394 THE HUMAN BODJ. " 



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 (p. 363); 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 i 

 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 bot- 

 tom 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 out rush ; 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 respira- 

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

 circumstances, being excited; but, to discharge a nervous 

 impulse along the efferent respiratory nerves, they have to 

 overcome a certain resistance. The nervous impulses have 



