Overview Of The Thorax

Anatomically, the thorax is typically divided into compartments; there are two bilateral pulmonary cavities; each contains a lung with its pleural covering (Fig. 1). The space between the pleural cavities is the mediastinum, which contains all the other structures found in the thorax. The mediastinum is divided into the superior and inferior compartments by a plane referred to as the "transverse thoracic plane"; it passes through the mediastinum at the level of the sternal angle and the junction of the T4 and T5 vertebrae (Fig. 1).

Fig. 1. The left panel is a diagrammatic representation of the pulmonary cavities, one on each side of the thorax with the mediastinum between. The right panel illustrates the divisions of the mediastinum. Adapted from Figs. 1.14 (left) and 1.24 of Grant's Dissector, 12th Ed., E. K. Sauerland (ed.). © 1999 Lippincott, Williams, and Wilkins, Philadelphia, PA.

Fig. 1. The left panel is a diagrammatic representation of the pulmonary cavities, one on each side of the thorax with the mediastinum between. The right panel illustrates the divisions of the mediastinum. Adapted from Figs. 1.14 (left) and 1.24 of Grant's Dissector, 12th Ed., E. K. Sauerland (ed.). © 1999 Lippincott, Williams, and Wilkins, Philadelphia, PA.

Fig. 2. The left panel illustrates the bones of the thorax from a posterior view. The right panel is an anterior view of the bony thorax.

The superior mediastinum contains the major vessels supplying the upper extremity, the neck, and the head. The inferior mediastinum, the space between the transverse thoracic plane and the diaphragm, is further divided into the anterior, middle, and posterior mediastinum. The middle mediastinum is the space containing the heart and pericardium. The anterior mediastinum is the space between the pericardium and the sternum. The posterior mediastinum extends from the pericardium to the posterior wall of the thorax.

The inferior aperture of the thorax is formed by the lower margin of the ribs and costal cartilages and is closed off from the abdomen by the respiratory diaphragm (Fig. 1). The superior aperture of the thorax leads to the neck and the upper extremity. It is formed by the first ribs and their articulation with the manubrium and first thoracic vertebra. The root of the neck is open to the superior aperture of the thorax, and numerous structures pass from the neck to the thoracic cavity. The clavicle crosses the first rib at its anterior edge close to its articulation with the manubrium. Structures exiting the superior thoracic aperture and communicating with the upper extremity pass between the first rib and clavicle.

3. BONES OF THE THORACIC WALL 3.1. The Thoracic Cage

The skeleton of the thoracic wall is composed of the 12 ribs, the thoracic vertebra and intervertebral discs, and the sternum. Attached to the thorax are the bones of the pectoral girdle, the clavicle and the scapula (Fig. 2). Of these, the clavicle is particularly important because it forms, with the first rib, the thoracic outlet to the upper extremity.

Fig. 3. The T6 vertebra as viewed from above (upper left) and laterally (upper right) and a typical rib (bottom).

Costal groove'

Fig. 3. The T6 vertebra as viewed from above (upper left) and laterally (upper right) and a typical rib (bottom).

The thoracic vertebrae comprise the middle portion of the posterior wall of the thorax. Each thoracic vertebra has a body anteriorly, two pedicles and two lamina that together form an arch creating the vertebral foramen; a relatively long spinous process projecting posteriorly and inferiorly; and two transverse processes projecting laterally and somewhat posteriorly (Fig. 3). Each thoracic vertebra articulates with at least one rib.

The first through 9th thoracic vertebrae have a set of costal facets on their bodies for articulation with the head of the rib. These costal facets are also called demifacets. The superior demifacet articulates with the head of the rib of the same number as the vertebra. The inferior demifacet articulates with the head of the rib below. The head of rib 1 articulates only with the T1 vertebra. Thus, this vertebra has a single facet for articulation with rib 1 and a demifacet for articulation with rib 2. The heads of ribs 10-12 articulate only with the vertebra of the same number. The articular facet on vertebrae T10-T12 is located at the junction of the body and pedicle (T10) or fully on the pedicle (T11 and T12). The first 10 thoracic vertebrae also have costal facets on their transverse processes for articulation with the tubercles of the ribs of the same number. The transverse processes of the thoracic vertebrae get progressively shorter, and the transverse processes of T11 and T12 do not articulate with the tubercles of their respective ribs.

