488 EFFECTS OF MUSCULAR EXERCISE. [Book ii. 



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, the condition of blood merely modifies the natural 

 automatic action of the centre. 



§ 303. There are reasons for thinking that conditions of the 

 blood, other than variations in the amount of oxygen and car- 

 bonic acid, may also materially affect the working of the res- 

 piratory 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 move- 

 ments is carried to the spinal bulb, and not by any nervous 

 impulses sent up to the bulb 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 respiratory centre when the blood passes from the hind limbs 

 to be distributed by the heart to the bulb. Now when a muscle 

 contracts its consumption of oxygen and production of carbonic 

 acid, especially the latter (§ 60), are increased ; the blood leav- 

 ing 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 

 bulb from the left side of the heart in a more venous, less com- 

 pletely 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 of the heart in such cases, is not less arterialized but 

 if anything more arterialized than usual. The increased res- 

 piratory movements induced by the changed blood soon prove 

 sufficient or even more than sufficient to give the blood the 

 extra quantity of oxygen and to remove the extra quantity of 

 carbonic acid. Obviously the blood coming from the tetanized 

 muscles affects the respiratory centre by virtue of some quality 

 which, unlike that due to the deficiency of oxygen or excess of 

 carbonic acid, is not immediately affected by the passage through 

 the lungs. Whether the quality in question be dependent on 

 an excess of sarcolactic acid, or on some other product or prod- 

 ucts of muscular metabolism, we do not as yet know. But the 

 fact that substances in the blood may so affect the respiratory 



