The chest, or THORAX, is the upper part of the trunk. It is enclosed by the breastbone (sternum) and the 12 ribs which join the sternum by way of cartilages and are attached to the spine behind. At the top of the thorax, the opening in between the ﬁrst ribs admits the windpipe (TRACHEA), the gullet (OESOPHAGUS) and the large blood vessels. The bottom of the thorax is separated from the abdomen below by the muscular DIAPHRAGM which is the main muscle of breathing. Other muscles of respiration, the intercostal muscles, lie in between the ribs. Overlying the ribs are layers of muscle and soft tissue including the breast tissue.
Contents The trachea divides into right and left main bronchi which go to the two LUNGS. The left lung is slightly smaller than the right. The right has three lobes (upper, middle and lower) and the left lung has two lobes (upper and lower). Each lung is covered by two thin membranes lubricated by a thin layer of ﬂuid. These are the pleura; similar structures cover the heart (pericardium). The heart lies in the middle, displaced slightly to the left. The oesophagus passes right through the chest to enter the stomach just below the diaphragm. Various nerves, blood vessels and lymph channels run through the thorax. The thoracic duct is the main lymphatic drainage channel emptying into a vein on the left side of the root of the neck. (For diseases aﬀecting the chest and its contents, see HEART, DISEASES OF; LUNGS, DISEASES OF; CHEST, DEFORMITIES OF.)