MECHANICS OF RESPIRATORY MOVEMENTS 1169 



found to be accompanied by a fine rustling sound, the ' vesicular mur- 

 mur.' It is thought to be caused by the sudden dilatation of the air- 

 vesicles during inspiration or perhaps by the current of air passing from 

 the narrow terminal bronchioles into the wider infundibula. It is impor- 

 tant to remember that this sound is heard only during inspiration and 

 over healthy lungs. On listening over the larger air-passages, i.e. the 

 larynx, trachea, and bronchi, we hear a much louder sound which 

 accompanies both expiration and inspiration and may be compared to 

 a sharp whispered hah. This is known as the ' bronchial murmur.' 

 It can be heard also at the back of the chest between the scapulae at 

 the level of the fourth dorsal vertebra, where the trachea bifurcates. 

 In all other parts of the chest the healthy lung prevents the propaga- 

 tion of this sound to the chest wall. If, however, the lung is solid, 

 as occurs in pneumonia, it conducts the sound easily from the large 

 air-tubes to the chest wall. Bronchial breathing at any part of the 

 chest other than that immediately over the air-tubes is therefore a 

 distinctive sign of consolidation of the lung. Absence of breath- 

 sounds at any part of the chest implies either that air is not entering 

 that part of the lung, or that the lung is separated from the chest wall 

 by effused fluid. 



INTRATHORACIC PRESSURE. Even at the end of expiration 

 the lungs are in a stretched condition. This is shown by the fact 

 that if in an animal or in the corpse an opening be made into 

 the pleural cavity, air rushes into the opening and the lungs col- 

 lapse, driving a certain amount of air out through the trachea. 

 Since the lungs are always tending to collapse, it is evident that 

 they must exert a pull on the thoracic wall. This pull of the 

 lungs gives rise to a negative pressure in the pleural cavity. If we 

 connect a mercurial manometer with the pleural cavity, we find that 

 the pull of the lungs amounts in the corpse to 6 mm. of mercury. If 

 the lungs are fully distended, as after full inspiration, the elastic 

 forces are more brought into play, and the negative pressure in the 

 pleura may amount to 30 mm. Since the lungs are always tending 

 to collapse, respiration becomes impossible directly free openings are 

 made into the pleural cavities on both sides. With each inspiratory 

 movement air rushes in through these openings, so that the thoracic 

 movements can no longer exert any influence on the volume of the 

 lungs. The negative pressure in the thorax is diminished by any 

 factor decreasing the elasticity of the lung-tissue. Thus in an old 

 man, where the elastic tissue is degenerated and the alveoli are 

 enlarged, giving rise to the condition known as emphysema, the lungs 

 may collapse only slightly or not at all on opening the chest. The 

 lungs do not collapse on making an opening in the chest of a new-born 

 mammal ; but this is owing to the fact that they completely fill the 



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