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THORACIC CAGE AND THORACIC INLET NOTE

THORAX

The thorax is the superior part of the trunk between the neck and abdomen. Commonly the term chest is used as a synonym for thorax, but our concept of the chest is much more extensive than the thoracic wall and cavity contained within it. The chest is generally conceived as being broadest superiorly owing to the presence of the pectoral, or shoulder, girdle (clavicles and scapulae), with much of its girth accounted for by the associated pectoral and scapular (upper limb) musculature. Our concept of the well-formed chest is one that

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narrows inferiorly to the waist and, in adult females, gains further dimension from the breasts.

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Thoracic skeleton. A and B. The osteocartilaginous thoracic cage includes the sternum, 12 pairs of ribs and costal cartilages, and 12 thoracic vertebrae and intervertebral discs. The clavicles and scapulae form the pectoral (shoulder) girdle. The dotted line indicates the position of the diaphragm, which separates the thoracic and abdominal cavities. The thoracic cavity is much smaller than the surrounding thoracic cage.

The thoracic cavity and the wall specific to it are actually the opposite of this. They have the shape of a truncated cone, being narrowest superiorly, with the circumference increasing inferiorly, and reaching its maximum at the junction with the abdominal portion of the trunk. The wall of the thoracic cavity is relatively thin, essentially as thick as its skeleton. The thoracic skeleton takes the form of a domed birdcage, the thoracic cage (rib cage), with the

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horizontal bars formed by ribs and costal cartilages supported by the vertical sternum (breastbone) and thoracic vertebrae. Furthermore, the floor of the thoracic cavity (the diaphragm) is deeply invaginated inferiorly (i.e., is pushed upward) by viscera of the abdominal cavity. Consequently, nearly the lower half of the thoracic wall surrounds and protects abdominal rather than thoracic viscera. Thus the true thorax and especially the thoracic cavity are much smaller than one might expect based on external appearances of the chest.The thorax includes the primary organs of the respiratory and cardiovascular systems. The thoracic cavity is divided into three major spaces: The central compartment, or mediastinum, houses the conducting structures that make up the thoracic viscera, except for the lungs. The lungs occupy the lateral compartments or pulmonary cavities that lie on each side of the mediastinum. Thus the majority of the thoracic cavity is occupied by the lungs, which provide for the exchange of oxygen and carbon dioxide between air and blood, whereas most of the remainder of the thoracic cavity is occupied with structures involved in conducting the air and blood to and from the lungs. Nutrients (food) traverse the thoracic cavity via the esophagus, passing from the intake organ (mouth) to the site of digestion and absorption (abdomen).

THORACIC WALL

The true thoracic wall includes the thoracic cage and the muscles that extend between its elements as well as the skin, subcutaneous tissue, muscles, and fascia covering its anterolateral aspect; the same structures covering its posterior aspect are considered to belong to the back. The mammary glands of the breasts lie within the subcutaneous tissue of the thoracic wall. Although the shoulders are clearly part of the upper limb, the anterolateral thoracoappendicular muscles that overlie the thoracic cage and form the bed of the breast (pectoralis major and serratus anterior distinctly upper limb muscles based on function and innervation) are encountered in the thoracic wall and may be considered part of it. Similar thoracoappendicular muscles placed posteriorly (trapezius and latissimus dorsi) have commonly been considered to be superficial muscles of the back, although in terms of function they are clearly upper limb muscles.The domed shape of the thoracic cage provides remarkable rigidity, given the light weight of its components, enabling it to:

Protect vital thoracic and abdominal internal organs from external forces.

Resist the negative internal pressures generated by the elastic recoil of the lungs and inspiratory movements.

Provide attachment for and support the weight of the upper limbs.

Provide the anchoring attachment of many of the muscles that move and maintain the position of the upper limbs relative to the trunk as well as provide the attachments for muscles of the abdomen, neck, back, and respiration.

Although the shape of the thoracic cage provides rigidity, its joints and the thinness and flexibility of the ribs allow remarkable flexibility, permitting it to absorb many external blows and compressions without fracture and to change its shape repetitively and sufficiently as required for respiration. Because the most important structures within the thorax (heart,

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great vessels, lungs, and trachea) as well as its floor and walls are constantly in motion, the thorax is one of the most dynamic regions of the body. With each breath, the muscles of the thoracic wall working in concert with the diaphragm and muscles of the abdominal wall vary the volume of the thoracic cavity, first by expanding the capacity of the cavity, thereby causing the lungs to expand and draw air in and then (mostly owing to lung elasticity and muscle relaxation) decreasing the volume of the cavity, compressing the lungs, and causing them to expel air.

