Dominic Stone
The thorax is a complex structure that houses vital organs, including the heart and lungs. The thoracic wall, which forms the anterior, posterior, and lateral boundaries of the thorax, is composed of several muscles that play a crucial role in respiration. These muscles include the diaphragm, the primary muscle of inspiration, and the intercostal muscles, which consist of three layers: external, internal, and innermost intercostals. During inspiration, the external intercostals contract, raising the lateral part of the ribs and enlarging the thorax. The internal intercostals depress the ribs during forced expiration, reducing thoracic volume. The innermost intercostals, the deepest layer, originate from the medial aspect of the costal groove and insert into the superior surface of the rib below, stabilising the ribs during normal respiration. In addition to the diaphragm and intercostals, other muscles that contribute to the function and structure of the thorax include the levatores costarum, serratus posterior superior and inferior, subcostalis, and transversus thoracis.
| Characteristics | Values |
|---|---|
| Muscles that make up the thorax wall | External intercostal muscles, internal intercostal muscles, innermost intercostal muscles, subcostalis, and transversus thoracis |
| Number of intercostal muscles in each intercostal space | 11 pairs |
| Origin of external intercostal muscles | Inferior aspect of the rib above |
| Insertion point of external intercostal muscles | Superior aspect of the rib below |
| Direction of external intercostal muscles | Inferomedial |
| Attachment of internal intercostal muscles | Lateral aspect of the costal groove of the rib above |
| Insertion point of internal intercostal muscles | Superior aspect of the rib below |
| Direction of internal intercostal muscles | Perpendicular to the external intercostal muscles |
| Origin of innermost intercostal muscles | Medial aspect of the costal groove of the rib above |
| Insertion point of innermost intercostal muscles | Internal aspect of the rib below |
| Accessory muscles of respiration | Pectoralis major, minor, serratus anterior, and scalene muscles |
| Primary muscle of inspiration | Diaphragm |
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What You'll Learn
The diaphragm
Phrenic nerve damage is a common cause of diaphragm problems, which may occur due to cancer, autoimmune diseases, trauma, or surgery. Certain conditions, such as HIV, West Nile virus, and Lyme disease, can cause nerve inflammation. Neuromuscular disorders, including multiple sclerosis (MS) and ALS, can lead to diaphragmatic palsy or weakness of the diaphragm muscle. Regular check-ups are essential for individuals with a higher risk of diaphragm issues.
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External intercostals
The external intercostal muscles are a set of muscles located between adjacent ribs, attaching along the shafts of the ribs and extending obliquely from the lower border of one rib to the upper border of the adjacent rib below. There are 11 pairs of external intercostal muscles, with each muscle occupying its own intercostal space. They are the most superficial layer of the intercostal muscles, located superficially to the internal intercostal muscles.
The external intercostal muscles play a role in respiration, elevating the ribs during forced inhalation and increasing the transverse and anteroposterior diameter of the lungs, which in turn decreases the intrapleural pressure. This process expands the lungs and facilitates the entry of air into them. During inhalation, the internal intercostal muscles relax while the external muscles contract, causing the expansion of the chest cavity.
The external intercostal muscles are thicker than the internal intercostals, and their fibres are directed obliquely downward and laterally on the back of the thorax, and downward, forward, and medially on the front. The most posterior fibres of each muscle fuse with the superior costotransverse ligaments, while the most anterior fibres blend with the external intercostal membrane, which fills the remainder of the corresponding intercostal space from the costochondral junction to the sternum.
The external intercostal muscles receive innervation from intercostal nerves of corresponding intercostal spaces, which are the anterior rami of thoracic spinal nerves. They have an extensive vascular supply, with the first two posterior intercostal arteries arising from the supreme intercostal artery, a branch of the costocervical trunk, and the remaining nine arising from the posterior aspect of the thoracic aorta.
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Internal intercostals
The internal intercostals are a group of muscles that make up the middle layer of the intercostal muscle group. There are 11 internal intercostal muscles, which lie in the intercostal spaces between the ribs. They originate from the inferior margin of the costal cartilage and inner lip of the costal groove, extending between the inferior margin of the superior rib to the superior margin of the first rib below.
The internal intercostals are innervated by the intercostal nerves of the corresponding intercostal spaces. They are supplied by the muscular branches of the anterior and posterior intercostal arteries, as well as the intercostal branches of the internal thoracic, musculophrenic arteries, and costocervical trunk. The venous blood is conveyed by the anterior and posterior intercostal veins to either the brachiocephalic or azygos venous system, which both empty into the superior vena cava.
