INSPIRATION 



173 



with each inward or diminishing movement there is a 

 corresponding output, or expiration, of air. This air 

 movement is called respiration, and takes place only 

 when the chest wall, or diaphragm, or both are 

 moved. 



Inspiration. The enlargement of the chest in inspi- 

 ration is caused by the contraction of the muscles which 

 lead from the ribs to the shoulder girdle and to the 

 neck and head. The chest wall is readily lifted by 

 these muscles, because its stiff framework of ribs 

 is jointed at the spine. 

 Moreover, the ribs are con- 

 nected by thin layers of 

 muscle and connective tissue ; 

 hence, when one rib moves, 

 the others are carried with it. 

 The ribs are slanted forward 

 and downward so that, when 

 they are raised by the con- 

 traction of the muscles, the 

 chest cavity is enlarged at 

 the sides as well as in the 

 front. As the chest enlarges, 

 the lungs follow, and their air cavity is correspondingly 

 increased. The enlargement of the chest cavity in in- 

 spiration is further increased by the downward movement 

 of the dome-shaped diaphragm, due to the contraction 

 of its muscle fibres. 



Expiration. In expiration, the reverse process takes 

 place. The muscles which lifted the chest relax and 

 allow the ribs to descend. Their descent is greatly aided 

 by the weight of the chest walls and shoulders, and, 

 especially in the case of stout persons, by the weight of 

 the abdominal walls. The diaphragm also relaxes and 



FIG. 103. Diagram illustrating the 

 position of the chest in expira- 

 tion (A) and inspiration (B). 



