Overview of Thorax
The thorax is the part of the body between the neck and abdomen. Commonly the term chest is used as a synonym for thorax, but 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 skeleton takes the form of a domed birdcage. The thoracic cage (rib cage), with the horizontal bars formed by ribs and costal cartilages, is also supported by the vertical sternum (breastbone) and thoracic vertebrae (Fig. 1.1). 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 rather than thoracic viscera (e.g., liver). Thus the thorax and its cavity are much smaller than one might expect based on external appearances of the chest.
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, one side of which is included here to demonstrate the relationship between the thoracic (axial) and upper limb (appendicular) skeletons. The red dotted line indicates the position of the diaphragm, which separates the thoracic and abdominal cavities.
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. Additionally, nutrients (food) traverse the thoracic cavity via the esophagus, passing from the site of entry in the head to the site of digestion and absorption in the abdomen.
Although in terms of function and development the mammary glands are most related to the reproductive system, the breasts are located on and are typically dissected with the thoracic wall; thus they are included in this chapter.