THE RESPIRATORY SYSTEM. 407 



trolled by the central nervous system through nervea. The 

 medulla oblongata is chiefly concerned, and especially one 

 small part of it known as the respiratory center. It is possible, 

 even probable, that there are subordinate centers in the cord, 

 which, under peculiar circumstances, assume importance ; but 

 how far they act in concert with the medullary center, or 

 whether they act at all when normal conditions prevail, is an 

 open question. 



The vagus is the principal afferent respiratory nerve. The 

 efferent nerves are the phrenics, intercostals, and others supply- 

 ing the various muscles used in moving the chest-walls, etc. 



The respiratory center is automatic, but its action is sus- 

 ceptible of modification through afferent influences taking a 

 variety of paths, the principal of which is along the vagi nerves. 

 The respiratory, vaso-motor, and cardio-inhibitory centers seem 

 to act somewhat in concert. 



Blood-pressure is being constantly modified by the respira- 

 tory act, rising with inspiration and sinking with expiration. 

 In some animals the heart-beat also varies with these phases of 

 respiration, becoming slow and irregular during expiration. 

 Into the causation of these changes both mechanical and nerv- 

 ous factors enter, and make a very complex mesh, which we 

 can at present but imperfectly unravel. When the access of 

 air to the tissues is prevented, a series of stages of respiratory 

 activity and decline, accompanied by pronounced changes in 

 the vascular system, are passed through, known as asphyxia. 



Three stages are distinguishable : one of dyspnoea, one of 

 convulsions, and one of exhaustion — while at the same time 

 there is a rise of blood-pressure during the first two, and a 

 decline during the third, accompanied by marked alterations in 

 the cardiac rhythm. 



