Clinically Oriented Anatomy, 8th ed., Overview of Thorax section through The bottom line: Muscles and neurovasculature of thoracic wall.
Figure 12.1 Thoracic wall.
The walls surrounding the thoracic cavity are made from layers of muscles and fascia that are reinforced by the thoracic skeleton (“rib cage”). The thoracic wall is cone-shaped; narrower above and wider below. The bones and bulky muscles of the pectoral region, which are properly part of the upper limb, overlie and obscure the relatively small thoracic skeleton, made of ribs, costal cartilages, sternum, and thoracic vertebrae. The spaces between the ribs (intercostal spaces) are filled with muscles, nerves, and vessels. The thoracic wall is flexible and compressible, yet rigid enough to support the upper limb and resist pressures generated within the thoracic cavity.
The bony thorax
Composed of 12 thoracic vertebrae, 12 pairs of ribs, and the sternum. The vertebrae are covered in a separate chapter.
Ribs
Figure 12.2
The true ribs (1–7) are connected directly to the sternum via costal cartilages, forming complete loops between the vertebrae and sternum. The fusion of a rib to a costal cartilage produces a costochondral joint. Inflammation of this joint (costochondritis) is a common cause of chest pain that can occur after strenuous activity, such as lifting heavy objects.
The false ribs (8–12) fail to reach the sternum directly. The cartilages of ribs 8, 9, and 10 are joined to one another and to the cartilage of rib 7 above them, forming the costal margin.
A subset of the false ribs are the floating ribs (11 and 12). These short ribs have small costal cartilages that do not attach distally to other ribs and thus “float” within the fibromuscular body wall.
Features of typical ribs:
Figure 12.3 GRAY’S ANATOMY FOR STUDENTS, 3RD ED., FIGURE 3.21.
Head: Articulates with one or two vertebral bodies at the costovertebral joint. Usually one rib head articulates with two adjacent vertebrae
Tubercle: Knuckle-shaped, it contains a smooth facet that articulates with the tansverse process of an adjacent vertebra (costotransverse joint)
Body (shaft): Distal to the tubercle, the body curves and twists down and forwards until it meets the costal cartilage. The sharpest bend in the rib body is called the angle of the rib. Most rib fractures occur in this vicinity. The superior border of the rib is rounded, while the inferior border presents a sharp margin. Just internal to the inferior border of the rib is the costal groove, which contains and protects the intercostal nerve and vessels.
Atypical ribs
Ribs 1, 2, 10, 11, and 12 are considered atypical in one form or another. The most important of these is the first rib, which is broad and flat and the most curved of all the ribs. The first rib borders the superior thoracic aperture and its upper surface is a landmark for the subclavian artery.
Joints between ribs and vertebrae
Ribs articulate with vertebrae at two joints. The costovertebral joint (rib head + vertebral body) and costotransverse joint (rib tubercle + transverse process of vertebra) are synovial joints, both of which are involved in movements of the chest wall that facilitate respiration. This will be discussed in a later chapter.
Figure 12.4
Intercostal spaces
Intercostal spaces separate adjacent ribs and their costal cartilages. They are numbered (1–11) based on the rib above the space. Intercostal “spaces” are actually occupied by muscles, membranes, nerves, and vessels. They are clinically useful as they allow access to the thoracic cavity and are traversed by needles during procedures such as thoracentesis (pleural tap).
Sternum
Figure 12.5 GRAY’S ANATOMY FOR STUDENTS, 3RD ED., FIGURE 3.32.
Located in the midline, its entire length can be palpated.
The manubrium (Latin = handle [of a sword]) is the upper shield-shaped portion. Its thick superior border is indented by the suprasternal (jugular) notch. The first pair of ribs attaches to the manubrium, as does the clavicle at the sternoclavicular joint.
The body is the central portion to which the costal cartilages of ribs 2–7 attach.
The manubrium and body are joined at the sternal angle. In many individuals this is marked by a visible ridge. The sternal angle is an important landmark for locating:
Union of the second costal cartilages and sternum
Intervertebral disc between T-4 and T-5 vertebrae
Division between superior and inferior mediastinum
Superior limit of the pericardium
Beginning and ending of the arch of the aorta
Superior vena cava entering the heart
Superior limit of the pulmonary trunk
The xiphoid process (Latin = sword-shaped) is the inferior- most extension of the sternum. It lies within the angle formed by the left and right costal margins. Its size and position are variable.
The xiphisternal junction (junction of body and xiphoid process) is located at the level of the 6th costal cartilages and T-10 vertebra. It is a useful landmark for identifying the central tendon of the diaphragm, upper border of the liver, and inferior border of the heart.
Thoracic apertures
The thoracic wall is open above and below, producing two apertures.