SKELETON OF THE THORACIC WALL

The thoracic skeleton forms the osteocartilaginous thoracic cage which protects the thoracic viscera and some abdominal organs. The thoracic skeleton includes 12 pairs of ribs and associated costal cartilages, 12 thoracic vertebrae and the intervertebral (IV) discs interposed between them, and the sternum. The ribs and costal cartilages form the largest part of the thoracic cage.

RIBS, COSTAL CARTILAGES, AND INTERCOSTAL SPACES

Ribs are curved, flat bones that form most of the thoracic cage. They are remarkably light in weight yet highly resilient. Each rib has a spongy interior containing bone marrow (hematopoietic tissue), which forms blood cells. There are three types of rib:

True (vertebrocostal) ribs (1st - 7th ribs): They attach directly to the sternum through their own costal cartilages.

False (vertebrochondral) ribs (8th, 9th, and usually 10th ribs): Their cartilages are connected to the cartilage of the rib above them; thus their connection with the sternum is indirect.

Floating (vertebral, free) ribs (11th, 12th, and sometimes 10th ribs): The rudimentary cartilages of these ribs do not connect even indirectly with the sternum; instead they end in the posterior abdominal musculature.

Typical ribs (3rd - 9th) have the following components:

Head: wedge-shaped and has two facets, separated by the crest of the head one facet for articulation with the numerically corresponding vertebra and one facet for the vertebra superior to it.

Neck: connects the head with the body at the level of the tubercle.

Tubercle: at the junction of the neck and body and has a smooth articular part, for articulating with the corresponding transverse process of the vertebra, and a rough non-articular part, for attachment of the costotransverse ligament.

Body (shaft): thin, flat, and curved, most markedly at the costal angle where the rib turns anterolaterally the concave internal surface of the body has a costal groove paralleling the inferior border of the rib, which provides some protection for the intercostal nerve and vessels.

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Atypical ribs (1st, 2nd, and 10th - 12th) are dissimilar.

The 1st rib is the broadest (i.e., its body is widest and nearly horizontal), shortest, and most sharply curved of the seven true ribs. It has a single facet on its head for articulation with the T1 vertebra only and two transversely directed grooves crossing its superior surface for the subclavian vessels; the grooves are separated by a scalene tubercle and ridge, to which the anterior scalene muscle is attached.

The 2nd rib is more typical; its body is thinner, less curved, and substantially longer than the 1st rib, and its head has two facets for articulation with the bodies of the T1 and T2 vertebrae; its main atypical feature is a rough area on its upper surface, the tuberosity for serratus anterior, from which part of that muscle originates.

The 10th - 12th ribs, like the 1st rib, have only one facet on their heads and articulate with a single vertebra.

The 11th and 12th ribs are short and have no neck or tubercle.

Costal cartilages prolong the ribs anteriorly and contribute to the elasticity of the thoracic wall, providing a flexible attachment for their anterior or distal ends (tips). The cartilages increase in length through the first 7 and then gradually decrease. The first 7 cartilages attach directly and independently to the sternum; the 8th, 9th, and 10th articulate with the costal cartilages just superior to them, forming a continuous, articulated, cartilaginous costal margin. The 11th and 12th cartilages form caps on the anterior ends of these ribs and do not reach or attach to any other bone or cartilage. The costal cartilages of ribs 1-10 clearly anchor the anterior end (tips) of the rib to the sternum, limiting its overall movement as the posterior end rotates around the transverse axis of the rib.

Typical ribs. A. The 3rd- 9th ribs have common characteristics. Each rib has a head, neck, tubercle, and body (shaft). B. Cross section of a rib at midbody.

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Atypical ribs. The atypical 1st, 2nd, 11th, and 12th ribs differ from typical ribs (e.g., the 8th rib, shown in center). The 1st rib is short and flattened and the tubercle merges with the angle. The body of the 2nd rib bears a tuberosity for attachment of the serratus anterior. The 11th and 12th ribs lack a neck and tubercle, and the 12th rib is shorter than most ribs.

