200 THE MECHANISM OF 



tilage being longest ; the fibres pass backwards as they ascend. 

 Thus while the spinal segment of the diaphragm tends to elongate 

 the thorax in a vertical direction, the sterno-costal part pulls 

 forwards and downwards the abdominal viscera, increasing the 

 back-to-front diameter in the lower part of the thorax. The 

 resultant movement of the diaphragm is one in a downward and 

 forward direction ; the more the thorax is elevated the greater is 

 the forward visceral movement ; the more the downward move- 

 ment, the more are the abdominal viscera depressed. The writer 

 has observed in patients who were described clinically as neuras- 

 thenic, that the spinal part of the diaphragm may act forcibly 

 while the sterno-costal part is almost passive. Duchenne showed 

 conclusively that if descent of the abdominal viscera is restrained 

 the diaphragm spent its force in elevating the thorax. Dally 

 observed, and the observation has been frequently verified, that 

 the curvature of the domes of the diaphragm is scarcely altered 

 during even a profound inspiration. We have come to see that 

 the diaphragm, rendered semi-solid by the abdominal viscera and 

 having its circumferential zone kept constantly applied to the 

 inner wall of the lower part of the thorax by the negative intra- 

 thoracic pressure, acts as a true piston a piston moving in a 

 downward and forward direction, the lung expanding into the 

 space it vacates. The action of the diaphragm depends on which 

 group of muscles comes into play as its antagonists. If the 

 abdominal contents are rendered fixed by the abdominal muscu- 

 lature, the lower margin of the thorax moves towards the domes 

 of the diaphragm ; if, on the other hand, the ribs are fixed by the 

 intercostal muscles, and the abdominal musculature is reflexly 

 relaxed, the domes of the diaphragm move towards the lower 

 aperture of the thorax. In subjects of extreme visceroptosis 

 Wenckebach observed that the diaphragm was thrown out of 

 action by its visceral fulcrum being lost, and breathing was carried 

 on by an elevation of the upper part of the thorax. Seeing how 

 variable the action of the diaphragm is in the same individual, 

 and how much it differs in its action from individual to individual, 

 it is rather misleading to make any precise statement of the ampli- 

 tude of the movements of its domes. In quiet breathing Dally 

 found, by the use of the orthodiascope, that the mean descent 

 of the right dome in 100 individuals was 12-5 mm. ; the left 

 dome. 12 mm. ; the central part rather less than the left dome. 



