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The human rib cage is a bony framework that protects the thoracic organs and aids respiration. There are 12 pairs of ribs, and several muscles are attached to them. The intercostal muscles are the most well-known muscles connected to the ribs, and they consist of three layers: external, internal, and innermost intercostal muscles. Other muscles connected to the ribs include the diaphragm, serratus anterior, pectoralis major and minor, latissimus dorsi, scalenus anterior, posterior and medius, and rectus abdominis. These muscles play a crucial role in respiratory functions, rib cage stability, and protection of internal organs.
| Characteristics | Values |
|---|---|
| Number of pairs of ribs | 12 |
| Number of pairs of external intercostal muscles | 11 |
| Origin of external intercostal muscles | Lower border of each rib |
| Insertion point of external intercostal muscles | Upper border of the rib below |
| Direction of external intercostal muscle fibres | Oblique, downward, and forward |
| Number of pairs of internal intercostal muscles | 11 |
| Origin of internal intercostal muscles | Costal groove near the inferior border of the rib above |
| Insertion point of internal intercostal muscles | Upper border of the rib below |
| Direction of internal intercostal muscle fibres | Posteroinferior |
| Function of intercostal muscles | Stabilise the position of the ribs during normal respiration, elevate the ribs and increase thoracic volume during inspiration, depress the ribs and decrease thoracic volume during forced expiration |
| Innervation of intercostal muscles | Intercostal nerves (T1-T11) |
| Other muscles related to ribs | Diaphragm, serratus anterior, pectoralis major and minor, latissimus dorsi, scalenus anterior/posterior/medius, rectus abdominis |
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What You'll Learn
- Intercostal muscles: 11 pairs of muscles between ribs, aiding respiration
- Diaphragm: arises from the inner surfaces of the sixth rib's costal cartilage
- Serratus anterior: originates from the first to eighth ribs
- Pectoralis major and minor: arise from superior anterior ribs
- Latissimus dorsi: originates from the ninth to twelfth ribs
Intercostal muscles: 11 pairs of muscles between ribs, aiding respiration
The intercostal muscles are the muscles that present within the rib cage. They consist of three layers of muscles: external, internal, and innermost. These layers combine to fill the space between the ribs.
The external intercostal muscles are the outermost layer and lie directly under the skin. They originate from the lower border of the rib above and run obliquely to insert into the upper border of the rib below. When these muscles contract, they pull the ribs upward and outward, expanding the chest wall during inhalation. There are 11 pairs of external intercostal muscles, and they are the most superficial muscles in the area. They are attached to the shafts of the ribs and extend obliquely from the lower border of one rib to the upper border of the adjacent rib below.
The internal intercostal muscles are the intermediate layer. They originate from the costal groove near the inferior border of the rib above and attach to the upper border of the rib below. These muscles help to collapse the lungs during expiration, having the opposite effect of the external intercostal muscles. The 11 pairs of internal intercostal muscles are located immediately deep to the external intercostals.
The innermost intercostal muscles cross more than one intercostal space and assist the internal and external intercostals in their function. They are the deepest of the intercostal muscles and are similar in structure to the internal intercostals. They are separated from the internal intercostals by the intercostal neurovascular bundle.
Together, the intercostal muscles aid in respiration by stabilising the position of the ribs during normal respiration and elevating the ribs during inspiration, increasing thoracic volume.
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Diaphragm: arises from the inner surfaces of the sixth rib's costal cartilage
The diaphragm is a crucial muscle that plays a vital role in respiration. It arises from the inner surfaces of the costal cartilages on the sixth rib, also known as the costal diaphragm. The diaphragm is a dome-shaped muscle that separates the thoracic cavity, which contains the heart and lungs, from the abdominal cavity below.
The diaphragm is the primary muscle of respiration and is essential for breathing. When the diaphragm contracts, it flattens and moves downward, increasing the volume of the thoracic cavity. This expansion of the thoracic cavity creates a vacuum, causing air to rush in and fill the lungs. During forced inspiration, the diaphragm contracts, causing the ribs to elevate and the thoracic volume to increase, facilitating deeper inhalation.
Additionally, the diaphragm works in conjunction with other respiratory muscles, including the intercostal muscles located between the ribs. There are three layers of intercostal muscles: external, internal, and innermost. The external intercostal muscles originate from the lower border of each rib and insert onto the superior border of the rib below, running obliquely downward and forward. They are responsible for expanding the chest wall during inhalation, pulling the ribs upward and forward.
The internal intercostal muscles lie deep to the external intercostals and extend from the rib above to the one below, but in the opposite direction. They assist in collapsing the lungs during expiration, aiding in forced exhalation. The innermost intercostal muscles are the deepest layer, similar in structure to the internal intercostals, and they assist the internal and external intercostals in their function.
The diaphragm's connection to the sixth rib's costal cartilage highlights its integral role in respiration. This structural relationship allows for the diaphragm's efficient contraction and relaxation, facilitating the inhalation and exhalation processes, respectively. The diaphragm's origin from the inner surfaces of the sixth rib's costal cartilage contributes to its ability to regulate thoracic and abdominal cavity pressures, making it a key muscle in the respiratory system.