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Intercostal spaces separate the ribs and their costal cartilages from one another (Fig. 1.1A). The spaces are named according to the rib forming the superior border of the space—for example, the 4th intercostal space lies between rib 4 and rib 5. There are 11 intercostal spaces and 11 intercostal nerves. Intercostal spaces are occupied by intercostal muscles and membranes, and two sets (main and collateral) of intercostal blood vessels and nerves, identified by the same number assigned to the space. The space below the 12th rib does not lie between ribs and thus is referred to as the subcostal space, and the anterior ramus of spinal nerve T12 is the subcostal nerve. The intercostal spaces are widest anterolaterally, and they widen with inspiration. They can be further widened by extension and/or lateral flexion of the thoracic vertebral column to the contralateral side.

THORACIC VERTEBRAE

Thoracic vertebrae are typical vertebrae in that they are independent, have bodies, vertebral arches, and seven processes for muscular and articular connections. Characteristic features of thoracic vertebrae include

Bilateral costal facets (demifacets) on their bodies, usually occurring in inferior and superior pairs, for articulation with the heads of ribs.

Costal facets on their transverse processes for articulation with the tubercles of ribs, except for the inferior two or three thoracic vertebrae.

Long, inferiorly slanting spinous processes.

Superior and inferior costal facets, most of which actually occur in the form of demifacets, make up only a portion of a compound articular surface. They occur as bilaterally paired, planar surfaces on the superior and inferior posterolateral margins of typical (T2-T9) thoracic vertebrae. Functionally, the facets are arranged in pairs on adjacent vertebrae, flanking an interposed IV disc: an inferior (demi) facet of the superior vertebra and a superior (demi)facet of the inferior vertebra. Typically, two demi facets paired in this manner and the posterolateral margin of the IV disc between them form a single socket to receive the head of the rib that is assigned the same number as the inferior vertebra. Atypical thoracic vertebrae bear whole costal facets in place of demi facets:

The superior costal facets of vertebra T1 are not demifacets because there are no demifacets on the C7 vertebra above, and rib 1 articulates only with vertebra T1. T1 has a typical inferior costal (demi)facet.

T10 has only one bilateral pair of (whole) costal facets, located partly on its body and partly on its pedicle.

T11 and T12 also have only a single pair of (whole) costal facets, located on their pedicles.

The spinous processes projecting from the vertebral arches of typical thoracic vertebrae are long and slope inferiorly, usually overlapping the vertebra below. They cover the intervals

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between the laminae of adjacent vertebrae, thereby preventing sharp objects such as a knife from entering the vertebral canal and injuring the spinal cord. The superior articular facets of the superior articular processes face mainly posteriorly and slightly laterally, whereas the inferior articular facets of the inferior articular processes face mainly anteriorly and slightly medially. The bilateral joint planes between the respective articular facets of the adjacent vertebrae describe an arc, centering on an axis of rotation within the vertebral body. Thus small rotatory movements are permitted between adjacent vertebra, but they are limited by the attached rib cage.

Thoracic vertebrae. A. T1 has a vertebral foramen and body similar to a cervical vertebra. B. T5–T9 vertebrae have typical characteristics of thoracic vertebrae. C. T12 has bony processes and a body size similar to a lumbar vertebra. The planes of the articular facets of thoracic vertebrae define an arc (red arrows) that centers on an axis traversing the vertebral bodies vertically. D. Superior and inferior costal facets (demifacets) on the vertebral body, costal facets on the transverse processes, and long sloping spinous processes are characteristic of thoracic vertebrae.

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Costovertebral articulations of a typical rib. The costovertebral joints include the joint of head of rib, in which the head articulates with two adjacent vertebral bodies and the intervertebral disc between them, and the costotransverse joint, in which the tubercle of the rib articulates with the transverse process of a vertebra. The rib moves (elevates and depresses) around an axis that traverses the head and neck of the rib (arrows).

