Chest, or THORAX, in Anatomy, is the part of the body which lies beneath the neck and above the Abdomen (q.v.), constituting the uppermost of the two divisions of the trunk, or that which contains the heart and lungs. The chest is somewhat conical in form. Its sides are rounded, but in front and behind they are flattened. The apex, or upper end, is truncated, sloping downwards and forwards; of small size, it permits of the passage of the gullet, windpipe, certain large veins and nerves from the neck into the chest, and of certain large arteries out of the chest. The broad or lower end of the cone slopes downwards and backwards, and is shut in by the diaphragm—a large muscular partition which projects upwards from the lower ribs, being convex towards the chest, and concave towards the abdomen. In Respiration (q.v.) the diaphragm descends by its own muscular contraction, while at the same time the ribs are drawn upwards and outwards by the intercostal muscles.
The structures forming the walls of the chest are: (1) The backbone or spinal column; (2) twelve pairs of ribs; (3) the sternum or breastbone; (4) the Diaphragm (q.v.); and (5) the intercostal muscles. See SKELETON, SPINAL COLUMN, &c.
The contents of the chest are the heart, the great arteries and veins, the lungs, the trachea or windpipe, the bronchi or branches of the trachea, leading to the lungs, the oesophagus or gullet, and the thoracic duct, or general terminus of the lymphatic system of vessels, by which the chyle and lymph are discharged into the blood. For the organs of the chest, see the cut at ABDOMEN.
The very great importance of these parts to life, and their great liability to deranged action, renders the chest the seat of a large proportion of the diseases which afflict humanity, and especially of those which end in death; for of the three organs which Bichat called the 'tripod of life'—viz. the brain, heart, and lungs—the chest contains two. The diseases of the chest depend in some cases on alterations in its form, as by Rickets (q.v.) and other diseases affecting the bones in early childhood or in youth, as by too tight lacing in girls. What are commonly called chest diseases are mainly those of the lungs and air-tubes, of which the principal are consumption, pneumonia, pleurisy, and bronchitis. These are treated in special articles : and for the diseases of the heart and other organs of the chest, see HEART, AORTA, ÆSOPHAGUS, &c. The examination of the chest in disease is largely conducted by Auscultation (q.v.) and Percussion (q.v.). For measurement round the chest, see MAN. See also DIAGNOSIS.