Arthur F. Dalley (II) - Moores Clinically Oriented Anatomy

CLINICAL BOX KEY

The thorax is the part of the body between the neck and abdomen. Commonly, the term chest is used as a synonym for thorax; however, the chest is much more extensive than the thoracic wall and cavity contained within it. The chest is generally conceived as the superior part of the trunk that is broadest superiorly owing to the presence of the pectoral (shoulder) girdle (clavicles and scapulae), with much of its girth accounted for by the pectoral and scapular musculature and, in adult females, the breasts.

The thoracic cavity and its wall have the shape of a truncated cone, being narrowest superiorly, with the circumference increasing inferiorly, and reaching its maximum size 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 cage (rib cage), with the oblique (near horizontal) bars formed by ribs and costal cartilages, is also supported by the vertical sternum and thoracic vertebrae (). Furthermore, the floor of the thoracic cavity (thoracic 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 viscera (e.g., liver) rather than thoracic viscera. Thus, the thorax and its cavity are much smaller than one might expect based on the external appearance 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 that houses the thoracic viscera except for the lungs and, on each side, the right and left pulmonary cavities housing the lungs.

The majority of the thoracic cavity is occupied by the lungs, which provide for the exchange of oxygen and carbon dioxide between the air and blood. Most of the remainder of the thoracic cavity is occupied by the heart and structures involved in conducting the air and blood to and from the lungs. Also, the esophagus, a tubular structure carrying nutrients (food) to the stomach, traverses the thoracic cavity.

In terms of function and development, the breasts are related to the reproductive system; however, the breasts are located on and typically dissected with the thoracic wall and therefore are included in this chapter.

The true thoracic wall includes the thoracic cage and the muscles that extend between the ribs 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. The anterolateral axio-appendicular muscles (see ) that overlie the thoracic cage and form the bed of the breast are encountered in the thoracic wall and may be considered part of it but are distinctly upper limb muscles based on function and innervation. They are mentioned only briefly here.

The dome shape of the thoracic cage provides remarkable rigidity, given the light weight of its components, enabling it to perform the following functions:

Protect vital thoracic and abdominal organs (most air or fluid filled) from external forces

Resist the negative (subatmospheric) 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 (origin) 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 dome shape of the thoracic cage provides rigidity, its joints and the thinness and flexibility of the ribs allow it to absorb external blows and compressions without fracture and to change its shape for respiration. Because the most important structures within the thorax (heart, 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. This is accomplished first by expanding the capacity of the cavity, thereby causing the lungs to expand and draw air in, and then, due to lung elasticity and muscle relaxation, decreasing the volume of the cavity and causing them to expel air.

The Bottom Line: Overview of Thorax

The thorax, consisting of the thoracic cavity, its contents, and the wall that surrounds it, is the part of the trunk between the neck and abdomen. The shape and size of the thoracic cavity and thoracic wall are different from that of the chest (upper trunk or torso) because the latter includes some proximal upper limb bones and muscles and, in adult females, the breasts. The thorax includes the primary organs of the respiratory and cardiovascular systems. The thoracic cavity is divided into three compartments: the central mediastinum, occupied by the heart and structures transporting air, blood, and food, and the right and left pulmonary cavities, occupied by the lungs.

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; both are identified numerically, from the most superior (1st rib or costal cartilage) to the most inferior (12th).

Ribs (L. costae ) 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 ribs that can be classified as typical or atypical: