THE MECHANISM OF BREATHING. 



211 



wards the center and end in a tendinous sheet of tissue called the 

 central tendon of the diaphragm. "When these fibers are relaxed, 

 the diaphragm is pushed up into the thoracic cavity, forming a 

 dome-shaped arch. This is caused by the pressure of the abdomi 

 nal organs, supported by the muscular walls of the abdomen, on 

 its lower surface, a suction pressure on the upper surface of the 

 diaphragm being maintained by the natural tendency of the 

 lungs to contract. The central tendon is pulled downwards and 

 the arched dome is flattened on contraction of the diaphragm, 



Fig. 31. Diagram to show movement of diaphragm during respiration : 

 I, expiration ; II, normal inspiration ; III, forced inspiration. 



thus increasing the size of the thoracic cavity (Fig. 31). An- 

 other result of the lowering of the diaphragm is the slight pro- 

 trusion of the abdomen due to the pressure exerted on the vis- 

 cera. This type of Joreathing is therefore known as abdominal or 

 diaphragmatic breathing. 



THE PART PLAYED BY THE THORAX. The action of certain 

 muscles attached to the ribs also produces an enlargement of the 

 thoracic cavity. Each pair of corresponding ribs, which are ar- 



