284 



HUMAN PHYSIOLOGY 



as this adds space to the thoracic cavity and it would 

 be natural to say that it must subtract space from the 

 abdomen. But this is not strictly true. The abdominal 

 contents are incompressible, so if the diaphragm is to 

 descend at all it must do so by crowding the viscera out 

 of place and room has to be made for them by stretching 

 the abdominal wall. The abdomen is not made smaller 

 but its shape is changed. 



If the descent of the diaphragm is opposed, the chief 

 effect of its contraction may be to pull its marginal at- 



FIG. 63. The diaphragm between empty body cavities. The 

 ventral border has been cut away to some extent. (See text.) 



tachments upward toward a stationary center. If the 

 ribs could move freely in any direction this would narrow 

 the lower part of the chest. But since, in reality, they 

 cannot rise without spreading, the movement is still 

 inspiratory. In such circumstances the diaphragm has 

 merely taken its place among the other muscles which 

 are adapted to elevate the ribs. If one inspires as deeply 

 as possible it will probably be noticed that up to a certain 

 point the abdominal wall is pushed steadily out; before 

 the movement is carried to its limit the wall has begun 

 to fall back. The protrusion indicates that the dia- 

 phragm is bearing down upon the viscera; the reversal 



