RESPIRATION 85 



which the lung is surrounded, the arrangement being simi- 

 lar to that of the pericardium. Properly speaking, there 

 are two pleurae, one for each lung, and these, besides 

 inclosing the lungs, partition off a middle space which is 

 occupied by the heart (Fig. 40). They also cover the 

 upper surface of the diaphragm, from which they deflect 

 upward, blending with the pericardium. A small amount 

 of liquid is secreted by the pleura, which prevents friction 

 as the surfaces glide over each other in breathing. 



The Thorax. The force required for breathing is sup- 

 plied by the box-like portion of the body in which the 

 lungs are placed. This is known as the thorax, or chest, 

 and includes that part of the trunk between the neck arid 

 the abdomen. The space which it incloses, known as the 

 thoracic cavity, is a variable space and the walls surround- 

 ing this space are air-tight. A framework for the thorax 

 is supplied by the ribs which connect with the spinal 

 column behind and with the sternum, or breast-bone, in 

 front. They form joints with the spinal column, but con- 

 nect with the sternum by strips of cartilage. The ribs do 

 not encircle the cavity in a horizontal direction, but slope 

 downward from the spinal column both toward the front 

 and toward the sides, this being necessary to the service 

 which they render in breathing. 



How Air is Brought into and Expelled from the Lungs. 

 The principle involved in breathing is that air flows from 

 a place of greater to a place of less pressure. The con- 

 struction of the thorax and the arrangement of the lungs 

 within it provide for the application of this principle in a 

 most practical manner. The lungs are suspended from 

 the upper portion of the thoracic cavity, and the trachea 

 and the upper air passages provide the only opening to 

 the outside atmosphere. Air entering the thorax must on 