Superior thoracic aperture
Bounded by the first pair of ribs, the superior border of the manubrium, and the first thoracic vertebra. This passageway connects the neck and thorax, and structures such as the esophagus and trachea traverse it.
Inferior thoracic aperture
Bounded by the 12th thoracic vertebra, 12th ribs, and costal margins. It is closed by the diaphragm.
Figure 12.6
Muscles of the thoracic wall
Figure 12.7 GRAY’S ANATOMY FOR STUDENTS, 3RD ED., FIGURE 3.27.
Muscles of the body wall are arranged in three layers: external, internal, and innermost. In the thoracic region the continuity of these muscle layers are interrupted by the ribs. Thus, these muscles occupy the intercostal spaces.
Muscle fibers slant downward and forward from one rib to another (“hands in front pockets”). They begin posteriorly at the rib tubercle and extend as far anteriorly as the costochondral junction. Function: elevate the ribs during inspiration, thus increasing the size of the thoracic cavity.
Muscle fibers are at 90 degree angle to external intercostal muscle, slanting upward and forward. They begin posteriorly at the rib angle and extend anteriorly to the sternum. Thus, the muscle fibers visible just lateral to the sternal body are those of the internal intercostal muscles. Function: depress the ribs during active (forced) expiration.
Poorly developed and does not completely fill the intercostal spaces. Instead, this layer consists of three disconnected muscle groups:
the subcostal muscles posteriorly
the innermost intercostal muscles at the midpoint of the intercostal space (midaxillary line)
the transversus thoracis muscles anteriorly, behind the sternum and costal cartilages.
Of these, the innermost intercostal muscles are functionally the most important. Their fiber direction and function are similar to those of the internal intercostal muscles.
Innervation
All of the intercostal muscles are supplied by the intercostal nerves.
Intercostal space and neurovascular plane
Figure 12.8 GRAY’S ANATOMY FOR STUDENTS, 3RD ED., FIGURE 3.26.
Each intercostal space contains the external, internal, and innermost intercostal muscles with deep fascia separating them. Deep to these is the parietal layer of pleura.
The neurovascular plane of the body wall is the fascial space that contains the nerves and vessels. In the thorax it is located between the second and third muscle layers = between internal intercostal and innermost muscle layers.
The neurovascular bundle is arranged within the costal groove of the rib from superior to inferior as “VAN” (intercostal Vein, Artery, Nerve).
The location of the neurovascular bundle in the costal groove has clinical significance.
Question
Where do you think a needle should be inserted through an intercostal space for aspiration of the pleural cavity: along the upper border or lower border of a rib?
Question
Where should the needle tip be placed in order to anesthetize an intercostal nerve?
Innervation of thoracic wall
Recall that spinal nerves divide into dorsal and ventral rami. Dorsal rami supply the back. The thoracic wall is innervated by the ventral rami of spinal nerves T-1 to T-12.
T-1 to T-11 are known as intercostal nerves, while T-12 is the subcostal nerve. These are mixed somatic nerves, supplying motor and sensory fibers to all of the tissues of the body wall as well as the parietal layer of pleura. Intercostal nerves also contain sympathetic fibers that are motor to smooth muscle in blood vessels in the body wall, arrector pili muscles of hair follicles, and sweat glands in the skin.
At the angle of the rib, the ventral ramus of the spinal nerve passes into the fascial space between the internal and innermost intercostal muscles = the neurovascular plane.
Here it is located inferior to the intercostal artery.
Figure 12.10 GRAY’S ANATOMY FOR STUDENTS, 3RD ED., FIGURE 3.32.
At the midaxillary line, the intercostal nerves give off lateral cutaneous branches to the skin. These pass between the costal attachments of the serratus anterior muscle.
The upper six intercostal nerves pass forward to the lateral edge of the sternum. Here they terminate as anterior cutaneous branches which penetrate the internal intercostal and pectoralis major muscles to enter the skin.
The lower five intercostal nerves leave the intercostal spaces distally and pass into the abdominal wall, becoming thoraco-abdominal nerves.
Intercostal nerves provide motor innervation to all the thoracic wall muscle layers = external, internal, and innermost.
Segmental innervation of the thoracic wall:
Dermatomes in the trunk are arranged in regular bands of skin around the body wall. The T-2 dermatome is at the level of the sternal angle and that of T-10 crosses the umbilicus (“belly button”). There is considerable overlap of adjacent dermatomes in the trunk, such that damage to one spinal nerve would probably not produce serious symptoms.
Likewise, myotomes in the trunk are arranged in segmental bands of muscle. Intercostal nerves supply the muscles within individual intercostal spaces,as well as strips of muscle in the abdominal wall.
Figure 12.11
Blood supply of thoracic wall
Arteries
Veins
Figure 12.12 GRAY’S ANATOMY FOR STUDENTS, 3RD ED., FIGURE 3.29.
