Muscles of the Pectoral, Back and Shoulder Regions

Joints of the upper limb (2) | Main Anatomy Index | Muscles of the arm

Muscles of the Pectoral, Back and Shoulder Regions

The Pectoral Muscles

• The pectoral region contains four muscles, all of which are attached to the pectoral girdle and are associated with movements of the pectoral girdle and upper limb.

The Pectoralis Major Muscle (p. 507)

• This large, thick, fan-shaped muscle covers the superior part of the thorax.
• Its lateral border forms the anterior axillary fold and most of the anterior wall of the axilla.

• The fascial sheath enclosing the pectoralis major muscle is attached at its origin to the clavicle and sternum.
• It leaves the lateral border of this muscle to form the axillary fascia in the floor of the axilla.

• The pectoralis major and the deltoid muscles diverge slightly from each other superiorly and, and along with the clavicle, form the deltopectoral triangle (infraclavicular fossa).

• The cephalic vein, one of the two major superficial veins of the upper limb, occupies the furrow between the deltoid and pectoris major muscles before it enters the deltopectoral triangle.

• Proximal attachments are: clavicular head-anterior surface of the medial half of clavicle; sternocostal head-anterior surface of sternum, superior six costal cartilages, and aponeurosis of the external oblique muscle.
• Distal attachments are: lateral lip of intertubercular groove of humerus.
• Innervation: lateral and medial pectoral nerves; clavicular head (C5 and C6), sternoclavicular head (C7, C8, and T1)

• When both parts of the muscle act together, the pectoralis major adducts and medially rotates the humerus at the shoulder joint.
• Acting alone, the clavicular head helps to flex the humerus and from this position the sternocostal head extends the humerus. When the arm is flexed, the sternocostal head raises the ribs during forced inspiration.

Back to top

The Pectoralis Minor Muscle (p. 507)

• This triangular muscle lies in the anterior wall of the axilla, where it is largely covered by the much larger pectoralis major.
• The pectoralis minor is the landmark of the axilla.
• Along with the coracoid process of the scapula, it forms an arch deep to which pass the vessels and nerves to the upper limb.

• The pectoralis minor is surrounded by clavipectoral fascia, a thin sheet of fibrous tissue that runs from the clavicle superiorly to the axillary fascia inferiorly.

• Proximal attachments are: ribs 3 to 5 near their costal cartilages.
• Distal attachments are: medial border and superior surface of coracoid process of scapula.
• Innervation: medial pectoral nerve (C8 and T1)

Back to top

The Serratus Anterior Muscle (pp. 507, 510)

• This is a large, foliate muscle that overlies the lateral portion of the thorax and the intercostal muscles.
• It was given its name (L. serratus, a saw) because the saw-toothed appearance of the fleshy digitations at its proximal attachments.

• Proximal attachments are: external surface of lateral parts of ribs 1 to 8.
• Distal attachments are: anterior surface of medial border of scapula.
• Innervation: long thoracic nerve (C5, C6, and C7).

• The serratus anterior protracts the scapula and holds or fixes it against the thoracic wall.
• Because it is active during punching, it has been called "the boxer's muscle".
• By fixing the scapula to the thorax, it acts as an anchor for this bone and permits other muscles to use it as a fixed bone to produce movements of the humerus. Inferior fibres of the serratus anterior help to raise the glenoid fossa of the scapula.

Injuries involving the Serratus Anterior Muscle

• When a person's serratus anterior muscle is paralysed owing to injury to the long thoracic nerve, the medial border of the scapula stands out, especially its inferior angle, giving it the appearance of a wing when the person presses anteriorly.
• Consequently this condition is called a "wing scapula". When the arm is raised, the scapula is pulled away from the thoracic wall.
• In addition the arm cannot be abducted farther than the horizontal position because the serratus anterior is unable to rotate the scapula and raise the glenoid fossa.

Back to top

The Back and Shoulder Region

Click here for the Intrinsic Muscles of the Back.

• The following back muscles described are the extrinsic back muscles, which attach the upper limb to the axial skeleton. The muscles of the shoulder are divided into three groups: (1) the superficial extrinsic muscles (trapezius and latissimus dorsi); (2) the deep extrinsic muscles (levator scapulae, rhomboids, and serratus anterior); and (3) the intrinsic muscles (deltoid, supraspinous, infraspinous, teres minor, teres major, and subscapularis).

Extrinsic Muscles of the Back

• These muscles are supplied by the ventral rami of cervical nerves, not by dorsal rami.
• The explanation for this is that the superficial back muscles develop in the embryo as a ventrolateral sheet that migrates posteriorly to gain attachment to the vertebral column.

The Trapezius Muscle (p. 530)

• This is a large, flat, triangular muscle that covers the posterior aspect of the neck and superior half of the trunk.
• It was given its name because the muscles of the two sides form a trapezion (G. irregular four-sided figure).

