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In mammals, the thorax is the region of the body formed by the sternum, the thoracic vertebrae, and the ribs. It extends from the neck to the diaphragm, and does not include the upper limbs. The heart and the lungs reside in the thoracic cavity, as well as many blood vessels. The inner organs are protected by the rib cage and the sternum
In other animals the number of thoracic vertebrae can vary greatly; [2] for example, most marsupials have 13, but koalas have only 11. [3] 12 to 15 is common among mammals, with 18 to 20 in horses, tapirs, rhinoceroses, and elephants, and extremes in mammals are marked by certain sloths with 25 and cetaceans with 9. [4]
The thoracic diaphragm, or simply the diaphragm (/ ˈ d aɪ ə f r æ m /; [1] Ancient Greek: διάφραγμα, romanized: diáphragma, lit. 'partition'), is a sheet of internal skeletal muscle [2] in humans and other mammals that extends across the bottom of the thoracic cavity.
This is an upwardly domed sheet of muscle that separates the thoracic cavity from the abdominal cavity. When it contracts, the sheet flattens, (i.e. moves downwards as shown in Fig. 7) increasing the volume of the thoracic cavity in the antero-posterior axis. The contracting diaphragm pushes the abdominal organs downwards.
The thoracic cavity (or chest cavity) is the chamber of the body of vertebrates that is protected by the thoracic wall (rib cage and associated skin, muscle, and fascia). The central compartment of the thoracic cavity is the mediastinum .
Humans have two lungs, a right lung and a left lung. They are situated within the thoracic cavity of the chest. The right lung is bigger than the left, and the left lung shares space in the chest with the heart. The lungs together weigh approximately 1.3 kilograms (2.9 lb), and the right is heavier.
The thoracic ribs of birds possess a wide projection to the rear; this uncinate process is an attachment for the shoulder muscles. [9] Usually dogs have 26 ribs. Mammals usually also only have distinct ribs on the thoracic vertebra, although fixed cervical ribs are also present in monotremes.
The lungs are not capable of inflating themselves, and will expand only when there is an increase in the volume of the thoracic cavity. [6] [7] In humans, as in the other mammals, this is achieved primarily through the contraction of the diaphragm, but also by the contraction of the intercostal muscles which pull the rib cage upwards and outwards as shown in the diagrams on the right. [8]