Muscles of the Thorax

Overview
The muscles of the thorax are vital for breathing, maintaining posture, and safeguarding the organs within the chest (the thoracic organs). This section provides an introduction to the principal muscle groups that drive ventilation and support the structure of the thoracic wall.

Key Structures
This section covers the muscles forming the thoracic cage, which enable rib movement during respiration, as well as the diaphragm, the main muscle involved in breathing. Each topic outlines muscle attachments, innervation, and functional roles.

Clinical Relevance and Learning Focus
A clear understanding of thoracic muscle anatomy is important when assessing respiratory impairment, chest wall discomfort, and disorders of the diaphragm. This knowledge underpins effective clinical examination and accurate interpretation of imaging findings.

The Diaphragm

Contents

The diaphragm is a musculotendinous sheet with a double-domed shape, positioned at the lowest part of the rib cage. It performs two key roles:

• It separates the thoracic cavity from the abdominal cavity (the term diaphragm comes from the Greek diáphragma, meaning a partition).
• Through rhythmic contraction and relaxation, it alters the volume of the thoracic cavity and lungs, enabling inspiration and expiration.

This article examines the anatomy of the diaphragm, including its attachments, functions, and associated nerves and blood vessels.

Anatomical Position and Attachments

The diaphragm occupies the inferior boundary of the thoracic cage, closing the inferior thoracic aperture. It forms the floor of the thoracic cavity and the roof of the abdominal cavity. Its attachments are divided into peripheral and central components, with three main peripheral attachment sites:

• Lumbar vertebrae and arcuate ligaments
• Costal cartilages of ribs 7–10, with direct attachment to ribs 11–12
• Xiphoid process of the sternum

The portions of the diaphragm that attach to the vertebral column are tendinous and are referred to as the crura:

• Right crus – arises from vertebral levels L1–L3 and their intervertebral discs. Some fibres encircle the oesophageal opening, acting as a functional sphincter to reduce reflux of gastric contents into the oesophagus.
• Left crus – arises from L1–L2 and their intervertebral discs.

Muscle fibres from the diaphragm converge centrally to form the central tendon, which ascends and fuses with the inferior surface of the fibrous pericardium. On either side of the pericardium, the diaphragm rises to create the left and right domes. In the resting position, the right dome typically lies slightly higher than the left, largely due to the position of the liver beneath it.

Fig 1 – Diagram showing the left and right domes of the diaphragm and the vertebral attachments via the crura.

Pathways Through the Diaphragm

As the diaphragm separates the thoracic and abdominal cavities, any structure passing between these regions must traverse it. There are three major openings that serve as passageways:

A useful mnemonic for remembering these levels is based on letter count: vena cava has eight letters (T8), oesophagus has ten letters (T10), and aortic hiatus has twelve letters (T12).

Fig 2 – Inferior view of the diaphragm highlighting its three major openings.

Actions

The diaphragm is the principal muscle responsible for respiration. During inspiration, it contracts and flattens, increasing the vertical dimension of the thoracic cavity. This expansion lowers intrathoracic pressure, allowing the lungs to inflate and air to enter.

During expiration, the diaphragm relaxes and returns to its domed shape, reducing thoracic volume and facilitating the passive expulsion of air from the lungs.

Innervation and Vasculature

Each half of the diaphragm is supplied by its corresponding phrenic nerve. The left hemidiaphragm is innervated by the left phrenic nerve, and the right by the right phrenic nerve. These nerves arise from the cervical plexus in the neck and contain fibres from spinal nerve roots C3–C5.

Arterial blood supply to the diaphragm is provided primarily by the inferior phrenic arteries, which arise directly from the abdominal aorta. Additional contributions come from the superior phrenic, pericardiacophrenic, and musculophrenic arteries. Venous drainage generally mirrors the arterial supply.

Fig 3 – Course of the phrenic nerves supplying the diaphragm.

