INNERVATION OF THE RESPIRATORY MOVEMENTS. 697 



considerable reduction may be made in the oxygen of the blood 

 without noticeably affecting the respirations. A similar conclusion 

 may be drawn from Haldane's experiments* with carbon monoxid. 

 He found upon breathing mixtures of this gas that no distinct effects 

 were observable until the blood was about one-third saturated with 

 the gas, that is, had lost one-third of its oxygen. Zuntz's ex- 

 periments, in which the C0 2 in the air breathed was increased, while 

 the oxygen remained normal, gave quite different results, as follows : 



Normal air yolume breathed per minute, 7,433 c.c. 



Air of 20.2 per cent. O, 0.95 per 



cent. CO 2 " " " " 9.060 " 



Air of 18.06 per cent. O, 2.97 per 



cent. C0 2 " " " " 11,326 " 



Air of 18.42 per cent. O, 11.5 per 



cent. CO 2 " " " 32,464 " 



These and similar results f show that small differences in the 

 amount of the carbon dioxid in the blood have a distinct effect 

 upon the activity of the respiratory center. Under normal con- 

 ditions the respiratory center receives blood containing 19 to 20 

 volumes per cent, of oxygen, while the venous blood flowing away 

 from the center still holds 10 to 12 per cent. Considering the 

 small effect of lowering this oxygen supply by one-third, it is 

 difficult to believe that normally the amount of oxygen is so 

 deficient for the normal metabolism as to set up a constant 

 stimulus. The trend of recent work favors rather the view that 

 the normal stimulus to the respiratory center is the carbon dioxid. 

 When this substance is present above a certain amount or tension 

 it acts as a stimulus, directly or indirectly, and gives rise to the 

 moderate movements of normal inspiration. If the tension of 

 the carbon dioxid is increased, the stimulus becomes stronger and 

 leads to the production of a condition of hyperpnea and dyspnea. 

 On the other hand, if for any reason, such as active ventilation of 

 the lungs, the tension of the carbon dioxid in the blood falls below 

 a certain value, estimated by Zuntz as lying between 19 and 24 

 mms., no stimulation occurs, the center is in a condition of apnea 

 and respiratory movements cease. Accepting the view that car- 

 bon dioxid in the blood circulating in the medulla gives rise to the 

 normal stimulus to the respiratory center, one naturally inquires 

 why a deficient supply of oxygen should also stimulate the center. 

 It is true, as stated above, that the supply of oxygen may be 

 diminished considerably before any augmented action of the center 

 is observed, but there seems to be no question that dyspneic move- 

 ments result when the oxygen tension falls below a certain point. 

 One explanation has been suggested which may be accepted pro- 



* Haldane, "Journal of Physiology," 18, 442, 1895. 



t See Haldane and Priestley, "Journal of Physiology," 32, 225, 1905. 



