KREATHING.] 



PHYSIOLOGY. 



97 



traction is flattened and descends, somewhat 

 as is shown in Fig. 13, -^. 



Fig. 13. — Diagrammatic Sections of the Body in 



A. inspiration; B. expiration. Tr. trachea; St. sternum; D, diaphragm; 

 Ab. abdominal walls. The shading roughly indicates the stationary air. 

 The unshaded portion at the top of A is the tidal air. 



The descent of the diaphragm in inspiration 

 is caused by a contraction of its muscular 

 fibres. During expiration the diaphragm is 

 at rest ; its muscular fibres relax ; and it 

 goes up because it is partly drawn up by the 

 lungs, partly pushed up by the contents of 

 the abdomen, 



H 



