94 



THE HOESE IN HEALTH AND DISEASE 



External respiration consists of inspiration, a movement in 

 which the lungs are filled with air, and expiration, by which the 

 inspired air is expelled from the lungs and these organs are ven- 

 tilated. 



Inspiration is brought about by a contraction of the diaphragm, 

 which causes this partition to flatten, particularly at the sides, so 

 that the length of the thorax is increased (Fig. 28). The ribs are 



Fig. 28. — Horizontal section of chest to show the movements of diaphragm: 

 o, Right lung; b, left lung; 1, position of the diaphragm during deep expiration; 

 c, liver; d, stomach during deep expiration; e, spleen during deep expiration; 

 2, position of the diaphragm during deep inspiration; c', liver; d', stomach; e', 

 spleen;/, posterior vena cava as it passes through its foramen in the diaphragm. 

 (Sussdorf.) 



rotated outward and forward by the external intercostal muscles 

 attached to them. As a result of these changes both the girth and 

 transverse diameter of the chest is increased, and the thorax is 

 greatly expanded, so as to permit of full inspiration. 



Expiration is made possible by the relaxation of the muscles 

 employed in inspiration. Normally it is mainly a passive act. The 

 diaphragm springs forward rapidly, due to pressure from the ab- 

 dominal viscera. This causes the lungs to be compressed and 



