Bones of the Chest and Upper Limb and Muscles of the Back, Chest, and Shoulder

Learning Outcomes

Describe the major anatomical features of the bones of the pectoral girdle (specifically the clavicle and scapula) and the upper limb.
Describe the origins, insertions, and actions of the muscles of the back, chest wall, and shoulder region.
Describe the origins, insertions, and actions of the muscles of the arm.

Anatomy of the Upper Limb

The upper limb is divided into four distinct regions:
- Brachium [arm proper]: The region situated between the shoulder and elbow joints.
- Antebrachium [forearm]: The region extending from the elbow joint to the wrist joint.
- Wrist region.
- Hand [carpus].
The upper limb is connected to the trunk of the body by the pectoral girdle, which consists of the scapula and the clavicle.

Pectoral Girdle: The Scapula

The scapula is a flat bone located on the postero-lateral aspect of the thorax, overlying Ribs $2$ through $7$ .
Borders and Angles:
- Borders: Superior, medial, and lateral.
- Angles: Superior, inferior, and lateral.
Posterior Features:
- Spine: A prominent transverse ridge on the posterior surface.
- Supraspinous fossa: The area located above the spine.
- Infraspinous fossa: The broad surface area located below the spine.
Superior Features:
- Suprascapular notch: Located on the superior border; it provides passage for nerves and vessels.
Anterior (Ventral) Features:
- Subscapular fossa: The concave anterior surface of the scapula.
Lateral Angle Features:
- Acromion process: An extension of the spine that forms the apex of the shoulder. It articulates with the clavicle and serves as the point of attachment for the scapula and upper limb to the rest of the skeleton.
- Coracoid process: A projection on the ventral surface that serves as an attachment site for muscles.
- Glenoid cavity: A shallow socket that articulates with the head of the humerus to form the glenohumeral joint.
Clinical Considerations:
- The scapula is surrounded and protected by layers of muscles.
- Fractures are rare, accounting for less than $1\%$ of all bone fractures.
- Fractures most commonly occur in the body of the scapula and can result from direct trauma.

Pectoral Girdle: The Clavicle

The clavicle is an S-shaped long bone formed through intramembranous ossification.
Ossification and Position:
- It is the first bone to appear in the embryo (Week $5$ or $6$ intrauterine) but is the last bone to ossify.
- It extends horizontally across the root of the neck towards the shoulder.
- It is subcutaneous throughout its entire length.
Anatomy and Curvatures:
- Characterized by two curvatures: the medial $2/3$ is convex anteriorly, and the lateral $1/3$ is concave.
- The medial end is more rounded for articulation with the manubrium of the sternum.
- The lateral end is more flattened where it forms the acromioclavicular joint.
- The lower surface is roughened due to various ligament attachments.
Function:
- Acts as a prop or rigid strut between the sternum and the arm.
- Enables the limb to swing clear of the trunk.
Clinical Considerations:
- It is the most frequently fractured bone in the body.
- Fractures commonly occur at the junction of the lateral and intermediate thirds.
- In a fracture, the weight of the arm pulls the scapula down while the Sternocleidomastoid (SCM) muscle pulls the medial end of the clavicle upwards.

The Sternum

A bony plate located anterior to the heart and palpable (subcutaneous) in parts.
Regions:
- Manubrium.
- Body (gladiolus).
- Xiphoid process.
Key Features:
- Jugular notch.
- Manubriosternal junction or joint (also known as the sternal angle).
- Articular facets for Ribs $1$ through $7$ .

The Humerus

Proximal End:
- Hemispherical head: Articulates with the glenoid cavity of the scapula.
- Anatomical neck.
- Greater and lesser tubercles.
- Intertubercular sulcus: Contains the biceps tendon.
- Surgical neck: A common site for fractures.
- Deltoid tuberosity: Located on the shaft.
Distal End:
- Capitulum: Articulates with the head of the radius.
- Trochlea: Articulates with the ulna.
- Lateral and medial epicondyles.
- Olecranon fossa: Receives the olecranon process of the ulna.
- Coronoid fossa.
- Radial fossa.
Clinical Considerations:
- Humeral fractures are not uncommon and can occur at any site.
- Causes include falls on outstretched hands, direct trauma, epiphyseal separation in adolescents, osteoporosis, and metastases.
- Nerve damage may occur, involving the axillary nerve, musculocutaneous nerve, radial nerve, or ulnar nerve.

