THE LUNGS AND RESPIRATION. 237 



enlarged, the lung is correspondingly expanded. But such 

 an expansion of the lung at once affects those sensory 

 nerves in it which, when stimulated, inhibit the center; 

 that is, cause it to stop. So, as the inspiration proceeds 

 and the lungs expand more and more these nerves are more 

 and more stimulated, the center is more and more in- 

 hibited, and is finally brought to a standstill. As soon as 

 this happens the muscles of respiration relax and the chest 

 collapses of its own accord; that is, a passive expiration 

 follows. But in this passive expiration the lung is com- 

 pressed, and this compression of the lung affects the second 

 pair of sensory fibres which serve to stimulate the center to 

 greater activity. Thus, as the lungs collapse more and 

 more, these stimuli become stronger and stronger, and so 

 finally arouse the center to a renewed contraction. 



(4) In addition to these two sensory nerves, which 

 in the manner just indicated so materially influence the 

 activity of this center, there is one additional nerve which 

 exercises a marked effect. This is the nerve which goes to 

 the larynx. This nerve when stimulated, strongly inhibits 

 the center, and so tends to make an inspiration impossible. 

 As this nerve is no doubt stimulated in the varying acts of 

 swallowing, singing, talking, and so on, there seems some 

 reason for this arrangement. Thus, in the act of swallow- 

 ing the center is inhibited, and so the possibility of choking 

 is materially reduced, while in the act of talking or singing 

 stimulation of this nerve tends to check the rate of breath- 

 ing, a circumstance which materially helps to sustain the 

 voice. 



When all these rather remarkable niceties in the nervous 

 control of the movements of respiration are stated, there yet 

 remains much that requires further investigation, and it is 

 possible that the researches of the immediate future may 

 materially modify and clarify our present knowledge of this 

 subject. 



