THE RESPIRATION 295 



The accumulation of other Waste Products in the blood 



undoubtedly stimulates these nerve cells. Muscular exertion 

 increases the activity of respiration. That this is really due 

 to the accumulation of products of muscle waste in the blood 

 and not to any reflex influence through the nerves is shown 

 by the fact that it occurs in a dog when the spinal cord is 

 cut across, and the muscles of the hind limb are made to 

 contract by stimulating the nerves with electricity. The fact 

 that the injection of acid into the blood of animals, which 

 are unable rapidly to neutralise it, increases the respirations, 

 suggests the possibility that sarcolactic acid may be among 

 the substances which can stimulate these nerve cells. 



Yet another factor of importance in modifying the activity 

 of this centre is the temperature of the animal. Increase in 

 temperature accelerates the rate of the heart so, too, it 

 accelerates the rate of the respirations, and in about the 

 same proportion, as is seen in feverish attacks, where pulse 

 and respiration are proportionately quickened so that their 

 ratio remains unaltered. When the respiratory rate rises 

 out of proportion to the rate of the pulse, it is usually an 

 indication that some pulmonary irritation is present. 



The lungs and heart being packed tightly together in the 

 air-tight thorax, and both undergoing periodic changes, 

 necessarily influence one another. At the same time, the 

 close proximity of the respiratory and cardio-motor centres 

 in the medulla seems to lead to the activity of one influencing 

 the other. 



Influence of Respiration on Circulation. The circulation 

 is modified in two ways by respiration. First, the rate of 

 the heart ; and second, the arterial blood pressure undergo 

 alterations. 



1st. Rate of Heart. If a sphygmographic trace giving the 

 pulse waves during the course of two or three respirations be 

 examined, it will be found that during inspiration the heart 

 is acting more rapidly, while during expiration its action is 

 slowed. 



If the vagus is cut these changes are not seen, showing 

 that the inspiratory acceleration is not the result simply of 

 the larger amount of blood which enters the heart during 