THE STERNUM

The sternum is the flat, elongated bone that forms the middle of the anterior part of the thoracic cage. The sternum consists of three parts: manubrium, body, and xiphoid process.The manubrium is a roughly trapezoidal bone. The manubrium is the widest and thickest of the three parts of the sternum. The easily palpated concave center of the superior border of the manubrium is the jugular notch (suprasternal notch). The notch is deepened in an articulated skeleton (and in life) by the medial (sternal) ends of the clavicles, which are much larger than the relatively small clavicular notches in the manubrium that receive them, forming the sternoclavicular (SC) joints. Inferolateral to the clavicular notch, the costal cartilage of the 1st rib is tightly attached to the lateral border of the manubrium the synchondrosis of the first rib. The manubrium and body of the sternum lie in slightly different planes superior and inferior to their junction, the manubriosternal joint hence, their junction forms a projecting sternal angle (of Louis).The body of the sternum, which is longer, narrower, and thinner than the manubrium, is located at the level of the T5-T9 vertebrae. Its width varies because of the scalloping of its lateral borders by the costal notches. In young people, four sternebrae (primordial segments of the sternum) are obvious. The sternebrae articulate with each other at primary cartilaginous joints (sternal synchondroses). These joints begin to fuse from the inferior end between puberty (sexual maturity) and age 25. The nearly flat anterior surface of the body of the

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sternum is marked in adults by three variable transverse ridges which represent the lines of fusion (synostosis) of its four originally separate sternebrae.The xiphoid process, the smallest and most variable part of the sternum, is thin and elongated. It lies at the level of T10 vertebra. Although often pointed, the process may be blunt, bifid, curved, or deflected to one side or anteriorly. It is cartilaginous in young people but more or less ossified in adults older than age 40. In elderly people, the xiphoid process may fuse with the sternal body.The xiphoid process is an important landmark in the median plane because

Its junction with the sternal body at the xiphisternal joint indicates the inferior limit of the central part of the thoracic cavity projected onto the anterior body wall; this joint is also the site of the infrasternal angle (subcostal angle) of the inferior thoracic aperture.

It is a midline marker for the superior limit of the liver, the central tendon of the diaphragm, and the inferior border of the heart.

Sternum. A. The felt-like covering of the sternum—formed as the thin, broad membranous bands of the radiate sternocostal ligaments pass from the costal cartilages to the anterior and posterior surfaces of the sternum—is shown on the upper right side. B. Observe the thickness of the superior third of the manubrium between the clavicular notches. C. The relationship of the sternum to the vertebral column is shown.

THORACIC APERTURES

While the thoracic cage provides a complete wall peripherally, it is open superiorly and inferiorly. The much smaller superior opening is a passageway that allows communication with the neck and upper limb. The larger inferior opening provides the ring-like origin of the diaphragm, which completely occludes the opening, and the excursions of which primarily

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control the volume/internal pressure of the thoracic cavity, providing the basis for tidal respiration.

Superior Thoracic ApertureThe superior thoracic aperture, the anatomical thoracic inlet, is bounded as follows:

Posteriorly, by vertebra T1 (protruding or convex posterior boundary/landmark).

Laterally, by the 1st pair of ribs and their costal cartilages.

Anteriorly, by the superior border of the manubrium (anterior landmark).

Structures that pass between the thoracic cavity and the neck through the oblique, kidney-shaped superior thoracic aperture include the trachea, esophagus, nerves, and vessels that supply and drain the head, neck, and upper limbs. The adult superior thoracic aperture measures approximately 6.5 cm anteroposteriorly and 11 cm transversely. To visualize the size of this opening, note that it is slightly larger than necessary to allow the passage of a 2—4 piece of lumber. Because of the obliquity of the 1st pair of ribs, the aperture slopes anteroinferiorly.

INFERIOR THORACIC APERTURE

The inferior thoracic aperture, the anatomical thoracic outlet, is bounded as follows:

Posteriorly, by the 12th thoracic vertebra (posterior landmark).

Posterolaterally, by the 11th and 12th pairs of ribs.

Anterolaterally, by the joined costal cartilages of ribs 7- 10, forming the costal margins.

Anteriorly, by the xiphisternal joint (anterior landmark).

