Chest. The human body being commonly divided into head, trunk, and limbs, these again are subdivided into head and face, chest and abdomen, upper and lower limbs. The chest, then, is the upper portion of the trunk, to which are attached externally the breasts in front, and the arms and shoulders laterally and posteriorly, and the cavity of which contains internally the heart and lungs. The walls of the chest are composed mainly of the ribs and the muscles pertaining to the ribs; the dorsal portion of the spinal column completes the walls of the chest behind, and the sternum completes them in front. The neck connects the chest with the' head, and the midriff or diaphragm divides internally the cavity of the chest from that of the abdomen. The upper portion of the chest, between the spine, first ribs, and the sternum, forms an aperture through which the trachea passes into the chest, and the great arteries and veins of the head, neck, and arms pass out of the chest from the heart and main vessels. The spaces between these vessels and the bones are occupied by the oesophagus, by certain nerves and muscles, and by what is termed cellular or connective tissue.

The whole chest thus forms a cavity closed on all sides, but allowing the passage of certain tubes or vessels through the upper and the lower portion of its walls, such as the trachea, oesophagus, blood vessels, nerves, etc. This cavity contains within it three subordinate cavities, the middle one containing the heart sheathed in the pericardium, and the two cavities at the sides containing the lungs, sheathed by the pleural serous membranes, and called the pleural cavities. Each lung is suspended, as it were, in its appropriate cavity by its bronchus from the trachea, and by the trunks of its pulmonary artery and veins, which, bound together by connective tissue, form what is called the root of the lung. For the sake of easy motion, the wall of the cavity is lined, and the surface of the lung is covered by a smooth membrane called the pleura, the contiguous surfaces of which are moistened with serous fluid to prevent friction from alternate respiratory motions of the chest and lungs. The pleural cavities being completely closed on all sides, no air can enter them, but the lung in each cavity communicates with the external air by means of its bronchial tubes leading to the trachea and larynx, through which the air passes, not into the cavity of the chest, but into the interior of the lung.

In their medium state, the lungs of a person of ordinary size and in good health contain about 200 cubic inches of air. In easy breathing, about 20 cubic inches is drawn into them at each inspiration; but this may vary considerably, according to the efforts made in muscular exertion, or during singing, or before coughing. The limit which the rigidity of the walls of the chest sets to the elastic collapse of the lungs is never reached in ordinary respiration, and the lungs accordingly are never entirely emptied of air.