Thoracic inlet
The thoracic inlet, also known as the superior thoracic aperture, refers to the opening at the top of the thoracic cavity. It is also clinically referred to as the thoracic outlet, in the case of thoracic outlet syndrome; this refers to the "superior" thoracic aperture, and not to the lower, larger opening, the inferior thoracic aperture. Structure. The thoracic inlet is essentially a hole surrounded by a bony ring, through which several vital structures pass. The thoracic inlet is bounded by: the first thoracic vertebra (T1) "posteriorly"; the first pair of ribs "laterally", forming lateral C-shaped curves  posterior to anterior; and the costal cartilage of the first rib and the superior border of the manubrium "anteriorly". Dimensions. The adult thoracic inlet is around 6.5 cm antero-posteriorly and 11 cm transversely. Because of the obliquity of the first pair of ribs, the aperture slopes antero-inferiorly. Relations. The clavicle articulates with the manubrium to form the anterior border of the thoracic inlet. Superior to the thoracic inlet is the root of the neck, and the superior mediastinum is inferiorly related. The brachial plexus is a superolateral relation of the thoracic inlet. The brachial plexus emerges between the  anterior and middle scalene muscles, superior to the first rib, and passes obliquely and inferiorly, underneath the clavicle, into the shoulder and then the arm. Impingement of the plexus in the region of the scalenes, ribs, and clavicles is responsible for thoracic outlet syndrome. Function. Structures that pass through the thoracic inlet include: This is not an exhaustive list. There are several other minor, but important, vessels and nerves passing through, and an abnormally large thyroid gland may extend inferiorly through the thoracic inlet into the superior mediastinum. The oesophagus lies against the body of the T1 vertebra, separated from  it by the prevertebral fascia, and the trachea lies in front of the oesophagus, in the midline, and may touch the manubrium. The apices of the lungs lie to either side of the oesophagus and trachea, and is separated from them by the other vessels and nerves listed above. Furthermore, they extend slightly superior past the level of the inlet (e.g. the horizontal plane of the first rib).