HUMAN BIOLOGY 



lower than its middle (Fig. 73) ; the lungs descend with it, 

 thus lengthening the chest from top to bottom ; at the 



(Esophagus 



Aorta 



FIG. 74. DIAPHRAGM (or midriff), seen from below. (Cunningham.) 



The central portion (light) is tendinous. As the diaphragm descends, it acts like the piston 

 of a great pump and the blood is forced up through the vena cava, and the lymph through 

 the thoracic duct (Fig. 66). 



same time the ribs are raised upward and outward (Fig. 

 76) by the contraction of the outer set of muscles between 

 the ribs. Thus the chest is made longer, broader, and 

 deeper from front to back. The lungs expand when the 

 chest expands, and the air rushes in. Why is this ? The 

 lungs contain no muscles and cannot expand themselves ; 

 the air cannot be pulled in, for its parts do not stick to- 

 gether. The true reason is that the air has weight. The 



