ACTION OF THE DIAPHRAGM. 371 



in a limited degree it is true, the action of the external 

 intercostals. 



The phenomena referable to the abdomen, which coincide 

 with the descent of the diaphragm, can easily be observed 

 in the human subject. As the diaphragm is depressed, 

 it necessarily pushes the viscera before it, and inspiration 

 is therefore accompanied by protrusion of the abdomen. 

 This may be rendered very marked by a forced or deep in- 

 spiration. 



The action of the diaphragm may be illustrated by a very 

 simple yet striking experiment. In an animal just killed, 

 after opening the abdomen, if we take hold of the structures 

 which are attached to the central tendon, and make traction, 

 we imitate, in a rough way, the movements of the diaphragm 

 in respiration, and the air will pass into the lungs, sometimes 

 with a distinctly audible sound. 



The effects of the action of the diaphragm upon the size 

 of its orifices are chiefly limited to the oesophageal opening. 

 The anatomy of the parts is such that contraction of the 

 muscular fibres has a tendency to close this orifice. When 

 we come to treat of the digestive system, we shall see that this 

 is auxiliary to the action of the muscular walls of the oeso- 

 phagus itself, by which the cardiac opening of the stomach 

 is regularly closed during inspiration. This may become 

 important when the stomach is much distended ; for descent 

 of the diaphragm compresses all the abdominal organs, and 

 might otherwise cause regurgitation of a portion of its con- 

 tents. 



The contractions of the diaphragm are animated almost 

 exclusively, if not exclusively, by the phrenic nerve ; a nerve 

 which, having the office of supplying the most important 

 respiratory muscle, derives its filaments from a number of 

 sources. It arises from the third and fourth cervical nerves, 

 receiving a branch from the fifth, and sometimes from the 

 sixth ; it passes through the chest, penetrates the diaphragm, 

 and is distributed to its under surface. This nerve was the 



