Intercostal Muscles

The intercostal spaces between the ribs are occupied by intercostal muscles attaching to the ribs above and below each space. The muscles here are arranged in three layers, but in practice, only the two outer layers are functionally significant.

1. External Intercostal Muscles:
   This forms the outer layer and their fibres run forwards and downwards (inferoanteriorly). Each muscle extends anteriorly from the tubercle of the rib as far forward as the costochondral junction where it is replaced by a fibrous aponeurosis.

2. Internal Intercostal Muscles:
   This forms the middle layer and their fibres run backwards and downwards (inferoposteriorly). These muscles extend from the sternum anteriorly to the angles of the ribs where they are replaced by aponeurotic tissue.

   The fibres of external and internal intercostal muscles run at right angles to each other.

3. Innermost Intercostal Muscles:
   This forms the inner most layer of muscles and it is usually incomplete. Muscle is only present in the lower intercostal spaces and in the middle part of the ribs. Their fibres are orientated in the same direction as the internal intercostal muscles and function with them.

The intercostal muscles play a major role in ventilation. Both external and internal intercostal muscles elevate the ribs, but the internal intercostal muscles can also depress the ribs, depending on the action of other fixator muscles.

Intercostal Neurovascular Bundles

The main vascular and nervous supply to the tissues of the intercostal space and adjacent tissues runs as a neurovascular bundle just below the costal groove of the ribs forming the upper boundary of each intercostal space.

The nerves and blood vessels are protected by the sharp inferior margin of the rib. The bundle runs deep to the internal intercostal muscles.

The intercostal arteries supply the skin and subcutaneous tissues over the side of the chest wall and give branches to the intercostal muscles and underlying pleura. Perforating branches pierce the chest wall to supply the pectoral muscles and breast. In the lactating female, these vessels may be of considerable size. Corresponding veins run with the arteries.

The intercostal nerves are the ventral primary rami of the upper 11 thoracic spinal nerves. Each pair of spinal nerves emerges from the intervertebral foramina formed between two adjacent vertebrae. On each side, the nerve immediately splits into two branches, the ventral and the dorsal primary rami. The dorsal primary rami supply the muscles acting on the vertebral column and the skin immediately overlying the vertebrae posteriorly. The ventral primary rami pass anteriorly to form the intercostal nerves in the intercostal spaces running with the intercostal vessels. A lateral cutaneous branch emerging near the mid-axillary line supplies sensory innervation to the skin over the side of the chest and an anterior cutaneous branch near the edge of the sternum supplies the skin on the front of the chest. Several motor branches supply the intercostal muscles. There are also pleural branches carrying sensory information from the parietal pleura.

The lower intercostal spaces are much shorter than those between the true ribs above. However, the intercostal nerves do not stop where the intercostal space is closed by the costal cartilages, but carry on across the abdominal wall to supply it. The lower six pairs of intercostal nerves carry motor axons to the abdominal muscles and sensory processes from the skin of the abdominal wall and peritoneum lining the abdominal cavity.