236 THE OFFICE OF THE DIAPHRAGM. 



The cavity of the chest is now enlarged. But this is a closed cavity, and 

 between its contents and the parietes of the chest a vacuum would be formed ; or 

 rather an inequality of atmospheric pressure is produced from the moment the 

 chest begins to dilate. As the diaphragm recedes, there is nothing to counter- 

 balance the pressure of the atmospheric air communicating with the lungs 

 through the medium of the nose and mouth, and it is forced into the respiratory 

 tubes already described, and the lungs are expanded and still kept in contact 

 with the receding walls of the chest. There is no sucking, no inhalent power 

 in the act of inspiration ; it is the simple enlargement of the chest from the 

 entrance and pressure of the air. 



From some cause, as inexplicable as that which produced the expansion of 

 the chest, the respiratory nerves cease to act ; and the diaphragm, by the inhe- 

 rent elasticity of its tendinous expansion and muscular fibres, returns to its na- 

 tural form, once more projecting its convexity into the thorax. The abdominal 

 muscles, also, which had been put on the stretch by the forcing of the viscera 

 into the posterior part of the abdomen by means of the straightening of the dia- 

 phragm, contract, and accelerate the return of that muscle to its quiescent 

 figure ; and the ribs, all armed with elastic cartilages, regain their former situa- 

 tion and figure. The muscles of the shoulder and the chest relax, a portion of 

 the lungs are pressed on every side, and the air with which they were distended 

 is again forced out. There is only one set of muscles actively employed in 

 expiration, namely, the abdominal : the elasticity of the parts displaced in 

 inspiration being almost sufficient to accomplish the purpose. 



The lungs, however, are not altogether passive. The bronchial tubes, so far 

 as they can be traced, are lined with cartilage, divided and subdivided for the 

 purpose of folding up when the lungs are compressed, but elastic enough to 

 afford a yielding resistance against both unusual expansion and contraction. In 

 their usual state the air-tubes are distended beyond their natural calibre ; for if 

 the parietes of the thorax are perforated, and the pressure of the atmosphere 

 rendered equal within and without them, the lungs immediately collapse. 



THE PLEURA. 



The walls of the chest are lined, and the lungs are covered, by a smooth 

 glistening membrane, the pleura. It is a serous membrane, so called from the 

 nature of its exhalation, in distinction from the mucous secretion yielded by the 

 membrane of the air-passages. The serous membrane generally invests the 

 most important organs, and always those that are essentially connected with 

 life ; while the mucous membrane lines the interior of the greater part of them. 

 The pleura is the investing membrane of the lungs, and a mucous membrane 

 the lining one of the bronchial tubes. 



Among the circumstances principally to be noticed, with regard to the pleura, 

 is the polish of its external surface. The glistening appearance of the lungs, 

 and of the inside of the chest, is to be attributed to the membrane by which 

 they are covered, and by means of which the motion of the various organs is 

 freer and less dangerous. Although the lungs, and the bony walls which con- 

 tain them, are in constant approximation with each other, both in expiration and 

 inspiration, yet in the frequently hurried and violent motion of the animal, and, 

 in fact, in every act of expiration and inspiration, of dilatation and contraction, 

 much and injurious friction would ensue if the surfaces did not glide freely ovei 

 each other by means of the peculiar polish of this membrane. 



Every serous membrane has innumerable exhalent vessels upon its surface, 

 from which a considerable quantity of fluid is poured out. In life and during 

 health it exists in the chest only as a kind of dew, just sufficient to lubricate the 

 surfaces. When the chest is opened soon after death, we recognize it in the 



