Introduction,Thoracic Wall Proper,Intercostal Muscles

Topic Overview

Introduction

• The thorax is covered externally by the muscles of the pectoral region and internally by intercostal muscles and membranes that fill the gaps between adjacent ribs and cartilages.

• These muscles give rigidity and protection to the thoracic wall.

• Each intercostal space has one right and one left thoracic nerve—together they form true dermatomes of the thoracic region.

• The posterior intercostal vein, artery, and nerve (V-A-N) lie in the costal groove, arranged from above downward.

• The sympathetic trunk originates from the lateral horns of the T1–T12 spinal cord segments and continues down to the L2 level.

Coverings of the Thoracic Wall (superficial to deep):

1. Skin

2. Superficial fascia

3. Deep fascia

4. Extrinsic muscles (derived from upper limb, back, and abdomen)

Extrinsic muscles covering the thorax include:

• Upper limb group: Pectoralis major, pectoralis minor, serratus anterior, trapezius, latissimus dorsi, levator scapulae, rhomboid major, rhomboid minor, serratus posterior superior, serratus posterior inferior.

• Abdominal group: Rectus abdominis, external oblique.

• Back group: Erector spinae (sacrospinalis).

• Additional muscles from the head, neck, and abdomen attach to the margins of the thoracic apertures.

Thoracic Wall Proper

• The thoracic cage forms the skeletal framework of the thoracic wall.

• The spaces between ribs are called intercostal spaces—filled by intercostal muscles, vessels, nerves, and lymphatics.

• There are 11 intercostal spaces posteriorly and 9 anteriorly (the lower two ribs—11th and 12th—are floating).

• The upper ten spaces correspond to true and false ribs, forming the muscular part of the wall.

Intercostal Muscles

Each intercostal space contains three layers of muscles:

1. External intercostal muscles

   • Direction: Downward and forward (“hands-in-pockets” direction).

   • Extent: From the rib tubercle posteriorly to the costochondral junction anteriorly, where replaced by external intercostal membrane.

   • Function: Elevate ribs → aid inspiration.

2. Internal intercostal muscles

   • Direction: Downward and backward, opposite to external intercostals.

   • Extent: From sternum anteriorly to angle of rib posteriorly, replaced by posterior intercostal membrane.

   • Function: Depress ribs → aid expiration.

3. Innermost intercostal muscles (deep layer)

   • Incomplete and best seen in lower intercostal spaces.

   • Includes:

     • Innermost intercostal (lateral portion)

     • Subcostal muscles (posteriorly, crossing more than one space)

     • Transversus thoracis (anteriorly, on deep surface of sternum and costal cartilages).

   • Function: Assist internal intercostals in expiration.

Nerve Supply:

• All intercostal muscles are supplied by intercostal nerves (T1–T11).

Blood Supply:

• From posterior and anterior intercostal arteries.

Actions Summary:

• External intercostals: Inspiration (raise ribs).

• Internal and innermost intercostals: Expiration (depress ribs).