The inferior thoracic aperture is much more spacious than the superior thoracic aperture and is irregular in outline. It is also oblique because the posterior thoracic wall is much longer than the anterior wall. In closing the inferior thoracic aperture, the diaphragm separates the thoracic and abdominal cavities almost completely. Structures passing from or to the thorax from the abdomen pass through openings that traverse the diaphragm (e.g., esophagus and inferior vena cava), or pass posterior to it (e.g., aorta).Just as the size of the thoracic cavity (or its contents) is often overestimated, its inferior extent (corresponding to the boundary between thoracic and abdominal cavities) is often incorrectly estimated because of the discrepancy between the inferior thoracic aperture and the location of the diaphragm (the floor of the thoracic cavity) in living persons. Although the diaphragm takes origin from the structures that make up the inferior thoracic aperture, the domes of the diaphragm rise to the level of the 4th intercostal space, and abdominal visceral, including the large liver, spleen, and stomach, lie superior to the plane of the inferior thoracic aperture, within the thoracic wall.

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JOINTS OF THE THORACIC WALL

Although movements of the joints of the thoracic wall are frequent for example, in association with normal respiration the range of movement at the individual joints is relatively small; nonetheless, any disturbance that reduces the mobility of these joints interferes with respiration. During deep breathing, the excursions of the thoracic cage (anteriorly, superiorly, or laterally) are considerable. Extending the vertebral column farther increases the anteroposterior (AP) diameter of the thorax. Joints of the thoracic wall occur between the

Vertebrae (intervertebral joints).

Ribs and vertebrae (costovertebral joints: joints of heads of ribs and costotransverse joints).

Ribs and costal cartilages (costochondral joints).

Costal cartilages (interchondral joints).

Sternum and costal cartilages (sternocostal joints).

Sternum and clavicle (sternoclavicular joints).

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Parts of the sternum (manubriosternal and xiphisternal joints) in young people the manubriosternal (and sometimes the xiphisternal) joint usually fuses in the elderly.

MOVEMENTS OF THE THORACIC WALL

Movements of the thoracic wall and diaphragm during inspiration produce increases in the intrathoracic volume and diameters of the thorax. Consequent pressure changes result in air being alternately drawn into the lungs (inspiration) through the nose, mouth, larynx, and trachea and expelled from the lungs (expiration) through the same passages. During passive expiration, the diaphragm, intercostal muscles, and other muscles relax, decreasing intrathoracic volume and increasing the intrathoracic pressure. Concurrently, intra-abdominal pressure decreases and abdominal viscera are decompressed. This allows the stretched elastic tissue of the lungs to recoil, expelling most of the air.The vertical dimension (height) of the central part of the thoracic cavity increases during inspiration as contraction of the diaphragm causes it to descend, compressing the abdominal viscera. During expiration, the vertical dimension returns to the neutral position as the elastic recoil of the lungs produces sub-atmospheric pressure in the pleural cavities, between the lungs and the thoracic wall. As a result of this and the absence of resistance to the previously compressed viscera, the domes of the diaphragm ascend, diminishing the vertical dimension. The AP dimension of the thorax increases considerably when the intercostal muscles contract: Movement of the ribs (primarily 2nd- 6th) at the costovertebral joints around an axis passing through the necks of the ribs causes the anterior ends of the ribs to rise the pump-handle movement. Because the ribs slope inferiorly, their elevation also results in anterior posterior movement of the sternum, especially its inferior end, with slight movement occurring at the manubriosternal joint in young people, in whom it has not yet synostosed.The transverse dimension of the thorax also increases slightly when the intercostal muscles contract, raising the middle (lateral-most parts) of the ribs the bucket-handle movement. The combination of all these movements moves the thoracic cage anteriorly, superiorly, and laterally

MUSCLES OF THE THORACIC WALL

Several upper limb (thoracoappendicular) muscles attach to the thoracic cage including the pectoralis major, pectoralis minor, subclavius, and serratus anterior muscles anteriorly and latissimus dorsi muscles posteriorly as do the anterolateral abdominal muscles and some back and neck muscles. The thoracoappendicular muscles usually act on the upper limbs. But several, including the pectoralis major and pectoralis minor and the inferior part of the serratus anterior may also function as accessory muscles of respiration, helping elevate the ribs to expand the thoracic cavity when inspiration is deep and forceful. The scalene muscles of the neck, which descend to the 1st and 2nd ribs, also serve as accessory respiratory muscles by fixing these ribs and enabling the muscles connecting the ribs below to be more effective in elevating the lower ribs during forced inspiration.The serratus posterior, levatores costarum, intercostal, subcostal, and transverse thoracic are muscles of the thoracic wall.

