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The muscles that cover the ribs are called intercostal muscles. They are a group of intrinsic rib cage muscles that occupy the 11 intercostal spaces between the ribs. There are two types of intercostal muscles: the internal intercostal muscles and the external intercostal muscles. The internal intercostal muscles are responsible for depressing the ribs and bending them inward, while the external intercostal muscles elevate the ribs and bend them outward.
| Characteristics | Values |
|---|---|
| Type | Intercostal muscles |
| Number of muscles | 11 pairs |
| Location | Between the ribs |
| Direction | Inferoanteriorly from the rib above to the rib below |
| Origin | Lower border of each rib |
| Insertion | Superior border of the rib below |
| Innervation | Intercostal nerves (T1-T11) |
| Blood supply | Anterior and posterior intercostal arteries |
| Function | Stabilises the position of the ribs during normal respiration; elevates the ribs and increases the thoracic volume during forced inspiration |
| Layers | External intercostal muscles, internal intercostal muscles, innermost intercostal muscles |
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What You'll Learn
External intercostal muscles
The external intercostal muscles are a set of muscles located between adjacent ribs. They are the most superficial set of muscles, occupying the 11 intercostal spaces. They are thicker than the internal intercostals, with their fibres directed obliquely downward and laterally on the back of the thorax, and downward, forward, and medially on the front.
The external intercostal muscles are also known as intercostales externi or intercostalis externus. They originate on ribs 1–11 and have their insertion on ribs 2–12. Each external intercostal muscle attaches superiorly to the lower border of one rib and attaches inferiorly to the upper border of the adjacent rib below. They extend from the rib tubercle posteriorly and attach to the costochondral junction anteriorly.
The external intercostal muscles are involved in respiration. They elevate the ribs and bend them more open, thus expanding the transverse dimensions of the thoracic cavity during inhalation. They work in unison with the internal intercostal muscles during inhalation, with the internal muscles relaxing while the external muscles contract, causing the expansion of the chest cavity and an influx of air into the lungs.
The external intercostal muscles, along with the internal and innermost intercostal muscles, maintain the stability of the thoracic cage. They are innervated by the intercostal nerves and supplied by the intercostal arteries.
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Internal intercostal muscles
The internal intercostal muscles are a muscle of the thoracic wall, located in the intercostal spaces between the ribs. They are part of the intermediate layer of the intercostal muscles group, which also includes the external and innermost intercostals. These muscles assist in breathing and support the integrity of the thoracic cage.
The internal intercostal muscles are responsible for depressing the ribs and bending them inward, thus decreasing the transverse dimensions of the thoracic cavity. They originate from the lateral edge of the costal groove of the rib above and insert onto the superior surface of the rib below. Their fibres are oriented in an inferior and posterior direction, forming a right angle with the external intercostal muscle.
The internal intercostal muscles are innervated by the intercostal nerves of the corresponding intercostal spaces. These nerves are the anterior rami of thoracic spinal nerves. The muscles are supplied by the muscular branches of the anterior and posterior intercostal arteries, and the venous blood is conveyed by the anterior and posterior intercostal veins.
The internal intercostal muscles are involved in forced expiration, such as coughing or during exercise, but not in relaxed breathing. During exhalation, the interosseous portions of these muscles depress and retract the ribs, compressing the thoracic cavity and expelling air.
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Innermost intercostal muscles
The innermost intercostal muscles, also known as the intercostalis intimus, are the deepest layer of the intercostal muscles. They are found in the most lateral portion of the intercostal spaces, originating from the medial aspect of the costal groove of the rib above and inserting onto the internal aspect of the rib below. The muscle fibres of the innermost intercostal muscles are directed downwards, forwards, and laterally, similar to the internal intercostal muscles.
The innermost intercostal muscles are separated from the internal intercostal muscles by the intercostal neurovascular bundle. They are similar in structure to the internal intercostals, and their vascularization and innervation follow the same pattern. The blood supply to these muscles comes from the anterior and posterior intercostal arteries, the costocervical trunk, the internal thoracic arteries, and the musculophrenic arteries.