The Forearm (Antebrachium)

Consists of two long bones: the ulna (medial) and the radius (lateral).
Interosseous membrane: A fibrous sheet that connects the lateral margin of the ulna to the radius.
The Ulna (Medial Bone):
- Olecranon: Located at the proximal end; forms the point of the elbow. It fits into the olecranon fossa of the humerus when the elbow straightens (extension). It also forms the superior lip of the trochlear notch.
- Trochlear notch: A U-shaped notch that articulates with the trochlea of the humerus.
- Coronoid process: Forms the inferior lip of the trochlear notch; fits into the coronoid fossa of the humerus when the elbow bends (flexion).
- Radial notch: A flat notch lateral to the coronoid process; articulates with the head of the radius to form the proximal radio-ulnar joint.
- Head of ulna (ulnar head): Located at the distal end; articulates with the radius to form the distal radio-ulnar joint.
- Ulnar styloid process: A short process pointing inferiorly; attaches to the articular disc (cartilage) that separates the ulnar head from the wrist bones.
The Radius (Lateral Bone):
- Head: Disc-shaped proximal end; articulates with the capitulum of the humerus and fits into the radial fossa during flexion.
- Neck: The area between the head and the radial tuberosity.
- Radial tuberosity: A roughened area below the neck where the biceps brachii muscle attaches.
- Ulnar notch: Located on the medial surface of the distal end; articulates with the head of the ulna.
- Radial styloid process: Located on the distal lateral surface to stabilize the wrist joint.

The Wrist and Hand

The Wrist (Carpal Bones):
- Consists of eight carpal bones arranged in two rows.
- Four proximal carpal bones: Scaphoid, Lunate, Triquetrum, Pisiform.
- Four distal carpal bones: Trapezium, Trapezoid, Capitate, Hamate.
- These allow the wrist to bend and twist.
- Carpal Tunnel Syndrome: Inflammation of connective tissues between the flexor retinaculum and carpal bones; compresses the median nerve, causing pain, weakness, and reduced mobility.
The Hand (Metacarpals and Phalanges):
- Metacarpals: Five long bones numbered $I$ through $V$ from lateral to medial. They articulate with distal carpals and proximal phalanges.
- Phalanges: Finger bones.
  - Pollex (thumb): Contains two phalanges (proximal and distal).
  - Other four fingers: Contain three phalanges each (proximal, middle, and distal).

Muscle Fundamentals

Attachments:
- Origin: The stationary attachment point.
- Insertion: The movable attachment point.
- Muscles can attach to bone, fascia, tendons of other muscles, or skin.
- Rule: Any muscle that crosses a joint will act on that joint.
Functional Groups (Muscle Action):
- Action: The effect produced by a muscle to create or prevent movement.
- Prime mover: The muscle producing most of the force during a specific joint action.
- Synergist: A muscle that aids the prime mover.
- Antagonist: A muscle that opposes the prime mover; it prevents excessive movement and can relax to give the prime mover control.
- Antagonistic pairs: Muscles that act on opposite sides of a joint.
- Fixator: A muscle that prevents the movement of a bone.

Muscles of the Neck and Back

Muscles Acting on the Head:
- Attach to the vertebral column, thoracic cage, pectoral girdle, and skull.
- Actions: Flexion (forward bending), extension (holding head erect), lateral flexion (moving to one side), and rotation.
Axial Muscles (Vertebral Column):
- Erector spinae (Superficial layer): Composed of the Spinalis group, Longissimus group, and Iliocostalis group.
- Erector spinae (Deep layer): Composed of the Semispinalis group, Multifidus, Interspinales, Intertransversarii, and Rotatores.
Spinal Flexors:
- Neck region: Longus capitis and longus colli (rotate and flex the neck).
- Lumbar region: Quadratus lumborum (flexes the vertebral column and depresses the ribs).

