602 EFFECTS OF MUSCULAR EXERCISE. [BOOK 11. 



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 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 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 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 

 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 nutritive conditions 

 of the diluted blood the medulla 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 

 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 

 measure 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 shewn 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 ( 63), are increased ; the blood leaving the muscle 

 is more venous than usual. Hence when many muscles are con- 

 tracting 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 



