THE LUNGS AND RESPIRATION. 235 



cumulating their force, so to speak, until finally such stim- 

 uli give rise to a respiratory impulse. Or, to take another 

 illustration. We may imagine a box, the bottom of which is 

 held in place by a spring. If, now, water be poured into this 

 box and the spring have an appreciable amount of strength , 

 it will not at once drop, but the water will accumulate in 

 the box, rise to a higher and higher level, until finally the 

 weight of the water in the box will become greater than the 

 strength of the spring supporting the bottom, when the 

 bottom will drop out, and the water flow out with one 

 gush. As soon as the vessel has thus emptied itself the 

 spring again pushes the bottom in place, and the process 

 repeats itself. Here, too, we see how a source that is con- 

 stant is changed into an effect which is periodical. 



2. Nervous Control of the Respiratory Center. But this 

 center reflexly stimulated by the blood which traverses it, 

 may be controlled within certain limits by nerves going 

 to it. 



(1) Those nerves from the brain which are under the 

 control of the will reach it, and every one is familiar with 

 the fact than within a wide range he is able to control his 

 movements of respiration, but that as soon as this range is 

 passed the movements go on independently of the will. 



(2) Sensory nerves of the body which, when violently 

 stimulated, affect this center. Thus, excessive pain in 

 almost any region of the body at once influences the rate of 

 breathing. 



(3) The most important influences reaching this center 

 come from the sensory nerves of the lung itself. These 

 sensory nerves run in the large vagus trunk. 



That these sensory nerves play a most important role in 

 controlling the center, may be understood from the follow- 

 ing experiment: If the vagus on one side be cut, no marked 

 effect follows; but cutting both vagi, there at once results 

 a very much slowed breathing. The number of breaths may 

 sink to one-fourth, or even one-sixth of the normal. The 



