Fig. 2.5C) running forwards and laterally away from it. The conoid tubercle and trapezoid line give attachment to the conoid and trapezoid parts of the coracoclavicular ligament binding the clavicle and scapula together. Laterally is a small oval facet for the acromion: it faces obliquely downwards and laterally.

The medial (sternal) end of the clavicle is enlarged and faces downwards and medially. The lower three-quarters is bevelled and articulates with the clavicular notch of the manubrium and the costal cartilage of the first rib, forming the sternoclavicular joint. The cylindrical clavicle projects above the shallow notch on the sternum; this can be confirmed by palpation. The superior quarter of the sternal end is roughened for the attachment of the intra-articular disc and ligaments. Between the lateral and medial ends, the superior surface is smooth, while the inferior surface is marked by a rough subclavian groove centrally and a large oval roughened area for the costoclavicular ligament medially. The anterior and posterior borders are roughened by muscle attachments.

The clavicle is often fractured by the direct violence of a blow, or by indirect forces transmitted up the limb following a fall on the outstretched arm. The fracture usually occurs at the junction of the two curvatures, and the resultant fracture appearance is caused by the weight of the arm pulling the shoulder downwards and medially so that the medial fragment of the clavicle overrides the lateral at the fracture site.

Palpation

In a slender subject the whole length of the clavicle can often be seen directly beneath the skin. Initially, the enlarged medial end of the clavicle can be palpated with the fingers, with the line of the sternoclavicular joint also being identified. Moving laterally, almost the whole length of the shaft of the clavicle can be gripped between finger and thumb. At the lateral end, the bulk of deltoid may require deeper pressure; nevertheless the line of the acromioclavicular joint should be palpable, particularly from above.

Ossification

The clavicle ossifies in membrane, being the first bone in the body to begin ossification. Two primary centres appear during the fifth week in utero which unite, with ossification spreading towards the ends of the bone. A secondary centre appears in the medial end between 14 and 18 years, fusing with the main part of the bone as early as 18–20 years in females and 23–25 years in males. An additional centre may appear in the lateral end at puberty; however, it soon fuses with the main bone.

The humerus

Largest bone in the upper limb (Fig. 2.6), being a typical long bone with a shaft (body) and two extremities (ends). Proximally, it articulates with the glenoid fossa of the scapula forming the shoulder joint, and distally with the radius and ulna forming the elbow joint.

Figure 2.6 Right humerus: (A) posterior view, (B) anterior view.

Proximally, the major feature is the almost hemispherical head of the humerus with its smooth, rounded articular surface facing upwards, medially and backwards; it is considerably larger than the socket formed by the glenoid fossa. The head is joined to the upper end of the shaft by the anatomical neck, a slightly constricted region encircling the bone at the articular margin, separating it from the tubercles.

The greater tubercle is a prominence on the upper lateral part of the bone, next to the head. It merges with the shaft below and is marked by three distinct impressions for muscular attachment. The greater tubercle projects laterally past the margin of the acromion and is the most lateral bony point at the shoulder.

The smaller lesser tubercle is a distinct prominence on the anterior aspect below the anatomical neck. It has a well-marked impression on its medial side for muscular attachment. Between these two tubercles, and passing onto the shaft of the humerus, is the deep intertubercular groove (sulcus). The crests of the greater and lesser tubercles continue down from the anterior borders of the tubercles to form the lateral and medial lips of the groove. Between the two lips is the floor of the groove.

Below where the head and tubercles join the shaft there is a definite constriction. This region is termed the surgical neck because fractures often occur here, particularly in the elderly.

The shaft of the humerus is almost cylindrical above, becoming triangular in its lower part with distinct medial and lateral borders. It presents three borders (anterior, medial, lateral), although they are frequently rounded and indistinct, between which are the three surfaces (anteromedial, anterolateral, posterior) of the shaft. The intertubercular groove is continuous with the anteromedial surface, the medial border beginning as the crest of the lesser tubercle and ending by curving towards the medial epicondyle. The smooth anterolateral surface is marked about its middle by the deltoid tuberosity. The posterior surface is crossed obliquely from superomedial to inferolateral by the spiral (radial) groove, which reaches the lateral border below the deltoid tuberosity, but is often poorly marked.

