CHAP. IL] RESPIRATION. 633 



through the capillaries of the bulb at the moment preceding 

 that breath, it is not to be imagined that each breath is the 

 result of the lack of oxygen felt immediately before. On the 

 contrary, as we have previously urged, the respiratory centre~like 

 the cardiac substance is an automatic centre, the respiratory 

 impulses issue from it in rhythmic series as a result of the 

 molecular changes, of the metabolism going on in its substance ; 

 and whatever affects that rhythm, whether few or many beats 

 be influenced, produces its result by modifying that metabolism. 

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

 afferent fibre, both affect the centre by modifying its metabolism ; 

 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, it is true, imagine that a lack of oxygen 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 

 substance of the respiratory centre is that its metabolism is so 

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

 such as muscle, rendered unstable and more explosive, not simply 

 diminished or deadened by a lack of oxygen ; and possibly this 

 feature may in a less marked degree be recognized in other parts 

 of the central nervous system. But these as yet are matters of 

 speculation. 



We may perhaps add that, under various nutritive conditions, 

 the sensitiveness of the metabolism of the respiratory centre to 

 lack of oxygen may vary widely. Thus while undoubtedly under 

 the normal nutritive conditions afforded by the ordinary supply 

 of normal blood to the bulb, lack of oxygen in that blood at once 

 provokes increased respiratory movements, it need not do so under 

 other nutritive conditions of the bulb. By transfusion a large 

 proportion of the haemoglobin-holding blood may in an animal be 

 gradually replaced by hsemoglobinless normal saline solution. In 

 such a case the amount of oxygen brought to the bulb by the 

 diluted blood must be greatly diminished, and yet, if the change 

 be made sufficiently slowly, no conspicuous dyspnoea is produced ; 

 under the new strange nutritive conditions of the diluted blood 

 the bulb is not affected in the same way as before by. lack of 

 oxygen. 



373. There are reasons for thinking that conditions of the 

 blood, other than variations in the amount of oxygen and carbonic 

 acid, may also materially affect the working of the respiratory 

 centre. It is a matter of common experience that muscular 



F. II. 41 



