

THORAX 3 



tip of the eleventh costal cartilage ; thence it proceeds up- 

 wards, backwards, and medially to the vertebral column. In 

 the first part of its extent it is formed by the cartilages of the 

 seventh, eighth, ninth, tenth and eleventh ribs, and in the 

 second part by the lower border of the twelfth rib. 



The lower margin of the thorax gives attachment to the 

 diaphragm, a highly vaulted or dome-shaped musculo-tendinous 

 partition, which intervenes between the cavity of the thorax 

 above and that of the abdomen below. It forms a convex 

 floor for the thorax, and a concave roof for the abdomen. 

 By its upward projection it greatly diminishes the general 

 vertical depth of the thoracic cavity (Figs. 9, 10, n, 12). 



As a result of this arrangement the peripheral margins of 

 the inferior part of the thorax overlap the upper part of the 

 abdomen, especially at the sides and behind. 



But the diaphragm does not form an unbroken partition. 

 It presents three large openings, by means of which structures 

 pass to and from the thorax, viz. (i) for the aorta, thoracic 

 duct, and vena azygos ; (2) for the oesophagus and vagi 

 nerves ; (3) for the inferior vena cava. Besides these there 

 are other smaller apertures which will be mentioned later. 



During life the movements of the thoracic wall produce 

 alterations in the capacity of the thoracic cavity and play an 

 essential part in the function of respiration. When inspira- 

 tion takes place, that is when a "breath is taken," the sternum 

 and the anterior parts of the ribs ascend and move forwards, 

 with the result that the antero-posterior extent of the thoracic 

 cavity is increased. At the same time, on account of the 

 peculiarities of the articulations of the ribs, the lower borders 

 of the majority of the ribs rotate outwards and the transverse 

 diameter of the cavity is increased. The vertical extent of 

 the thoracic cavity is increased by the contraction of the 

 dome -shaped diaphragm. The central upper part of the 

 dome which lies below the heart, and is tendinous, remains 

 relatively stationary but the muscular peripheral parts 

 contract. As they contract and pass from a more curved 

 to a less curved position, they press down the contents of 

 abdomen and distend the abdominal walls ; consequently, as 

 the floor of the thorax descends, the vertical extent of its 

 cavity is increased. 



As the cavity of the thorax is entirely enclosed, an increase 

 of its extent tends to produce a vacuum in its interior, but 



