634 EFFECTS OF MUSCULAR EXERCISE. [BOOK n. 



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 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 thoracic 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 bulb 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 ( 63), are increased ; 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 bulb 

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

 anything more arterialized than usual. The increased respiratory 

 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 products of muscular metabolism, 

 we do not as yet know. But the fact that substances in the blood 

 may so affect the respiratory centre is interesting since it shews 

 by how many safeguards the working of the respiratory centre is 

 carefully adapted to the needs of the economy ; this is not deter- 

 mined by the lack of oxygen only. 



Thus a change in the circumstances surrounding an animal 

 body, or a change in the body itself, may in one or more of 

 several ways, by acting as a stimulus to some afferent nerves and 

 so sending up afferent nervous impulses to the respiratory centre, 

 or by interfering with the interchange of gases in the lungs, or by 

 otherwise altering the proportion of the gases present in the blood 



