36 



PHYSIOLOGY 



circle (Fig. 21). As each fiber contracts, the entire cir- 

 cle becomes smaller. When the fibers are relaxed the 



Fie. 21.— Front view of diaphragm and its attachments. V.c, Vena cava ; S., 

 Steruum ; C, Cartilages of ribs: Ao. y Aorta; D.L, Central tendon of dia- 

 phragm ; Es., (Esophagus; D.p., Pillars of diaphragm. (From Fitz.) 



diaphragm is too large to make a flat partition across 

 the barrel-like body, and we should expect it to hang 

 down, but apparently against the laws of gravity it 

 hangs up (Fig. 22, A). When the fibers contract the 

 diaphragm flattens out and the upper cavity becomes 

 longer than it was before (Fig. 22, B). 



At the same time the muscles in the outside wall of 

 this cavity contract and lift the ribs. Because of the 

 peculiar shape of the ribs, the cavity becomes larger both 



