CHEMICAL PROPERTIES OF ADRENAL SECRETION. 



73 



centers, and particularly with the view that these centers "may 

 be excited both by blood that is rich in carbon dioxide and by 

 blood that is poor in oxygen." And yet there probably does 

 not exist in the whole domain of physiology a question ad- 

 mittedly more obscure, and in which ingenious theories have 

 proven more sterile. A reduction of the oxygen in the blood 

 is thought to impair central functions, although there is 

 ample experimental evidence to prove that such is not the 

 case. The blood of an animal may in great part be removed 

 and replaced by saline solution and its breathing continue as 

 quietly and regularly as before the procedure. The same re- 

 sults have been obtained even in frogs from which the heart 

 had been removed. As elsewhere in the organism, the func- 

 tions of the medullary centers occur in virtue of the molecular 

 changes, which must normally be enhanced, if at all, by an 

 excess of oxygen and inhibited by an excess of carbonic acid. 

 How can, therefore, the latter give rise to increased respiratory 

 activity? 



That considerable uncertainty prevails in this connection 

 is shown by the following lines by Professor Foster 149 : "A 

 lack of oxygen in the blood, or a nervous impulse along an 

 afferent fiber, both affect the center by modifying its metab- 

 olism; but each probably affects it in a different way. It is 

 beyond our present knowledge to explain how either the one 

 or the other acts. We may imagine that a lack of oxygen, on 

 the other hand, has a more profound effect in modifying the 

 whole complex series of metabolic changes, the whole chain of 

 building-up and breaking-down processes, thus in some way or 

 other rendering the whole edifice, so to speak, more unstable; 

 and that an afferent augmenting impulse (and possibly an 

 excess of carbonic acid) acts rather after the fashion of what 

 we are accustomed to call a stimulus, and fires off a larger 

 amount of the already stored up explosive compounds. And 

 we may further imagine that the special feature of the sub- 

 stance of the respiratory center is that the metabolism is so 

 arranged as to be thus, unlike that of other living substances, 

 rendered unstable and more explosive, not simply diminished 



149 Foster: "Text-book of Physiology," p. 383. 