The lower end of the humerus is expanded laterally, flattened anteroposteriorly, and curves slightly forwards. It presents two articular surfaces separated by a ridge. The lateral articular surface, the capitulum, is situated anteroinferiorly and is a rounded, convex surface, being less than a hemisphere in size. The capitulum articulates with the radius, making its greatest contact with the radius when the elbow is fully flexed.

Medial to the capitulum is the trochlea, the articular surface for the ulna. The trochlea is a grooved surface rather like a pulley, the medial edge projecting further distally and anteriorly than the lateral. This causes the ulna also to project laterally and results in a carrying angle between the humerus and ulna (see Fig. 2.79B).

On the medial side of the trochlea is the large medial epicondyle; its posterior surface is smooth with a shallow groove for the ulnar nerve. The sharp medial supracondylar ridge, comprising the lower third of the medial border, runs upwards onto the shaft. On the lateral side of the capitulum is the lateral epicondyle with the lateral supracondylar ridge, comprising the lower third of the lateral border, also running upwards onto the shaft.

Just above the articular surfaces, the lower end of the humerus presents three fossae for the bony processes of the radius and ulna. Posteriorly is the deep olecranon fossa, which on full extension of the elbow receives the olecranon process of the ulna. Anteriorly, there are two fossae, the lateral radial and medial coronoid fossae, which on full flexion of the elbow receive the head of the radius and coronoid process of the ulna respectively. Many of the bony features previously described can be seen in Figs. 2.7 and 2.9.

Figure 2.7 Radiograph of left shoulder, anteroposterior view with the arm laterally rotated and slightly abducted.

Palpation

At the upper end of the humerus the most lateral bony point at the shoulder is the greater tubercle, whose quadrilateral superior, anterior and posterior surfaces can be felt. Further differentiation can be made by palpating the lateral margin of the acromion (p. 41) and then running the fingers off its edge onto the greater tubercle. The rounded lesser tubercle can be felt through the deltoid, and is just lateral to the tip of the coracoid process. To the lateral side of the lesser tubercle the impression of the intertubercular sulcus can usually be felt. The shaft of the humerus is covered with thick muscle, but can be palpated on its medial and lateral sides. At the lower end, the prominent medial epicondyle is the most obvious bony landmark. The ulnar nerve can be rolled in the groove behind it (the ‘funny bone’). Running upwards from the medial epicondyle the sharp medial supracondylar ridge can be palpated. The lateral epicondyle can be palpated at the base of a dimple on the lateral aspect of the elbow, as can the lateral supracondylar ridge running upwards from it. Posteriorly, the olecranon fossa can be felt through the triceps tendon, if the relaxed elbow is flexed.

Ossification

A primary ossification centre appears in the shaft in the eighth week in utero and spreads until, at birth, only the ends are cartilaginous. Secondary centres appear in the head early in the first year, in the greater tubercle at about 3 years and in the lesser tubercle at about 5 years. These fuse to form a single cap of bone between the ages of 6 and 8 years, finally fusing with the shaft between 18 and 20 years in females and 20 and 22 years in males.

At the lower end of the humerus, secondary centres appear for the capitulum during the second year, for the trochlea between 9 and 10 years, and for the lateral epicondyle between 12 and 14 years. These join together at about 14 years, fusing with the shaft at 15 years in females and 18 years in males. A separate centre for the medial epicondyle appears between 6 and 8 years and fuses between 15 and 18 years with a spicule of bone projecting down from the shaft medial to the trochlea. This latter ossification centre lies entirely outside the joint capsule. Most of the growth in length of the humerus occurs at its upper end.

The bones of the forearm are the radius laterally and the ulna medially, which articulate proximally with the humerus at the elbow joint and contribute to the wrist joint distally. They are connected by a strong interosseous membrane between their shafts, and synovial pivot joints at each end. Both are long bones, the ulna being expanded proximally and the radius distally (Fig. 2.8A,B). The shaft of the radius is convex laterally, allowing it to move around the ulna carrying the hand with it in pronation of the forearm.

Figure 2.8 Right radius and ulna: (A) posterior view, (B) anterior view, (C) inferior view.

The radius

The radius lies lateral to the ulna and is the shorter of the two bones. It articulates proximally with the capitulum of the humerus, distally with the scaphoid and lunate bones of the proximal row of the carpus, and at each end with the ulna. It has a shaft and two ends, the inferior being the larger.

