CHAP. IL] RESPIRATION. 601 



shewn by direct analysis, is very largely increased. On the 

 contrary, the increase in the respiratory movements may after a 

 while pass off, the animal becoming unconscious, and appearing 

 to be suffering rather from a narcotic poison than from simple 

 dyspnoea; the excess of carbonic acid in the blood appears 

 to affect other parts of the central nervous system, and especially 

 portions of the brain, more profoundly than it does the respi- 

 ratory centre. It has been maintained by some that while a 

 deficiency of oxygen promotes inspiratory movements, an excess 

 of carbonic acid stimulates the expiratory movements, the nervous 

 mechanisms being so arranged that a lack of oxygen leads 

 to an effort to get more of it and a too great load of carbonic 

 acid to an effort to get rid of it ; but the facts are opposed to 

 the existence of any such teleological adaptation. It is obvious 

 however that a lack of oxygen and an excess of carbonic acid 

 affect the respiratory centre in very different ways, and that 

 in ordinary cases of interference with the interchange in the 

 lungs, as in deficient aeration, it is the lack of oxygen which plays 

 the principal part in developing the abnormal respiratory move- 

 ments. We may infer that it too is chiefly concerned in regulating 

 the more normal respiration, but cannot as yet say what is the 

 exact share to be attributed to the carbonic acid. 



We may here point out that it is not to be supposed that 

 each breath is determined by the condition of the blood flowing 

 through the capillaries of the medulla 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 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 substance of the respiratory centre is that its metabolism is 

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



