THE NERVOUS MECHANISM OF RESPIRATION. 383 



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 modi- 

 fying the whole complex series of metabolic changes, the whole chain of 

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

 dering 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 respira- 

 tory centre 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 or deadened by a lack of oxygen. But these as yet are matters 

 of speculation. 



We may, perhaps, add that, under various nutritive conditions, the sensi- 

 tiveness of the metabolism of the respiratory centre to lack of oxygen may 

 vary widely. Thus, while undoubtedly under the normal nutritive condi- 

 tions afforded by the ordinary supply of normal blood to the medulla, lack 

 of oxygen in that blood at once provokes increased respiratory movements, 

 it need not do so under other nutritive conditions of the medulla. By trans- 

 fusion 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 medulla 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 nutri- 

 trive conditions of the diluted blood the medulla is not affected in the same 

 way as before by lack of oxygen. 



316. There are reasons for thinking that conditions of the blood, other 

 than variations in the amount of oxygen and carbonic acid, may also mate- 

 rially affect the working of the respiratory centre. It is a matter of common 

 experience that muscular exertion, especially if at all excessive, increases 

 the respiratory movements ; violent exercise soon puts a man " out of 

 breath." This increased activity of the respiratory centre is in large meas- 

 ure at all events caused by the character of the blood which during and for 

 some little time after the movements is carried to the medulla, and not by 

 any nervous impulses sent up to the medulla from the contracting muscles. 

 This is shown by the fact that if in an animal the spinal cord be divided in 

 the dorsal or lumbar region and the hind limbs be powerfully tetanized, the 

 respiratory movements are increased ; the animal pants as it would do if it 

 had been running. In such a case the only connection between the hind 

 limbs and the respiratory centre is through the blood ; it must be some 

 change in the blood caused by the muscular contractions which affects the 

 medulla when the blood passes from the hind limbs to be distributed by the 

 heart to the medulla. Now when a muscle contracts, its consumption of 

 oxygen and production of carbonic acid, especially the latter ( 60), are in- 

 creased ; the blood leaving the muscle is more venous than usual. Hence, 

 when many muscles are contracting powerfully, the blood carried to the 

 right side of the heart is more venous than usual ; and we might expect 

 that it is this unusually venous blood failing to be adequately arterialized in 

 the lungs, and hence reaching the medulla from the left side of the heart in 

 a more venous, less completely arterialized, condition than usual, which stirs 

 up the respiratory centre to increased activity. 



On examination, however, it is found that the blood leaving the left side 



