The Thorax is the area of the torso between the base of the neck and the diaphragm. Muscles that support the abdominal organs, connect to and are supported by, the bony structure of the thorax, which forms part of the axial skeleton

The thoracic cavity, which contains the heart and the lungs, is surrounded by a protective framework of bones, formed by twelve pairs of ribs, which articulate with the thoracic vertebrae in the back and with the sternum at the front of the chest. 

The sternum is made mainly of bone, with some cartilage, and the ends of the seven pairs of ribs that connect to the sternum are also made of cartilage.  This creates some elasticity in the structure of the thorax. The ribs and the intercostal muscles between them, allow the thorax to be mobile, so that it can expand and contract with each breath.  Problems can arise when factors such as mis-use distort our posture so that the curvature of the spine increases to cause kyphosis or lordosis. When this happens, the freedom of the ribs to expand and contract as we breathe, is much reduced and conditions such as RSI are more likely to develop.

As Alexander noted, he had a 'tendency to lift the chest' when reciting, which 'went with a tendency to increase the arch of the spine and thus bring about what I now call a “narrowing of the back”.  This, I saw, had an adverse effect on the shape and functioning of the torso itself'

F M Alexander ~ The Use of the Self p. 14

Another natural type of movement available to the thorax, is rotation.  This movement can be performed quite easily when we allow the body to be free and lengthened. However, problems can be created if someone continually twists their body round, for instance to look at a computer screen which is set off centre.  This can lead to the rib cage becoming habitually twisted so that it is held forwards on one side of the body. Sometimes there is also a degree of scoliosis. When this happens, one side of the chest can have a reduced capacity to inflate and the lung is less able to function properly. With improved use, such problems may be modified or eliminated.

As Dr. Wilfred Barlow, who studied under Alexander and ran one of the first Teacher Training Schools noted: 

'such chest twists and rotations are frequently unobserved by doctors, and many patients suffer distressing pain in the chest…. which does not respond to physiotherapy, and leaves lurking doubts that there may be heart or lung pathology in spite of all tests showing them to be clear…. (even if there is pathology, it can rarely do anything but good to also consider the manner of USE'.)

Wilfred Barlow~ The Alexander Principle p.168