Muscles of the Thorax: Anatomy, Attachments, and Functions

Muscles of the Thorax

The thoracic region contains several important muscle groups that play key roles in respiration and movement of the upper limb. This section covers the main thoracic muscles, their origins, insertions, innervation, and functions.

Pectoral Muscles

The pectoral muscles are located on the anterior thoracic wall and are involved in movements of the shoulder and upper limb, as well as assisting in respiration.

• Pectoralis Major (musculus pectoralis major): A large, fan-shaped muscle with three parts (clavicular, sternocostal, and abdominal).

  • Origin: Medial two-thirds of the clavicle, anterior surface of the sternum, and costal cartilages of ribs II-VII.

  • Insertion: Crest of the greater tubercle of the humerus.

  • Function: Adducts and medially rotates the arm; assists in inspiration when the arms are fixed.

  • Example: Pushing movements, such as during a push-up.

• Pectoralis Minor (musculus pectoralis minor): A thin, triangular muscle beneath the pectoralis major.

  • Origin: Ribs III-V near their costal cartilages.

  • Insertion: Coracoid process of the scapula.

  • Function: Draws the scapula forward and downward; assists in forced inspiration.

• Subclavius (musculus subclavius): A small muscle beneath the clavicle.

  • Origin: First rib and its cartilage.

  • Insertion: Inferior surface of the clavicle.

  • Function: Stabilizes and depresses the clavicle; elevates the first rib during forced inspiration.

Sternal Muscles

• Sternalis (musculus sternalis): An inconstant muscle, present in some individuals, running parallel to the sternum.

  • Origin: Manubrium of the sternum.

  • Insertion: Fascia over the pectoralis major.

  • Function: Unclear; may assist in movement of the chest wall.

  • Additional info: Not present in all individuals.

Intercostal Muscles

The intercostal muscles are located between the ribs and are essential for the mechanics of breathing.

• External Intercostal Muscles (musculi intercostales externi)

  • Origin & Insertion: Extend from the lower border of one rib to the upper border of the rib below, from the tubercle of the rib posteriorly to the costal cartilage anteriorly.

  • Function: Elevate the ribs during inspiration.

• Internal Intercostal Muscles (musculi intercostales interni)

  • Origin & Insertion: Deep to the external intercostals, running at right angles, from the sternum to the angle of the ribs.

  • Function: Depress the ribs during forced expiration.

• Innermost Intercostal Muscles (musculi intercostales intimi)

  • Location: Deepest layer, separated from internal intercostals by intercostal nerves and vessels.

  • Function: Assist internal intercostals in depressing the ribs.

Other Thoracic Muscles

• Subcostal Muscles (musculi subcostales)

  • Location: Inner surface of the lower ribs, spanning one or more intercostal spaces.

  • Function: Depress the ribs; assist in forced expiration.

• Transversus Thoracis (musculus transversus thoracis)

  • Origin: Posterior surface of the lower sternum and xiphoid process.

  • Insertion: Costal cartilages of ribs II-VI.

  • Function: Depresses the ribs; assists in expiration.

• Levatores Costarum (musculi levatores costarum)

  • Origin: Transverse processes of C7-T11 vertebrae.

  • Insertion: Rib below the vertebra of origin.

  • Function: Elevate the ribs; assist in inspiration.

Diaphragm

The diaphragm is the primary muscle of respiration, separating the thoracic and abdominal cavities.

• Origin: Lumbar vertebrae (via crura), lower six ribs (costal part), and xiphoid process (sternal part).

• Insertion: Central tendon of the diaphragm.

• Openings: For the aorta (aortic hiatus), esophagus (esophageal hiatus), and inferior vena cava (caval opening).

• Function: Contracts to increase thoracic volume and decrease intrathoracic pressure, drawing air into the lungs.

• Additional functions: Assists in increasing intra-abdominal pressure for actions such as defecation, urination, and childbirth.

Summary Table: Thoracic Muscles and Their Functions

Muscle	Origin	Insertion	Function
Pectoralis Major	Clavicle, sternum, ribs II-VII	Humerus (greater tubercle)	Adducts, medially rotates arm; assists inspiration
Pectoralis Minor	Ribs III-V	Coracoid process of scapula	Draws scapula forward/down; assists inspiration
Subclavius	First rib	Clavicle (inferior surface)	Stabilizes clavicle; elevates first rib
External Intercostals	Lower border of rib	Upper border of rib below	Elevate ribs (inspiration)
Internal Intercostals	Lower border of rib	Upper border of rib below	Depress ribs (expiration)
Innermost Intercostals	Lower border of rib	Upper border of rib below	Assist expiration
Subcostals	Inner surface of lower ribs	Rib above	Depress ribs
Transversus Thoracis	Sternum, xiphoid	Costal cartilages II-VI	Depress ribs
Levatores Costarum	Transverse processes C7-T11	Rib below	Elevate ribs
Diaphragm	Lumbar vertebrae, ribs, xiphoid	Central tendon	Main muscle of inspiration

Key Terms and Definitions

• Inspiration: The process of drawing air into the lungs, primarily by increasing thoracic volume.

• Expiration: The process of expelling air from the lungs, primarily by decreasing thoracic volume.

• Origin: The fixed attachment point of a muscle.

• Insertion: The movable attachment point of a muscle.

• Aponeurosis: A flat, broad tendon.

Formulas and Equations

• Boyle's Law (relevant to respiration):

$P_1 V_1 = P_2 V_2$

This law explains how increasing thoracic volume (by contraction of the diaphragm and external intercostals) decreases intrathoracic pressure, allowing air to flow into the lungs.