234 STUDIES IN ADVANCED PHYSIOLOGY. 



off, and the supply of oxygen becomes short and this waste 

 product, therefore, accumulates, the center becomes at once 

 irritated by it to a greater and greater extent, until finally 

 the movements of respiration are ushered in. 



That condition in which the blood is so richly supplied 

 with oxygen that there is no tendency to breathe, is called 

 apnce. The condition in which the center is normally stim- 

 ulated is called eupnce, while finally that condition in which 

 there is not an adequate supply of oxygen, and which leads 

 to the phenomena of suffocation and to the convulsions 

 accompanying the same is called dyspnce. From these defi- 

 nitions it will be seen that the condition of respiration be- 

 fore birth is that of apnce. But it not infrequently happens 

 that even before birth the circulation with the placenta is 

 interrupted, the oxygen supply of the foetus is cut off, and 

 so dyspnce results, which may lead to real active respiratory 

 movements. 



As the tissues are always in action and as this irritating 

 substance, therefore, must continually be forming, we are 

 forced to believe that this center must be continually irri- 

 tated by this substance. The question then at once arises, 

 why this center which is being constantly and without in- 

 terruption irritated will give rise to impulses which are 

 periodic. Why would not a continued stimulation of this 

 center produce a tetanus just in the same way as a continued 

 stimulation of the nerve of the muscle would produce a con- 

 tinued tetanic contraction? This is explained by supposing 

 that there is a kind of resistance in this center, and that an 

 impulse does not result until this intrinsic resistance to act 

 is overcome. 



This center is, using a rather far-fetched metaphor, a 

 kind of intermittent spring. Such an intermittent spring 

 will discharge its waters periodically in spite of the fact 

 that its supply flows into it at a constant and uniform rate. 

 So in this center. The stimuli from this irritating substance 

 normally found in venous blood, and possibly, as was stated, 

 a waste product of the active tissues, keep flowing in, ac- 



