Thorax – Gross Anatomy

The mediastinum is the thoracic area between the 2 pleural cavities. The mediastinum contains vital structures of the circulatory, respiratory, digestive, and nervous systems including the heart and esophagus, and major thoracic vessels including the superior vena cava, inferior vena cava, pulmonary arteries, pulmonary veins, and aorta. The mediastinum extends from the upper thoracic aperture to the diaphragm and is bordered by the lungs.

The chest wall consists of skin, fat, muscles, bones, and cartilage. The bony structure of the chest wall is composed of the ribs, sternum, and thoracic vertebrae. The chest wall serves as armor for the vital intrathoracic organs and provides the stability necessary for the movement of the shoulders and arms. The interconnections between the bones, cartilage, and muscles allow for the rhythmic expansion and reduction of the chest wall during breathing, which facilitates changes in intrathoracic pressure to allow expansion of the lungs during inspiration. The extrinsic muscles have 2 bony attachments; the intrinsic muscles only attach to the thoracic skeleton.

The diaphragm is a large, dome-shaped muscle that separates the thoracic cavity from the abdominal cavity. The diaphragm consists of muscle fibers and a large central tendon, which is divided into right and left parts. As the primary muscle of inspiration, the diaphragm contributes 75% of the total inspiratory muscle force, and its contraction leads to flattening of the dome of the diaphragm. This flattening increases the volume of the thoracic cavity and allows the lungs to expand during inspiration.

The breasts are found on the anterior thoracic wall and consist of mammary glands surrounded by connective tissue. The mammary glands are modified apocrine sweat glands that produce milk, which serves as nutrition for infants. Breasts are rudimentary and usually nonfunctioning in men. The shape and size of the breasts change during a woman’s life and menstrual cycles. Breasts are supplied by the axillary, internal thoracic, and intercostal arteries, and they are innervated by branches of the cervical plexus.

The back is composed of several muscles of varying sizes and functions, which are grouped into intrinsic (or primary) back muscles and extrinsic (or secondary) back muscles. This division is based on the functionality and embryologic origin of these muscle groups. The extrinsic muscles comprise the superficial and intermediate muscle groups, while the intrinsic muscles comprise the deep muscles. The deep muscles are further subdivided into superficial, intermediate, and deep muscle layers.

The human spine, or vertebral column, is the most important anatomical and functional axis of the human body. It consists of 7 cervical vertebrae, 12 thoracic vertebrae, and 5 lumbar vertebrae and is limited cranially by the skull and caudally by the sacrum. The vertebral column is the foundation for the trunk, provides an attachment for muscles and ligaments, and protects the spinal cord and nerve roots.