JHECHANISM OP RESPIRATION. 101 



They are covered externally by a smooth, shining mem- 

 brane called the pleura the same kind of membrane, 

 that lines the walls of the chest and abdomen, and forms 

 the external coat of the alimentary canal. While in 

 action, they completely fill the cavity of the chest, that 

 is not occupied by other organs, so that no empty space 

 is ever left. 



Emily. -What an admirable system of machinery is 

 here ! I never imagined however, that the lungs were 

 sufficiently large to fill almost the whole cavity of the 

 chest. 



Dr. B.^To understand how the air gets admittance 

 into the lungs, we must first examine the structure and 

 mechanism of the chest. Its form is conoidal, the apex 

 being at the top, and the base at the bottom. Behind, 

 it is bounded by the spine ; at the sides, by the ribs ; in 

 front, by the sternum or breast bone ; below, by a thin 

 membranous muscle, called the diaphragm or midrif, 

 which forms the floor of its base, and separates it from 

 the cavity of the abdomen. .These parts are joined to- 

 gether in such a manner, that considerable motion is al- 

 lowed between them, and by this means the capacity of 

 the chest may be considerably enlarged and diminished. 

 The depression of the diaphragm enlarges the chest at the 

 expense of the abdomen, while the ribs being thrust for- 

 ward and upward by means of the numerous muscles, 

 that are attached to them, produce an enlargement in 

 the other directions. The lungs follow the variations of 

 capacity in the chest, expanding their air-cells, when 

 the latter is enlarged, so as still to fill up the whole cav- 

 ity, and collapsing, when the chest is contracting. Now 

 observe how they become filled with air. Suppose that 

 expiration has just taken place, the lungs in a collapsed 

 state, and the chest reduced to its smallest capacity. 

 This state of things can last but a few moments. The 

 necessity of fresh air is felt, and we instinctively expand 

 the chest. The lungs follow the expanding walls of the 

 chest, and consequently a vaciuim is produced in their 



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