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The serratus posterior muscles, extending from the vertebrae to the ribs, have traditionally been described as inspiratory muscles, but this function is not supported by electromyography or other evidence. The serratus posterior superior lies at the junction of the neck and back. It arises from the inferior part of the nuchal ligament in the neck and the spinous processes of the C6 or C7 through T2 or T3 vertebrae. This muscle runs inferolaterally and attaches by a series of digitations to the superior borders of the 2nd - 5th ribs, immediately lateral to their angles. On the basis of its attachments and disposition, the serratus posterior superior was said to elevate the superior four ribs, thus increasing the AP diameter of the thorax and raising the sternum.The serratus posterior inferior lies at the junction of the thoracic and lumbar regions. It arises from the spinous processes of the last two thoracic spinous processes, the first two lumbar spinous processes, and the thoracolumbar fascia. It runs superolaterally and attaches to the inferior borders of the inferior three or four ribs, also just lateral to their angles. On the basis of its attachments and disposition, the serratus posterior inferior was said to depress the inferior ribs, preventing them from being pulled superiorly by the diaphragm. However, recent studies suggest that these muscles, which span the superior and inferior thoracic apertures as well as the transitions from the relatively inflexible thoracic vertebral column to the much more flexible cervical and lumbar segments of the column, may not be primarily motor in function. Rather, they may have a proprioceptive function. These muscles, particularly the superior one, have been implicated as a source of chronic pain in myofascial pain syndromes.

Thoracoappendicular and anterolateral abdominal muscles overlying thoracic wall. The pectoralis major has been removed on the left side to expose the pectoralis minor, subclavius, and external intercostal muscles. When the upper limb muscles are removed, the domed shape of the thoracic cage is revealed.

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The levatores costarum muscles are attached to the transverse processes of the C7 and T1-T11 vertebrae and pass inferolaterally to attach to the ribs, close to their tubercles. As their name indicates, these 12 fan-shaped muscles elevate the ribs; but their role, if any, in normal inspiration is uncertain. They may play a role in vertebral movement and/or proprioception.The intercostal muscles occupy the intercostal spaces. The superficial layer is formed by the external intercostals, the inner layer by the internal intercostals. The deepest fibers of the latter, lying internal to the intercostal vessels are somewhat artificially designated as a separate muscle, the innermost intercostals.

The external intercostal muscles (11 pairs) occupy the intercostal spaces from the tubercles of the ribs posteriorly to the costochondral junctions anteriorly. Anteriorly, the muscle fibers are replaced by the external intercostal membranes. These muscles run inferoanteriorly from the rib above to the rib below. Each muscle attaches superiorly to the inferior border of the rib above and inferiorly to the superior border of the rib below. These muscles are continuous inferiorly with the external oblique muscles in the anterolateral abdominal wall. The external intercostals are most active during inspiration; they maintain or increase the tonus of the intercostal space. They elevate the ribs during forced inspiration.

The internal intercostal muscles (11 pairs) run deep to and at right angles to the external intercostals. Their fibers run inferoposteriorly from the floors of the costal grooves to the superior borders of the ribs inferior to them. The internal intercostals attach to the bodies of the ribs and their costal cartilages as far anteriorly as the sternum and as far posteriorly as the angles of the ribs. Between the ribs posteriorly, medial to the angles, the internal intercostals are replaced by the internal intercostal membranes. The inferior internal intercostals are continuous with the internal oblique muscles in the anterolateral abdominal wall. The internal intercostals—weaker than the external intercostal muscles are most active during expiration; they maintain or increase the tonus of the intercostal space. Their interosseous (vs. interchondral)

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portions may depress ribs during forced respiration. The interchondral portion of the internal intercostals appears to act with the external intercostals during active inspiration.

The innermost intercostal muscles are similar to the internal intercostals and are essentially their deeper parts. The innermost intercostals are separated from the internal intercostals by the intercostal nerves and vessels (Fig. 1.13). These muscles pass between the internal surfaces of adjacent ribs and occupy the lateralmost parts of the intercostal spaces. It is likely (but undetermined) that their actions are the same as those of the internal intercostal muscles.

Posterior aspect of anterior thoracic wall. The internal thoracic arteries arise from the subclavian arteries and have paired accompanying veins (L. venae comitantes) inferiorly. Superior to the 2nd costal cartilage, there is only a single internal thoracic vein on each side, which drains into the brachiocephalic vein. The continuity of the transverse thoracic with the transverse abdominal becomes apparent when the diaphragm is removed, as has been done here on the right side.

