CAVITY OF THE THORAX. 497 



CHAPTER XV. 



OF THE GENERAL CAVITY OF THE THORAX. 



Of the Form of the Cavity of the Thorax. 



THE osseous structure of the thorax is described in page 

 155. The cavity is completed by the intercostal muscles, which 

 close the vacuities between the ribs ; and by the diaphragm, 

 which fill up the whole space included within its lower 

 margin. 



If we except the apertures of the diaphragm, which are 

 completely occupied by the aorta, the vena cava, and the oeso- 

 phagus, &c., the only outlet of this cavity is above : it is formed 

 by the upper ribs, the first dorsal vertebra, and the sternum. 

 The figure of this aperture is between that of the circle and 

 the oval ; but it is made irregular by the vertebrae, and by the 

 upper edge of the sternum. 



When the superior extremities and the muscles appropriated 

 to them are removed, the external figure of the thorax is coni- 

 cal ; but the cavity formed by it is considerably influenced by 

 the spine, which protrudes into it ; while the ribs, as they pro- 

 ceed from the spine, curve backwards, and thus increase the 

 prominency of the cavity. 



The diaphragm has a great effect upon the figure of the 

 cavity of the thorax. It protrudes into it from below, with a 

 convexity of such form that it has been compared to an inver- 

 ted bowl ; so that although it arises from the lower margin of 

 the thorax, the central parts of it are nearly as high as the 

 fourth rib. 



The position of the diaphragm is also oblique. The anterior 

 portion of its margin, being'connected to the seventh and eighth 

 ribs, is much higher than the posterior portion, which is.attached 

 to the eleventh and twelfth. 



In consequence of the figure and position of the diaphragm, 

 42* 



