370 PHYSIOLOGY. 



are two: (1) the tension of carbonic acid on the respiration center 

 (a chemical theory), and (2) irritation of the expiratory fibers in 

 the vagus by distension of the lungs, and an irritation of the inspiratory 

 fibers in the vagus in sucking the air out of the lungs (a mechanical 

 theory.) 



The vagi, in reference to the movements of respiration, according 

 to F. H. Scott, must be regarded in the same light as the sensory 

 nerves of the muscle. Without the vagi the respiratory muscular 

 movements are excessive, and thus resemble the movements of an 

 ataxic limb. 



It is not the increase of oxygen which usually energizes the 

 center of respiration. Zuntz has shown that a reduction of one-half 



Fig. 134. Strong Irritation of the Central End of the Divided 

 Right Vagus of a Rabbit by an Induction Current. Arrest of Respira- 

 tory Movement in Inspiration. Read from left. (GLEY.) 



the oxygen in the air inhaled had little effect upon the rate of 

 respiration. 



Haldane found that the center of respiration was not affected until 

 the blood had lost one-third of its oxygen. When the tension of 

 oxygen falls below 13 per cent, of an atmosphere, then deficiency of 

 oxygen stimulates the center of respiration, for the tension of carbon 

 dioxide in the alveolus is then lower than normal. 



Haldane and Priestly found that the smallest increment of pres- 

 sure of carbonic acid (0.2 per cent.) in the inspired air is accompanied 

 by an increase in ventilation of the lungs sufficient to keep the per- 

 centage of carbon dioxide constant in the air of the alveolus. The 

 tension of carbon dioxide regulates the respiratory movements. 



Muscular exercise is the cause of an increased production of 

 carbon dioxide and its tension. Here the compensation is by an 

 increase in the alveolar ventilation in proportion to the increase of 