Muscles of the Chest and Upper Limb

Pectoralis major:
- Origins: Sternum, upper ribs, and clavicle.
- Insertion: Anterior aspect of the humerus.
- Actions: Flexion, adduction, and medial rotation of the arm. Used in climbing to bring the trunk upwards. Sternocostal fibers extend the arm from a flexed position.
Pectoralis minor:
- Origins: Ribs $3$ through $5$ .
- Insertion: Coracoid process of the scapula.
- Actions: Depresses the shoulder; acts as an accessory muscle of respiration.
Serratus Anterior:
- Origins: Lateral aspect of the upper $8$ ribs.
- Insertion: Medial border of the scapula.
- Actions: Protraction and rotation of the scapula.
- Clinical: Damage can lead to a "winged" scapula, where the medial border and inferior angle move outward while the arm is raised.
Trapezius (Back Muscle):
- Origins: Nuchal lines, ligamentum nuchae, cervical spines, and upper $6$ thoracic spines.
- Insertions: Lateral end of the clavicle, acromion process, and spine of the scapula.
- Actions: Neck extension (if muscle is fixed), elevation and rotation of the scapula, keeping shoulders braced (tonus), and suspending the upper limb.
- Clinical: Nerve damage results in drooping of the shoulders.
Latissimus dorsi:
- Origins: $T_{6-12}$ spines, lower $3$ to $4$ ribs, iliac crest, thoracolumbar fascia (lumbar/sacral spines), and inferior angle of scapula.
- Insertion: Anterior humerus (intertubercular sulcus).
- Actions: Adduction, extension, and medial rotation of the arm. Raises the body during climbing.
Deep Muscles of the Back:
- Levator scapulae:
  - Origins: Transverse processes of spines $C_{1}$ to $C_{4}$ .
  - Insertion: Upper medial border of the scapula (above the spine).
  - Actions: Elevation of the scapula and tilting the glenoid inferiorly.
- Rhomboid minor:
  - Origins: Spinous processes of $C_{7}$ , $T_{1}$ , and $T_{2}$ .
  - Insertion: Medial border of the scapula (opposite the spine).
  - Actions: Elevation and retraction of the scapula.
- Rhomboid major:
  - Origins: Spinous processes of $T_{2}$ through $T_{5}$ .
  - Insertion: Medial border of the scapula (below the spine).
  - Actions: Retraction and elevation of the scapula.

Muscles of the Shoulder Region

Deltoid:
- Origins:
  - Anterior fibers: Anterior border and upper surface of the clavicle.
  - Middle fibers: Lateral aspect of the acromion process.
  - Posterior fibers: Lower lip and posterior border of the spine of the scapula.
- Insertion: Deltoid tuberosity of the humerus.
- Actions:
  - All fibers: Abduction.
  - Anterior fibers: Flexion of the arm.
  - Middle fibers: Abduction of the arm.
  - Posterior fibers: Extension and hyperextension of the flexed arm.
  - Supports the shoulder joint by preventing dislocation of the humerus when carrying heavy weights.
Rotator Cuff Group:
- Consists of four muscles that form a 'cuff' around the shoulder joint; their tendons fuse with the joint capsule to provide stability to the glenohumeral joint and hold the humeral head in the glenoid cavity.
- Supraspinatus:
  - Origin: Supraspinous fossa.
  - Insertion: Upper facet on the greater tubercle of the humerus.
  - Action: Abduction of the humerus.
- Infraspinatus:
  - Origin: Infraspinous fossa.
  - Insertion: Middle facet on the greater tubercle of the humerus.
  - Action: Lateral rotation of the humerus.
- Teres minor:
  - Origin: Lateral border of the scapula.
  - Insertion: Lower facet on the greater tubercle of the humerus.
  - Action: Lateral rotation of the humerus.
- Subscapularis:
  - Origin: Costal surface of the scapula.
  - Insertion: Lesser tubercle of the humerus.
  - Action: Medial rotation of the humerus.

Other Shoulder Muscles

Teres major:
- Origin: Dorsal surface of the inferior angle/lateral border of the scapula.
- Insertion: Medial lip of the intertubercular sulcus.
- Action: Medial rotation of the humerus.