CHAPTER XXXVIII. 



THE INFLUENCE OF VARIOUS CONDITIONS UPON 

 THE RESPIRATIONS, 



The Effect of Muscular Work upon the Respiratory Move- 

 ments. It is a matter of common experience that muscular ex- 

 ercise increases the rate and amplitude of the respiratory move- 

 ments. Roughly speaking, the increase is proportional to the 

 amount of muscular work, and the relationship is evidently a bene- 

 ficial adaptation. The greater the amount of work done, the 

 larger will be the amount of C0 2 produced and the greater will be 

 the need of oxygen. The adaptation was formerly explained in 

 what seemed to be an entirely satisfactory way by assuming that 

 the increased consumption of O and the greater production of CO a 

 in the muscles resulted in rendering the blood more venous, and 

 consequently the respiratory center was stimulated more strongly , 

 and indeed proportionally to the muscular effort. Geppert and 

 Zuntz,* however, have shown by gas analyses that whatever may 

 be the condition of the venous blood during muscular exercise 

 the arterial blood sent out from the left heart shows no constant, 

 change in the quantity or tension of the contained gases. They 

 proved, also, that the effect on the center is not simply a reflex 

 from the nerves in the muscles, since when the hind limbs were made 

 to contract by stimulation the respiratory center was affected in 

 the usual way although all the nerve connections were destroyed. 

 They conclude, therefore, that the respiratory effect of muscular 

 work must be due to certain substances produced in the muscle and 

 given off to the blood. Other experiments (Lehmann) make it 

 probable that these substances are the acid products, lactic acid and 

 acid phosphates, known to be formed in muscle during contraction. 

 Dilute acids injected directly into the veins produce a similar 

 result. The adaptation is a most interesting one, since the products 

 that decrease the irritability of the muscle itself seem to cause an 

 increase in excitability of the group of nerve cells constituting the 

 respiratory center. 



The Effect of Variations in the Composition of the Air 

 Breathed. Variations in the amount of nitrogen in the inspired 



* Geppert and Zuntz, " Archiv f . die gesammte Physiologic, " 42, 189 > 

 1888. 



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