THE RESPIRATION 



155 



walls can also be lifted out at the sides and in front. This 

 is accomplished by muscles leading from the shoulders and 

 spinal column to the outer surface of the ribs (Fig. 124), 

 and by the intercostal muscles, or the muscles that connect 

 each rib with the rib above (Figs. 136, 140). Thus the 

 chest may be made deeper from front to back and from 

 side to side, and if the diaphragm acts at the same time, the 

 chest is elongated from top to bottom, and thus is enlarged 

 in all directions. 



FlGS. 137 and 138. Diagrams to show the Positions in Respiration of the Ster- 

 num, Diaphragm, and Abdominal Wall. 

 A, inspiration; 3, expiration; Tr, trachea; St, sternum; D, diaphragm; Ab, abdominal 



wall. The shaded part is to indicate the stationary air. The unshaded part shows the 



increased air space during inspiration. 



259. Inspiration. The lungs themselves contain no 

 muscular tissue ; therefore they cannot expand by any 

 force of their own. Yet they expand when the chest ex- 

 pands. How does the enlargement of the chest cause the 