• The trapezius muscle attaches the pectoral girdle to the skull and the vertebral column and assists in suspending it.

• Medial attachments are: medial third of superior nuchal line; external occipital protuberance, ligamentum nuchae, spinous processes of C7 to C12 vertebrae.
• Lateral attachments are: lateral third of clavicle, acromion and spine of scapula.
• Innervation: spinal root of accessory nerve (CN XI) and cervical nn. (C3 and C4).

• The superior fibres of the trapezius elevate the scapula; its middle fibres retract the scapula, and its inferior fibres depress the scapula and lower the shoulder.
• The superior and inferior fibres act together in the superior rotation of the scapula.
• The trapezius muscles brace the shoulders by pulling the scapulae posteriorly, hence weakness of these muscles result in drooping of the shoulders.

Back to top

The Latissimus Dorsi Muscle (pp. 531, 533)

• The name of this muscle is derived from the Latin meaning "widest of the back".
• This is large, wide, fan-shaped muscle passes between the trunk and the humerus and acts on the shoulder joint and indirectly on the pectoral girdle.

• Medial attachments are: spinous processes of the inferior six thoracic vertebrae, thoracolumbar fascia, iliac crest, and inferior 3 or 4 ribs.
• Lateral attachments are: floor of intertubercular groove of humerus.
• Innervation: thoracodorsal n. (C6, C7, and C8).

• The latissimus dorsi extends, adducts, and medially rotates the humerus at the shoulder joint.
• When climbing, these muscles raise the trunk. In conjunction with the pectoralis major muscle, the latissimus dorsi muscle raises the trunk to the arm, which occurs when performing chip-ups.

Back to top

The Levator Scapulae Muscle (p. 533)

• The superior third of this strap-like muscle lies deep to the sternocleidomastoid muscle; the inferior third is deep to the trapezius.

• Medial attachments are: posterior tubercles of transverse processes of C1 or C4 vertebrae.
• Lateral attachments are: superior part of medial border of scapula.
• Innervation: dorsal scapular (C5) and cervical (C3 and C4) nn.

• The levator scapulae elevates the scapula and helps to tilt its glenoid fossa inferiorly by rotating the scapula.
• It also helps to retract the scapula and fix it against the trunk and to flex the neck.

Back to top

The Scapular Muscles

• Six short muscles pass from the scapula to the humerus and act on the shoulder joint.

The Deltoid Muscle (p. 533)

• This thick, powerful shoulder muscle covers the shoulder joint and forms the rounded contour of the shoulder.
• As its name "deltoid" indicates, it is triangular in outline, i.e., it is shaped like an inverted Greek letter delta.

• Proximal attachments are: lateral third of clavicle, acromion and spine of scapula.
• Distal attachments are: deltoid tuberosity of humerus.
• Innervation: axillary nerve (C5 and C6)

• For descriptive purposes, the deltoid may be divided into three parts: anterior, middle, and posterior. It is capable of acting in part or as a whole.
• The anterior part is a strong flexor and medial rotator of the humerus.
• The middle part is the chief abductor of the humerus.
• The posterior part is a strong extensor and lateral rotator of the humerus.

• In performing these movements, the deltoid works with other muscles, e.g., the anterior part acts with the pectoralis major and coracobrachialis muscles in flexing the arm.

• The deltoid muscle tends to stabilise the shoulder joint and helps to hold the head of the humerus in the glenoid fossa of the scapula during arm movements.

Injuries involving the deltoid muscle

• The deltoid muscles atrophies when the axillary nerve is severely damaged (e.g., during a fracture of the surgical neck of the humerus).
• As the deltoid atrophies, the rounded contour of the shoulder disappears; this gives it a flattened appearance.
• To test the strength of the deltoid clinically, the patient's arm is abducted, and then the patient is asked to hold it in that position against resistance. Inability to do this indicates injury to the axillary nerve.

Back to top

The Teres Major Muscle (p. 533)

• The teres (L. round) major is a somewhat flattened rectangular muscle that forms a raised oval area on the dorsum of the scapula, beginning at the inferior angle.

• The inferior border of this rounded muscle forms the inferior border of the posterior wall of the axilla.
• The teres major and the tendons of the latissimus dorsi form the posterior axillary fold.

• Proximal attachments are: dorsal surface of inferior angle of scapula.
• Distal attachments are: medial lip of intertubercular groove of humerus.
• Innervation: lower subscapular nerve (C6 and C7)

• The teres major muscle adducts and medially rotates the arm. It can also help to extend it from the flexed position.
• It is furthermore an important stabiliser of the head of the humerus in the glenoid fossa during the abduction of the arm.

Back to top

Rotator Cuff Muscles

• Click here for the muscles of the rotator cuff or click for the individual muscles: supraspinatus, subscapularis, teres minor and infraspinatus.

Back to top

Get your free homepage with GeoCities!