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Serratus anterior: originates from the first to eighth ribs
The serratus anterior is a fan-shaped muscle that originates from the first to eighth or ninth ribs at the lateral wall of the thorax. It is located deep under the scapula and the pectoral muscles and is easily palpable between the pectoralis major and latissimus dorsi muscles. It can be felt by placing your hand just below the armpit.
The serratus anterior has three parts: the superior, middle, and inferior. The superior part originates from the first and second ribs and attaches to the superior angle of the scapula. The middle part originates from the second and third ribs and attaches to the medial border of the scapula. The inferior part originates from the fourth to eighth or ninth ribs and attaches to the medial border and inferior angle of the scapula. This lower attachment may extend to the tenth rib in approximately 10% of the population. The inferior part is the most prominent and powerful section of the muscle.
The serratus anterior is a key scapular stabiliser, keeping the shoulder blades against the rib cage when at rest and during movement. It acts on the scapula and is the prime mover in scapular protraction and upward rotation. When the shoulder blade is fixed, the serratus anterior lifts the rib cage and assists in respiration.
The serratus anterior is sometimes called the "big swing muscle" or the "boxer's muscle" because it is largely responsible for the protraction of the scapula, which occurs when throwing a punch.
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Pectoralis major and minor: arise from superior anterior ribs
The pectoralis major and minor muscles are among the muscles that arise from the superior anterior ribs. The pectoralis major is the largest and most superficial muscle in the chest area, arising from parts of the clavicle and sternum, costal cartilages of the true ribs, and the aponeurosis of the abdominal external oblique muscle. It receives dual motor innervation from the medial and lateral pectoral nerves. Its primary functions are flexion, adduction, and internal rotation of the humerus, and it is colloquially referred to as "pecs" or "chest muscle".
The pectoralis minor is a smaller, inferior muscle to the pectoralis major, and it is also one of the most superficial muscles on the anterior aspect of the chest or thoracic wall. It arises as three separate heads from the anterior surface of the 3rd, 4th, and 5th ribs near the corresponding costal cartilages and the fascia overlying the adjacent intercostal muscles. The pectoralis minor is crucial for stabilising the scapula by pulling it downward and anteriorly against the thoracic wall. It also plays a role in respiration, elevating the ribs for deep inspiration when the pectoral girdle is fixed or elevated.
The intercostal muscles are another group of muscles that are associated with the ribs. They are organised into three layers and lie in the intercostal spaces between the ribs. There are 11 pairs of external intercostal muscles that run inferoanteriorly from the rib above to the rib below. The internal intercostals lie deep to the external intercostals and extend from the rib above to the one below in the opposite direction. The innermost intercostals are the deepest layer and are similar in structure to the internal intercostals. They originate from the medial edge of the costal groove and insert into the superior surface of the rib below.
The ribs themselves form the bony framework of the thoracic cavity and serve to protect the thoracic organs, with their primary function being to aid respiration. There are twelve pairs of ribs, and they are classified into three groups based on their attachment to the sternum: true, false, and floating ribs. The true ribs, which include ribs 1-7, articulate directly with the sternum through their costal cartilages.
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Latissimus dorsi: originates from the ninth to twelfth ribs
The latissimus dorsi muscle, commonly known as the 'lats', is one of the largest muscles in the back, stretching across the lower posterior thorax. Its primary function is in upper extremity movement, but it also serves as a respiratory accessory muscle. The muscle is innervated by the thoracodorsal nerve, originating from the posterior cord of the brachial plexus (i.e., C6-8).
The latissimus dorsi originates from the ninth to twelfth ribs, the spinous processes of the seventh thoracic to the fifth lumbar vertebrae, the thoracolumbar fascia, the posterior iliac crest, and the inferior angle of the scapula. It inserts on the floor of the intertubercular sulcus of the humerus. The latissimus dorsi muscle works with the teres major and the pectoralis major muscles to depress the arm and adduct, extend, and internally rotate the shoulder and arm.
The latissimus dorsi is also active during deep inspiration and with forceful respiratory functions, such as coughing and sneezing. It assists in forced expiration by compressing the rib cage. The muscle is frequently used as a myocutaneous flap for reconstructive surgeries, including chest wall and post-mastectomy reconstruction, and head and neck reconstruction.
The latissimus dorsi is implicated in patients with restrictions of shoulder motion, including abduction, flexion, and lateral rotation. For patients with low back pain, it is important to assess the length and flexibility of the latissimus dorsi due to its attachments to the spine and pelvis. A decrease in length or an increase in stiffness of this muscle can lead to alterations in movement patterns and postures, exacerbating low back pain.
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Frequently asked questions
Intercostal muscles are the muscles that present within the rib cage. They consist of three layers of muscles: external, internal, and innermost. They combine to fill the space between the ribs.
The external intercostal muscles connect adjacent ribs and slope downward and forward. When they contract, the ribs are pulled upward and forward, resulting in an increase in both the anteroposterior and lateral diameters of the thorax.
The internal intercostal muscles originate from the costal groove near the inferior border of the rib above and attach to the upper border of the rib below. They help collapse the lung during expiration.
Other muscles that connect the ribs include the diaphragm, serratus anterior, pectoralis major and minor, latissimus dorsi, scalenus anterior, posterior and medius, and rectus abdominis.
