THE LUNGS AND RESPIRATION. 233 



how a thing which is absent may serve as a positive stimu- 

 lus. This savors just a little of a mental absurdity. It is 

 perfectly conceivable how the presence of the carbon diox- 

 ide in blood might stimulate the center, but experiments 

 have been made by passing different kinds of blood heavily 

 charged with carbon dioxide through the center, and yet 

 the center has failed to be materially stimulated by these 

 large quantities of that gas. Of late, therefore, physiolo- 

 gists have, looked elsewhere for the substance in question. 

 Quite a suggestive experiment was made when blood 

 which had just passed through a severely-exercised muscle 

 was then injected into the arteries which traverse the respir- 

 atory center. A very violent stimulation was at once the 

 result. This would seem to indicate that when tissues such 

 as the muscle are hard at work there is a production of waste 

 products of some kind which act as a powerful irritant to 

 this center. This irritating waste product may possibly be 

 eliminated in the lungs, or possibly destroyed when the 

 blood becomes arterial, either case explaining why increased 

 breathing will serve to lessen the stimulating effect of this 

 substance. If, for instance, the blood does not succeed in 

 passing to the lungs rapidly enough, this irritating waste 

 product accumulates in the blood, and so stimulates the 

 center to more and more activity. If, on the other hand, 

 by the process of breathing this substance is eliminated, 

 either through the lung or destroyed in the blood as soon 

 as it becomes arterial, we can understand why when such 

 blood passes through the center the center remains quiet 

 and inactive. This will explain why there are no respira- 

 tory movements before birth, for at this time the blood 

 stream is richly supplied with oxygen from the maternal 

 wall. This waste product is given no chance to accumu- 

 late in the blood, but by the oxygen from the placenta it is 

 continually removed. On account of this the center is not 

 at all stimulated and so remains perfectly dormant, sending 

 out not a single impulse to breathe. However, at the mo- 

 ment of birth, when the circulation with the placenta is cut 