The ribs form the largest part of the bony wall of the thorax (Fig. 2). Each rib articulates with one or two thoracic vertebrae, and the upper 10 ribs articulate directly or indirectly with the sternum anteriorly. The upper 7 ribs are referred to as "true" ribs because each connects to the sternum via its own costal cartilage. Ribs 8-10 are referred to as "false" ribs because they connect indirectly to the sternum. Each of these ribs is connected to the rib immediately above via their costal cartilage and ultimately to the sternum via the costal cartilage of the 7th rib. Ribs 11 and 12 are referred to as "floating" ribs because they do not connect to the sternum, but end in the musculature of the abdominal wall.

Each rib has a head that articulates with the thoracic vertebra and a thin flat shaft that is curved (Fig. 3). The costal angle, the sharpest part of the curved shaft, is located where the rib turns anteriorly. At the inferior margin of the shaft, the internal surface of the rib is recessed to form the costal groove. This depression provides some protection to the intercostal neu-rovascular bundle, something that must be considered when designing devices for intercostal access to the thorax. The heads of ribs 2-9 have two articular facets for articulation with the vertebra of the same level and the vertebra above. The heads of ribs 1, 10, 11, and 12 only articulate with the vertebra of the same number and consequently have only one articular facet. In ribs 1-10, the head is connected to the shaft by a narrowing called the "neck." At the junction of the head and the neck is a tubercle that has an articular surface for articulation with the costal facet of the transverse process. Ribs 11 and 12 do not articulate with the transverse process of their respective vertebra and do not have a tubercle or a neck portion.

The sternum is the flat bone that makes up the median anterior part of the thoracic cage (Fig. 2). It is composed of three parts: the manubrium, body, and xiphoid process. The manubrium (from the Latin word for handle, like the handle of a sword) is the superior part of the sternum; it is the widest and thickest part. The manubrium alone articulates with the clavicle and the first rib. The sternal heads of the clavicle can be readily seen and palpated at their junction with the manubrium. The depression between the sternal heads of the clavicle above the manubrium is the suprasternal, or jugular, notch.

The manubrium and the body of the sternum lie in slightly different planes and thus form a noticeable and easily palpated angle, the sternal angle (of Louis), at the point where they articulate. The second rib articulates with both the body of the sternum and the manubrium and can easily be identified just lateral to the sternal angle. The body of the sternum is formed from the fusion of segmental bones (the sternebrae). The remnants of this fusion can be seen in the transverse ridges of the sternal body, especially in young people. The third through sixth ribs articulate with the body of the sternum, and the seventh rib articulates at the junction of the sternum and xiphoid process.

The xiphoid process is the most inferior part of the sternum and is easily palpated. It lies at the level of thoracic vertebra 10 and marks the inferior boundary of the thoracic cavity anteriorly. It also lies at the level even with the central tendon of the diaphragm and the inferior border of the heart.

3.2. The Pectoral Girdle

Many of the muscles encountered on the wall of the anterior thorax are attached to the bones of the pectoral girdle and the upper extremity. Because movement of these bones can have an impact on the anatomy of vascular structures communicating between the thorax and upper extremity, it is important to include these structures in a discussion of the thorax.

The clavicle is a somewhat S-shaped bone that articulates at its medial end with the manubrium of the sternum and at its lateral end with the acromion of the scapula (Fig. 2). It is convex medially and concave laterally. The scapula is a flat triangular bone, concave anteriorly, that rests upon the posterior thoracic wall. It has a posterior raised ridge called the "spine" that ends in a projection of bone called the "acromion," which articulates with the clavicle. The coracoid process is an anterior projection of bone from the superior border of the clavicle that serves as an attachment point for muscles that act on the scapula and upper extremity. The head of the humerus articulates with the glenoid fossa of the scapula, forming the glenohumeral joint. The clavicle serves as a strut to hold the scapula in position away from the lateral aspect of the thorax. It is a highly mobile bone, with a high degree of freedom at the sternoclavicular joint that facilitates movement of the shoulder girdle against the thorax. The anterior extrinsic muscles of the shoulder pass from the wall of the thorax to the bones of the shoulder girdle.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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