The function of the innermost intercostal muscles is to assist in depressing the ribs during forced expiration, thereby decreasing the thoracic volume. They work alongside the internal intercostal muscles during this process. Deficient innermost intercostal muscles may not provide appropriate tension to the intercostal spaces during thoracic pressure changes, leading to respiratory issues.
The innermost intercostal muscles are accessory respiratory muscles that participate in the process of forced breathing, specifically aiding in forced expiration. They are one of the five muscles that make up the thoracic wall, along with the external and internal intercostal muscles, the subcostalis, and the transversus thoracis.
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Subcostalis
The subcostalis is a muscle that arises on the inner surface of a rib and is inserted into the inner surface of the second or third rib below. It is part of the group of muscles known as the subcostal muscles, which are most developed in the inferior portion of the thoracic wall. The subcostalis is present in abundance in the lower regions of the posterior thoracic wall.
The subcostalis is one of the muscles that make up the thoracic wall, along with the external intercostal muscles, internal intercostal muscles, innermost intercostal muscles, and transversus thoracis. These muscles are primarily responsible for changing the volume of the thoracic cavity during respiration. The subcostalis is part of the accessory respiratory musculature, as it aids in forced exhalation by depressing the ribs. This action enables the subcostalis to pull the ribs towards the central axis of the thorax, which then compresses the lungs and pushes air out of them.
The intercostal muscles are another group of muscles that run between the ribs and help form and move the chest wall. They are mainly involved in the mechanical aspect of breathing by helping to expand and shrink the size of the chest cavity. The external intercostal muscles aid in quiet and forced inhalation by raising the lateral part of the ribs, increasing the transverse diameter of the thorax. The internal intercostal muscles aid in forced expiration by depressing the ribs and bending them inward, thus decreasing the size of the chest cavity.
The subcostalis is similar to the innermost intercostal muscles in terms of the direction of its fibres. It crosses over two or three intercostal spaces, unlike the intercostal muscles that fill in only one intercostal space. The lateral borders of the subcostalis are in contact with the innermost intercostal muscles and may sometimes fuse with them. The posterior surfaces of the subcostalis lie over the inner surface of the ribs and the internal intercostal membranes, while the anterior surfaces are in relation to the endothoracic fascia and the parietal pleura.
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Transversus thoracis
The transversus thoracis is a muscle found on the inner surface of the anterior chest wall. It is one of the intrinsic muscles of the chest wall, along with the intercostals, subcostal, levatores costarum, and serratus posterior muscles. The transversus thoracis is organised into several slips that radiate from the body and xiphoid process of the sternum towards the costal cartilages of ribs 2 to 6. The lowest fibres of this muscle are horizontal and continuous with the transversus abdominis, while the intermediate fibres run obliquely in a superolateral direction and the superiormost fibres lie nearly vertically.
The transversus thoracis is an accessory respiratory muscle that is active during forced expiration. It pulls ribs 2 to 6 towards the sternum, resulting in the depression of those ribs. This action decreases the transverse diameter of the thoracic cage and, consequently, the anteroposterior diameter of the thoracic cavity. During inspiration, the transversus thoracis stiffens the thoracic wall, preventing paradoxical movements of the chest wall.
The muscle separates the pleura from the intercostal nerves and is innervated by the second to fifth intercostal nerves, which are the anterior rami of spinal nerves T2-T6. The blood supply to the transversus thoracis comes from the sternal branches of the internal thoracic artery, a branch of the subclavian artery.
The transversus thoracis plays a significant role in heart surgery due to its proximity to the internal thoracic artery, also known as the internal mammary artery (IMA). This artery is commonly used as a transplant vessel for coronary artery bypass surgery because it only needs to be anastomosed on one side and is easily accessible between the first rib and the highest insertion tendon of the transversus thoracis.
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Frequently asked questions
The intercostal muscles are a group of intrinsic rib cage muscles that occupy the 11 intercostal spaces. They are divided into three groups: external intercostal muscles, internal intercostal muscles, and innermost intercostal muscles.
The external intercostal muscles elevate the ribs during inhalation, increasing the thoracic volume.
The internal intercostal muscles depress the ribs during exhalation, decreasing the thoracic volume.