The head is a thick disc with a concave superior surface for articulation with the capitulum. The outer, articular surface of the head is flattened, articulating with a fibro-osseous ring formed by the radial notch of the ulna and the annular ligament. Below the head is the constricted neck, which slopes medially as it approaches the shaft. Where the shaft joins the neck it is round, but it becomes triangular lower down. Together with the neck, the shaft has a slight medial convexity in its upper quarter, with a lateral convexity in its remaining lower part. The radial tuberosity lies anteromedially on the upper part of the shaft at the maximum convexity of the medial curve. The majority of the shaft presents three borders (anterior, posterior, interosseous) and three surfaces (lateral, anterior, posterior).

The sharp interosseous border, to which the interosseous membrane attaches, faces medially. It extends from just below the radial tuberosity to the medial side of the lower end of the radius, splitting into two ridges which become continuous with the anterior and posterior margins of the ulnar notch. The anterior and posterior borders pass obliquely downwards and laterally from either side of the radial tuberosity to the roughened area for pronator teres lower down. The anterior border becomes distinct lower down, while the posterior border becomes more rounded. These borders enclose the lateral, anterior and flatter posterior surfaces.

The inferior end of the radius is expanded having five distinct surfaces. The lateral surface, which extends down to the styloid process, has a shallow groove anteriorly for the tendons of the abductor pollicis longus and extensor pollicis brevis. The medial surface forms the concave ulnar notch for articulation with the head of the ulna: it has a roughened triangular area superiorly. The posterior surface is convex and grooved by tendons. The prominent ridge in the middle of this surface is the dorsal (Lister’s) tubercle. The lateral half of this surface continues down onto the styloid process. The anterior surface is smooth and curves forward to a distinct anterior margin. The distal articular surface is concave and extends onto the styloid process; it is divided by a ridge into two areas, a lateral triangular area for articulation with the scaphoid and a medial quadrilateral area for the lunate (Fig. 2.8C).

Palpation

The head of the radius can be palpated in a ‘dimple’ on the posterolateral aspect of the elbow, particularly when the elbow joint is extended as it overhangs the capitulum; it can be felt rotating during pronation and supination. The shaft of the radius can be palpated on the lateral side in the lower half of the forearm. Distally, on the posterior aspect, the dorsal tubercle can be identified above the wrist, as can the styloid process laterally between the extensor tendons of the thumb within the ‘anatomical snuffbox’.

Ossification

A primary ossification centre appears in the shaft during the eighth week in utero, so that at birth only the head, inferior end and radial tuberosity are cartilaginous. The first secondary centre appears in the inferior end during the first year of life, fusing with the shaft between the ages of 20 and 22 years. The secondary centre for the head appears at about 6 years and fuses with the shaft between 15 and 17 years. A secondary centre usually appears in the radial tuberosity between 14 and 15 years, but soon fuses with the shaft.

The ulna

The ulna lies medial to the radius and is the longer of the two bones. It has a shaft and two ends, of which the superior is the larger presenting as a hook-like projection for articulation with the trochlea of the humerus. The smaller rounded distal end is the head of the ulna (Fig. 2.8B): it does not articulate directly with the carpus. The ulna articulates laterally at each end with the radius.

The large upper end of the ulna has two projecting processes, enclosing a concavity. The olecranon process is the larger of the two processes and forms the proximal part of the bone. It is beak-shaped and is directed forwards, being continuous inferiorly with the shaft. Posteriorly, it is smooth and subcutaneous while anteriorly it is concave, forming the upper part of the articular surface of the trochlear notch. The borders of the olecranon are thickened and rough.

The coronoid process projects from the front of the shaft and has an upper articular surface which completes the trochlear notch (Figs. 2.8B and 2.9). These two surfaces are often separated by a roughened non-articular area running horizontally across the notch. The trochlear notch is divided by a vertical ridge into a larger medial part and a smaller lateral part, the latter being continuous over its outer edge with the articular surface of the radial notch on the lateral side of the coronoid process. There is a small tubercle where the medial and anterior edges of the articular surface of the coronoid process meet; this gives attachment to the anterior part of the ulnar collateral ligament. The irregular, anterior surface of the coronoid ends inferiorly as the rough tuberosity of the ulna. Both this surface and the tuberosity give attachment to brachialis. At the upper medial part of the coronoid is the small sublime tubercle from which the pronator ridge runs downwards and laterally.