The subcostal muscles are variable in size and shape, usually being well developed only in the lower thoracic wall. These thin muscular slips extend from the internal surface of the angle of one rib to the internal surface of the second or third rib inferior to it. Crossing one or

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two intercostal spaces, the subcostals run in the same direction as the internal intercostals and blend with them. Thus it is assumed that the subcostals act with the internal intercostals, and that they may depress ribs.The transverse thoracic muscles, consist of four or five slips that attach posteriorly to the xiphoid process, the inferior part of the body of the sternum, and the adjacent costal cartilages. They pass superolaterally and attach to the 2nd - 6th costal cartilages. The transverse thoracic muscles are continuous inferiorly with the transverse abdominal muscles in the anterolateral body wall. These muscles appear to have a weak expiratory function, drawing downward on the costal cartilages to which they attach. They may provide proprioceptive information.Although the external and internal intercostals are active during inspiration and expiration, respectively, most activity is isometric; the role of these muscles in producing movement of the ribs appears to be related mainly to forced respiration. The diaphragm is the primary muscle of inspiration. Expiration is passive unless one is exhaling against resistance (e.g., inflating a balloon) or trying to expel air more rapidly than usual (e.g., coughing, sneezing, blowing one's nose, or shouting), the elastic recoil of the lungs and decompression of abdominal viscera expel previously inhaled air. The primary role of the intercostal muscles in respiration is to support (increase the tonus or rigidity of) the intercostal space, resisting paradoxical movement especially during inspiration when internal thoracic pressures are lowest (most negative). This is most apparent following a high spinal cord injury, when there is an initial flaccid paralysis of the entire trunk but the diaphragm remains active. In these circumstances, the vital capacity is markedly compromised by the paradoxical incursion of the thoracic wall during inspiration. Several weeks later, the paralysis becomes spastic; the thoracic wall stiffens and vital capacity rises.The mechanical action of the intercostal muscles in rib movement, especially during forced respiration, can be appreciated by means of a simple model. A pair of curved levers, representing the ribs bordering an intercostal space, are hinged posteriorly to a fixed, vertical (vertebral) column; their anterior ends (costal cartilages) are hinged to a moveable, vertical column (the sternum). The ribs (and intervening intercostal space) descend as they run anteriorly, reaching their low point approximately at the costochondral junction, and then ascend to the sternum. The concentric contraction of the muscle fibers that most closely approximate the slope of the ribs at their attachments rotate the ribs superiorly at their posterior axes, elevating the ribs and sternum. The concentric contraction of muscle fibers that are approximately perpendicular to the slope of the ribs at their attachment rotate the ribs inferiorly at their posterior axes, depressing the ribs and sternum. Fibers A represent the external intercostal muscles, and fibers B represent the interosseous portion of the internal intercostal muscles. Fibers A run nearly perpendicular to fibers B. Thus the external intercostal muscles elevate and the interosseous internal intercostal muscles depress the ribs. Note that even though the fibers of the interchondral portion of the internal interosseous muscles (fibers C) run parallel to the fibers of the interosseous portion (fibers B), they approximate the slope of the ribs (instead of being perpendicular to the slope). Therefore, the interchondral portion of the internal intercostal muscles (fibers C) act with the external intercostal muscles (fibers A) to elevate the ribs.The diaphragm is a shared wall (actually floor/ceiling) separating the thorax and abdomen. Although it has functions related to both compartments of the trunk, its most important (vital) function is serving as the primary muscle of inspiration.

FASCIA OF THE THORACIC WALL

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Each part of the deep fascia is named for the muscle it invests or the structure(s) to which it is attached. Consequently, a large portion of the deep fascia overlying the anterior thoracic wall, forming a major part of the bed of the breast is called pectoral or pectoralis fascia for its association with the pectoralis major muscles. Deep to the pectoralis major and its fascia is another layer of deep fascia suspended from the clavicle and investing the pectoralis minor muscle, the clavipectoral fascia. The thoracic cage is lined internally with endothoracic fascia. This thin fibroareolar layer attaches the adjacent portion of the lining of the lung cavities to the thoracic wall. It becomes more fibrous over the apices of the lungs as the suprapleural membrane.