The chest or thorax occupies the upper part of the trunk in front and is a dome-shaped cavity containing and protecting the heart and lungs. Its walls are formed by the dorsal vertebræ at the back, the ribs at either side, and the sternum and costal cartilages in front, all well covered with muscles. The floor is formed by the diaphragm. Through the upper opening of the chest pass the trachea, the esophagus, and many important vessels and nerves.

The shape of the chest may vary in disease. Thus, in rickets there is the prominent “pigeon” breast and the rosary, that is, a bead at the juncture of each rib with the costal cartilage, while in emphysema the chest is enlarged in all directions and barrel-shaped. In severe cases of lateral curvature it is distorted but may be improved by exercises.

The Sternum.—The sternum or breast-bone is a long narrow bone and has three parts, the manubrium or handle above, the gladiolus or sword, and the ensiform cartilage at the lower end. On either side are notches for the costal cartilages; for the first seven ribs as well as the clavicle articulate with it. Except for some muscles along the edges it lies directly under the skin and the ridge between the manubrium and the gladiolus can be felt in the living, a fact which assists in determining the position of the different ribs in cases of fracture, as the second rib articulates at this point.

The Ribs.—The ribs are twenty-four in number, twelve on each side, of which the upper seven, which articulate with the sternum by individual cartilages, are called true ribs, the other five false ribs. Of the false ribs the upper three articulate indirectly with the sternum through the seventh cartilage, with which their cartilages unite, while the other two have their anterior extremities free and are called floating ribs. All the ribs slope down toward the front and are by nature more freely movable in women than in men. Most of the ribs have a head divided by a little ridge into two facets for articulation with the dorsal vertebræ, a flattened neck, a tuberosity at the base of the neck with a facet for articulation with the transverse process of the vertebra below, an angle, and a shaft, which is externally convex and is grooved on its lower edge for the intercostal vessels and nerve. The first and second, eleventh and twelfth ribs, however, are somewhat peculiar, the first two being shorter, flatter and rather broader than the rest and the first having only one facet on the head, while the last two have only one facet on the head and no neck or tuberosity.

The costal cartilages serve to prolong the ribs and greatly increase the elasticity of the chest wall. They grow longer down to the seventh and then decrease again in length.

The ribs, except the first and second, which are protected by the clavicle, are frequently broken. Such a break causes pain in breathing and sometimes the end of a rib pierces the lung tissue and swelling all over the body results, due to the presence of air. Caries or death of the rib is also frequent. Fracture of the sternum occurs occasionally, generally from direct force, as from a blow with the knee in foot-ball, and there may be dislocation between the manubrium and gladiolus.

Muscles of the Chest.—The spaces between the ribs, from the tubercle of the rib behind to the cartilage in front, are filled by the external intercostal muscles, which pass downward and forward from the lower border of one rib to the upper border of the one below. There are, therefore, eleven pairs of these muscles. There are also eleven pairs of the internal intercostals, which commence at the sternum and extend back to the angle of the rib. These extend downward and backward. The external intercostals raise and evert the ribs in inspiration, the internal depress and invert them in expiration.

The chief respiratory muscle, however, is the diaphragm, a somewhat fan-shaped muscle that forms the floor of the chest cavity. It takes its origin from the ensiform cartilage, the six or seven lower ribs and their cartilages, and from the upper three or four lumbar vertebræ, that is, from the whole of the internal circumference of the thorax, and is inserted into the central cordiform tendon. It has several large and several small openings for the aorta, the esophagus, the venæ cavæ, the thoracic duct, and various nerves, and its surfaces are covered by serous membranes, by the two pleuræ and the pericardium above and by the peritoneum below. It partially supports the heart and lungs. Convex toward the chest, it becomes flattened in contraction and so increases the capacity of the chest. It aids in all expulsive acts, as sneezing, coughing, laughing, urinating, defecating, vomiting, and childbirth. Hiccough is spasm of the diaphragm.

The arteries of the chest are the intercostal branches of the subclavian and the thoracic aorta, the phrenic, mediastinal, and intercostal branches of the internal mammary, and the thoracic branches of the axillary.

The nerves are the intercostals and phrenics.

Mammary Glands.—On the outside of the chest walls, lodged in the fascia of the pectoral muscles, are the mammary glands, accessory organs of the generative system. They exist in both sexes but are only rudimentary in the male. In the female they are small before puberty but enlarge as the generative organs become more completely developed, forming two hemispherical eminences, one on either side, between the third and seventh ribs. During pregnancy they increase once more in size preparatory to the secretion of the milk, and in old age they atrophy. From the middle projects a small pinkish-brown conical eminence, the nipple, surrounded by a paler area, the areola. After the second month of pregnancy both nipple and areola become darker in color, a point of great diagnostic value in early pregnancy.

The mammary glands themselves consist of lobules of gland tissue with a central lactiferous tubule, the lobules being gathered into lobes with fatty tissue between. From the juncture of these tubules result fifteen or twenty excretory ducts, the tubuli lactiferi, which converge toward the areola. Beneath the nipple they dilate, forming the ampullæ, and then contract again to pass out through the nipple as straight tubes.

Breast abscess occurs most commonly in nursing mothers, as where a part is most active there is most danger of abscess. Many benign tumors of the breast, as the fibrous tumors, occur and are especially common in young women. If a fibrous tumor is allowed to develop it may become cancerous. Cancer, however, generally occurs after the age of forty and is usually due to some irritation, as to a blow from a ball.

The arteries of the breasts are the thoracic branches of the axillary, the intercostal, and the internal mammary.

The nerves are from the thoracic cutaneous.