Clinical Relevance

Paralysis of the Diaphragm

Diaphragmatic paralysis results from disruption of its nerve supply, which may occur at the level of the phrenic nerve, cervical spinal cord, or brainstem. The most common cause is injury to the phrenic nerve, which may result from:

• Mechanical trauma: accidental ligation or damage during surgery
• Compression: from intrathoracic tumours
• Myopathies: such as myasthenia gravis
• Neuropathies: including diabetic neuropathy

Paralysis leads to paradoxical movement of the affected hemidiaphragm: it ascends during inspiration and descends during expiration. Unilateral paralysis is often asymptomatic and may be detected incidentally on chest radiography. Bilateral paralysis, however, can cause reduced exercise tolerance, orthopnoea, and fatigue, with lung function tests demonstrating a restrictive pattern.

Management focuses first on identifying and treating the underlying cause. Symptomatic treatment may involve non-invasive ventilatory support, most commonly using continuous positive airway pressure (CPAP).

Fig 4 – Chest X-ray demonstrating paralysis of the right hemidiaphragm.

The Muscles of the Thoracic Cage

Contents

The thoracic wall forms the front, back, and sides of the thorax. It is made up of five muscle groups that work together to alter the volume of the thoracic cavity during breathing:

• External, internal, and innermost intercostal muscles
• Subcostal muscles
• Transversus thoracis

In addition, several muscles attach to the thoracic wall without forming part of it. These are known as accessory muscles of respiration and include the pectoralis major, pectoralis minor, serratus anterior, and scalene muscles.

This article reviews the anatomy of the thoracic wall muscles, focusing on their attachments, actions, and nerve supply.

Intercostal Muscles

The intercostal muscles occupy the spaces between adjacent ribs and are arranged in three distinct layers.

External Intercostals

There are eleven pairs of external intercostal muscles. Their fibres run downward and forward from the rib above to the rib below and are continuous with the external oblique muscle of the abdominal wall.

• Attachments: Originate from the lower border of each rib and insert onto the upper border of the rib immediately below.
• Actions: Stabilise the ribs during normal breathing and elevate them during forced inspiration, increasing thoracic volume.
• Innervation: Intercostal nerves (T1–T11).

Fig 1 – External and internal intercostal muscles of the thoracic wall.

Internal Intercostals

The internal intercostal muscles lie deep to the external intercostals. Their fibres run in the opposite direction, passing downward and backward between adjacent ribs. Inferiorly, they are continuous with the internal oblique muscle of the abdominal wall.

• Attachments: Originate from the lateral margin of the costal groove and insert onto the superior surface of the rib below.
• Actions: Stabilise the ribs during quiet respiration and depress them during forced expiration, reducing thoracic volume.
• Innervation: Intercostal nerves (T1–T11).

Innermost Intercostals

The innermost intercostals form the deepest layer of the intercostal muscles and closely resemble the internal intercostals in structure and function.

They are separated from the internal intercostals by the intercostal neurovascular bundle and are most prominent in the lateral portions of the intercostal spaces.

• Attachments: Originate from the medial edge of the costal groove and insert onto the superior surface of the rib below.
• Actions: Assist in rib stabilisation during breathing and depress the ribs during forced expiration.
• Innervation: Intercostal nerves (T1–T11).

Transversus Thoracis

The transversus thoracis muscle lies on the inner surface of the anterior thoracic wall and is continuous inferiorly with the transversus abdominis muscle.

• Attachments: Arises from the posterior surface of the lower sternum and adjacent costal cartilages. Its fibres ascend to attach to the internal surfaces of costal cartilages 2–6.
• Actions: Acts as a weak depressor of the ribs during expiration.
• Innervation: Intercostal nerves (T2–T6).

Fig 2 – Internal view of the thoracic wall showing the internal intercostal and transversus thoracis muscles.

Subcostal Muscles

The subcostal muscles are most prominent in the lower regions of the thoracic wall. Their fibre orientation is similar to that of the innermost intercostal muscles.

• Attachments: Originate from the inner surface of the lower ribs and insert onto the upper border of the second or third rib below.
• Actions: Stabilise the ribs during normal respiration and depress them during forced expiration, decreasing thoracic volume.
• Innervation: Intercostal nerves.