Intercostal arteries: Each intercostal space is supplied by two sets of intercostal arteries: anterior and posterior. These arteries anastomose in the intercostal space.
Posterior intercostal arteries are branches of the thoracic aorta. Exception: the first two intercostal spaces receive blood from branches of the subclavian artery in the neck.
Anterior intercostal arteries supplying the upper six intercostal spaces are branches of the internal thoracic artery. The lower intercostal spaces receive anterior intercostal arteries from the musculophrenic artery, one of the terminal branches of the internal thoracic artery.
Internal thoracic artery: Branches from the subclavian artery at the base of the neck. It descends inside the rib cage, internal to the costal cartilages just lateral to the sternum. Like the intercostal vessels, it too is located in the neurovascular plane, between the transversus thoracis muscle (innermost muscle layer) and the internal intercostal muscles. At the level of the 6th costal cartilage, the internal thoracic artery terminates, bifurcating into:
Superior epigastric artery: Passes downward into the abdominal wall.
Musculophrenic artery: Passes along the costal margin, supplying the diaphragm and the lower intercostal spaces with anterior intercostal arteries.
Figure 12.13 GRAY’S ANATOMY FOR STUDENTS, 3RD ED., FIGURE 3.30.
Anterior and posterior intercostal veins drain the thoracic wall and accompany the arteries of the same name.
Posterior intercostal veins are tributaries of the azygos vein on the right and hemiazygos veins on the left. Ultimately this blood reaches the superior vena cava.
Anterior intercostal veins drain into the internal thoracic veins. The later ultimately drain to the brachiocephalic veins.
Movements of respiration
One of the functions of the chest wall is to generate the forces needed for adjusting the volume of the thoracic cavity, thus allowing air to move into and out of the lungs (inspiration and expiration).
Figure 12.14 GRAY’S ANATOMY FOR STUDENTS, 3RD ED., FIGURE 3.34.Inspiration is an active process requiring contraction of muscles attaching to the rib cage. These produce movements that affect the size of the thoracic cavity in three dimensions:
The anteroposterior dimension is affected by elevation or depression of the upper ribs. Elevation of the ribs occurs with contraction of the external intercostal muscles. Since the anterior ends of the ribs attaching to the sternum are inferior to the posterior parts attaching to the vertebral column, elevation of the upper ribs moves the sternum forward and upward, like the motion of an old-fashioned “pump handle.”
The transverse (lateral) dimension is affected by elevation or depression of the lower ribs. Elevation is due to contraction of the external intercostal muscles. The lower ribs are arranged as loops, with their middle portions lower than their anterior and posterior ends. Elevation of these ribs swings their middle portions laterally, like the metal handle on a bucket of paint (“bucket handle” movement).
The vertical dimension is affected by elevation and depression of the diaphragm. When contracted, the diaphragm flattens and lowers, increasing the vertical dimension of the thoracic cavity. Relaxation of the diaphragm causes it to elevate, reducing the volume. The movements of the diaphragm are the most important factor in altering the volume of the thoracic cavity (accounting for 2/3 of the volume change).
Clinical correlation
Any muscles that attach to the ribs can potentially move them and act as accessory muscles of respiration. The scalene muscles in the neck (attached to ribs 1 and 2) and the pectoral muscles in the chest (when the arms are fixed) can be recruited to elevate the ribs. Patients in respiratory distress will often grasp their lower limbs or lean on their elbows to immobilize their arms so that the pectoral muscles can assist in inspiration.
Expiration is mainly passive in normal breathing, involving elastic recoil of the lungs and body wall after they have been stretched during inspiration. Forced expiration is accomplished by recruiting the internal intercostal muscles to depress the ribs and muscles of the abdominal wall to increase intra-abdominal pressure and cause the diaphragm to elevate.
The pressure within the closed pleural cavities is slightly below that of the atmosphere. Atmospheric pressure within the lungs, being higher than the “negative pressure” in the pleural cavity, keeps the visceral pleura against the parietal pleura. When the body wall and parietal pleura move, visceral pleura moves with it and air enters the lungs, inflating them within the expanding thoracic cavity.
Clinical correlation
Atmospheric air entering the pleural cavity produces a pneumothorax. This can be caused by a penetrating injury to the chest wall—but most often they are spontaneous, occurring without a known cause, perhaps because of a ruptured “bleb” on the surface of the lung. Air in the pleural cavity destroys the natural pressure gradients, preventing the lung from inflating (collapsed lung). If the compromised pleural cavity continues to fill with air, the increasing pressure could force the organs in the mediastinum to the unaffected side (mediastinal shift), compressing the heart and other lung. This emergency condition is called a tension pneumothorax.
