46 PERSONAL APPEARANCES IN HEALTH AND DISEASE. 



The cavity of the thorax, which extends from the root 

 of the neck to the lower end of the breast-bone in front, is 

 separated from the cavity of the abdomen by the midriff 

 or diaphragm, an expanded muscular structure, which 

 is dome-shaped, and on contracting tends to descend and 

 thus to increase the capacity of the thorax. This occurs 

 every time the lungs are filled with air in the process 

 of respiration, the descent of the diaphragm being one 

 main cause of the enlargement of the lungs so as to 

 allow them to receive more air. At the same time by 

 virtue of the muscular structures attached to them, 

 the ribs are raised and turned outwards, and thus the 

 thorax capacity is increased in a lateral direction. The 

 lungs are each of them contained in a thin membrane or 

 pleura, composed of two layers, the outer of which lines 

 the wall of the cavity of the chest, and the inner is re- 

 flected around the air-tubes or bronchi, and blood- 

 vessels entering the lung to cover the whole surface of 

 the lung itself. Although in the normal state- there is 

 no actual cavity between the lung and the chest-wall, but 

 only the virtual space between the smooth surface of the 

 costal pleura (as it is called) on the one hand and the 

 pulmonary pleura on the other, it is usual to speak of 

 the " pleural cavity " when we mean that half of the chest 

 cavity which contains a lung. The shape of this is roughly 

 that of half a cone, the apex being upwards behind the 



