188 THE MECHANISM OF 



The negative tension, in such an inspiration, falls first and most 

 in the basal part of the lung ; hence one finds very frequently a 

 horizontal groove (Harrison's sulcus) on each side of the thorax 

 corresponding to the level of the domes of the diaphragm and 

 to the zone of greatest negative pressure in those who have had 

 the respiratory passage obstructed. Meltzer found the degree of 

 negative pressure within the thorax of rabbits increased as the 

 diaphragm was approached ; it is least along the stationary walls 

 of the thorax. It is thus apparent that a certain degree of the 

 expanding force transmitted by the thorax to the surfaces of direct 

 expansion is lost as it passes through the lung to the surfaces of 

 indirect expansion. Further, the extent to which any part of the 

 lung is expanded depends on the distensibility of that part, being 

 greatest in the superficial zone of the lung and least in the root 

 zone. That the expansion of the lung does not take place instan- 

 taneously and equally throughout all its parts is well substantiated 

 by clinical observation. Huggard and Olive Riviere observed 

 independently that if percussion and auscultation of the lung 

 were carried out before a deep breath were taken, the apices of 

 the lungs, especially in those who were regarded as being the 

 subjects of a phthisical tendency, were less in action than the 

 rest of the lungs ; Lloyd Jones records that when a breath is 

 taken the resonance of the anterior part of the apex of the lung 

 increases much more than the posterior ; Gerhardt noticed that 

 when a pleural effusion was being absorbed the apex of the lung 

 was the last part to regain its normal resonance. It is a well- 

 known clinical fact that a localised consolidation is surrounded 

 by an area of increased resonance, showing that the diminution 

 of one part of the lung is not followed by an expansion of the 

 whole lung but only of the part immediately adjacent. When it 

 is remembered that those who lead sedentary lives use their lungs 

 to only 10 per cent, or even less of the full pulmonary capacity, 

 the importance of recognising that a thoracic movement only 

 affects that part of the lung directly subject to the movement will 

 become apparent. In those who have contracted a lazy habit of 

 body it is possible for the parts of the lung which are the most 

 remote from the surfaces of direct expansion and at the same 

 time of a low degree of distensibility, to pass into a condition of 

 partial or almost complete disuse ; such a part is that region of 

 the apex where phthisis so frequently commences. 