The internal intercostals are primarily muscles of exhalation. During exhalation, the interosseous portions of the internal intercostals depress and retract the ribs, compressing the thoracic cavity and expelling air. They are only used during forceful exhalation, such as coughing or during exercise, and not during relaxed breathing. Contraction of the internal intercostals has the opposite effect of the external intercostals, resulting in the lowering of the ribs and a reduction in the anterior-posterior dimension.
The internal intercostals also play a crucial role in speech and singing, as they are the muscles that propel air out through the mouth and nose. The greater the pressure of the escaping air, the louder one's voice.
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Innermost intercostals
The innermost intercostal muscles are one of the three intercostal muscles in the thorax, the other two being the external and internal intercostals. They are the deepest of the intercostal muscles, forming the third and deepest layer of intercostal muscles. They are located deep to the internal and external intercostals, filling the 11 intercostal spaces between the ribs. The fibres of the innermost intercostal muscles travel obliquely, in the same direction as the internal intercostal muscles and the opposite direction of the external intercostal muscles.
The precise function of the innermost intercostal muscles is not yet known, but it is likely to be the same as that of the internal intercostal muscles. They are thought to depress the ribs and aid in pushing air out of the lungs during forced exhalation. This places them in the category of accessory respiratory muscles that are only active during forced breathing. In addition to aiding respiration, these muscles are tonically contracted constantly, increasing the rigidity of the chest wall and making it suitable for manipulation by the diaphragm.
The innermost intercostal muscles originate from the medial edge of the costal groove and insert onto the superior surface of the rib below. They are innervated by the intercostal nerves, which are the anterior rami of the thoracic spinal nerves. The innermost intercostals are supplied by the vessels of the thoracic wall, including the muscular branches of the anterior and posterior intercostal arteries, the intercostal branches of the internal thoracic and musculophrenic arteries, and the costocervical trunk.
The innermost intercostal muscles are not equally developed in every intercostal space. They are typically most developed in the inferior intercostal spaces, and may blend with the subcostales in the same intercostal space. Even the most developed innermost intercostal muscles do not fill the entire length of the intercostal spaces, but rather occupy the middle two quarters.
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Subcostalis
The subcostal muscles, also known as subcostalis, are a group of small muscles that arise on the inner surface of a rib and insert into the inner surface of the second or third rib below. They are a part of the accessory respiratory musculature and aid in forced exhalation by depressing the ribs during respiration. This action enables them to pull the ribs towards the central axis of the thorax, compressing the lungs and pushing air out of them.
The subcostalis muscles are one of the five muscles that make up the thoracic wall, along with the external intercostal muscles, internal intercostal muscles, innermost intercostal muscles, and transversus thoracis. These muscles work together to change the volume of the thoracic cavity during respiration. The subcostal muscles are most developed in the lower part of the thoracic cage and are sometimes fused with the innermost intercostals.
The intercostal muscles, which are also attached to the thorax, are organised into three layers: external, internal, and innermost. The external intercostal muscles are the most superficial layer, originating from the rib above and inserting onto the rib below. The internal intercostal muscles form the intermediate layer, originating from the lateral aspect of the costal groove of the rib above and inserting into the rib below. The innermost intercostal muscles are the deepest layer, originating from the medial aspect of the costal groove and inserting into the rib below.
The subcostal muscles cross over two or three intercostal spaces, unlike the intercostal muscles that fill in only one intercostal space. The lateral borders of the subcostal muscles are in contact with the innermost intercostal muscles, and their posterior surfaces lie over the inner surface of the ribs and the internal intercostal membranes. The anterior surfaces of these muscles relate to the endothoracic fascia and the parietal pleura.
In summary, the subcostal muscles, or subcostalis, play an important role in respiration by aiding forced exhalation and changing the volume of the thoracic cavity. They are a part of the thoracic wall and work in conjunction with the intercostal muscles to facilitate respiration and maintain the position of the ribs.
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Frequently asked questions
The diaphragm and the external intercostals are the primary muscles that enlarge the thorax.
The diaphragm is a sheet of skeletal muscle that separates the thoracic cavity from the abdominal cavity. It is the main muscle of inspiration.
The external intercostals are the most superficial layer of the intercostal muscles. They contract and raise the lateral part of the ribs, increasing the transverse diameter of the thorax.
The other muscles of the thorax include the internal intercostals, innermost intercostals, subcostals, and transversus thoracis. The scalene muscles and the sternocleidomastoid muscle are also accessory muscles that attach to the thorax.
