ELEMENTARY EXPERIMENTAL PHYSIOLOGY 177 



DEMONSTRATION. The trachea of a dead rabbit is exposed, and a 

 ligature tied round it. The skin is divided over the thorax on one 

 side, and the ribs exposed. The intercostal muscles are carefully 

 separated between two ribs. Note that the lung is in contact with the 

 thoracic wall. The ligature round the trachea is now divided ; the air 

 escapes, and the lung, owing to its elasticity, will collapse. On opening 

 the pleural cavity the pressure within and without the lung becomes 

 atmospheric. The elasticity of the distended lung then comes into 

 play and causes its collapse. 



DEMONSTRATION. In the rabbit anaesthetised with ure thane or 

 chloral the skin is divided over an intercostal space. The intercostal 

 muscles are then separated with care, and a piece of rib removed, 

 while the parietal pleura is left quite uninjured. The lung will not 

 collapse so long as the pleural cavity is not opened. On the contrary 

 it will be seen gliding to and fro with each movement of respiration. 

 Note how easily the pleural surface of the lung glides over the 

 parietal pleura. A glass cannula attached to a water manometer is 

 pushed throughout the intercostal muscles until the end comes to 

 lie in the thoracic cavity. Notice the negative pressure indicated in 

 the manometer, which becomes greater in inspiration and less in expira- 

 tion. Note the immediate collapse of the lung on opening the pleural 

 cavity. 



CHAPTER XLV. 



VENTILATION OF THE LUNGS. 



THE SPIROMETER AND THE STETHOGRAPH. 



THE ventilation of the lungs is determined by a gas-meter through 

 which the subject breathes by means of an anaesthetic mask, provided 

 with inspiratory and expiratory valves. Meters with a very low 

 resistance are more convenient than the special instrument known as 

 the spirometer (Fig. 176), although the latter is very useful for some 

 experiments. 



The subject of the experiment should breathe through the mask and 

 meter for a minute or two before a record is taken, in order that he 

 may become accustomed to the novel conditions. Then the volume of 

 each breath and the number in periods of consecutive minutes should 

 be determined. A table should be made to show the results obtained 

 with each member of the class, for the differences in the rate and depth 

 of breathing in healthy men are considerable ; some men breathe slowly 



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