418 THE HUMAN BODY. 



next act. In natural quiet breathing there is no essential 

 difference between the expiration and the pause. The in- 

 spiration is the only active part; the expiration and the 

 pause are dependent on muscular inactivity and, there- 

 fore, on the cessation of the discharge of nervous impulses 

 from the respiratory centre. But then, we may ask, if in 

 accordance with the hypothesis made in the last paragraph, 

 the respiratory centre is constantly being excited, why is it 

 not always discharging ? why does it only send out nervous 

 impulses at intervals? This question, which is essentially 

 the same as that why the heart beats rhythmically, belongs 

 to the higher regions of Physiology and can only at present 

 be hypothetically answered. Let us consider, for a moment, 

 ordinary mechanical circumstances under which a steady 

 supply is turned into an intermittent discharge. Suppose a 

 tube closed water-tight below by a hinged bottom, which is 

 kept shut by a spring. If a steady stream of water is poured 

 into the tube from above, the water will rise until its weight 

 is able to overcome the pressure of the spring, and the bottom 

 will then be forced down and some water flow out. The 

 spring will then press the bottom up again, and the water 

 accumulate until its weight again forces open the bottom of 

 the tube, and there is another outrush; and so on. By 

 opposing a certain resistance to the exit we could thus turn 

 a steady inflow into a rhythmic outflow. Or, take the case 

 of a tube with one end immersed in water and a steady 

 stream of air blown into its other end. The air will emerge 

 from the immersed end, not in a steady current, but in a 

 series of bubbles. Its pressure in the tube must rise until 

 it is able to overcome the cohesive force of the water, and 

 then a bubble bursts forth; after this the air has again to 

 get up the requisite pressure in the tube before another 

 bubble is ejected; and so the continuous supply is trans- 

 formed into an intermittent delivery. Physiologists sup- 

 pose something of the same kind to occur in the respiratory 

 centre. Its nerve-cells are always, under usual circum- 

 tances, being excited; but, to discharge a nervous impulse 

 along the efferent respiratory nerves, they have to overcome 

 a certain resistance. The nervous impulses have to accumu- 

 late, or " gain a head," before they travel out from the 

 centre, and, after their discharge, time is required to attain 

 once more the necessary level of irruption before a fresh in- 



