370 THE RESPIRATION 



Although there is therefore no doubt that the C H of the blood may 

 be raised because of an increase in C0 2 in solution in the blood plasma 

 a C0 2 acidosis, as we may call it (see page 371) this does not prove that 

 the stimulation of the respiratory center is brought about solely by C H . 

 The increase in C0 2 might in itself also serve as a stimulus. That such 

 is actually the case was demonstrated by finding that, if C H of the blood 

 was first of all lowered by injecting alkali intravenously, hyperpnea still 

 developed in proportion as the C0 2 accumulated in the inspired air; and 

 that CH of the blood, when the hyperpnea was at its highest, was below 

 that of normal blood. Some other factor than C H must obviously be 

 responsible for this result. This is undoubtedly dependent on the C0 2 . 



Further corroboration of the claim that C0 2 has a specific stimu- 

 lating effect on th'e respiratory center that is independent of C H , has 

 been furnished by Hooker, Wilson and Connett. 23 These authors suc- 

 ceeded in keeping the centers of the medulla alive by perfusion with 

 defibrinated blood through the blood vessels of the brain, and found 

 that, although the respiratory movements of the diaphragm became de- 

 pressed with a decrease and excited with an increase in C H of the per- 

 fusion fluid, a greater activity of the center was produced when the fluid 

 contained a high tension of C0 2 than with another fluid of the same C H 

 but with a low tension of C0 2 . We conclude that, although the <7 H is the 

 important respiratory hormone, carbon dioxide per se also has a stimu- 

 lating influence. 



A similar conclusion had previously been arrived at by Lacquer and 

 Verzar 54 who studied the activity of the respiratory center in young rab- 

 bits perfused through the aorta with isotonic solutions in which the C H was 

 caused to vary by the addition of different acids. It was found that when 

 C H was about the neutral point (even slightly on the alkaline side), but 

 the solution contained C0 2 , much more marked stimulation occurred than 

 when C H was raised by adding some other acid. 



These conclusions are confirmed by observations on the influence of 

 C0 2 on other living cells than those of the respiratory center. Rona and 

 Neukirch 55 found that the isolated intestine when made to beat in oxy- 

 genated saline solution (page 497) is very sensitive to C0 2 , and Jacobs 56 

 has more recently made some very interesting observations on the toxic 

 effects of C0 2 as compared with that of other acids, on the tadpoles of 

 the toad, and on several species of protozoa. It was found that a saturated 

 solution of C0 2 is incomparably more toxic than are solutions of various 

 inorganic and organic acids of the same C H as the C0 2 solution. On the 

 other hand neutral solutions of NaHC0 3 are nontoxic, which indicates 

 that it can not be the HCO s -anion as some have supposed) that is the toxic 

 agent. The order of resistance of various protozoa to C0 2 was found to 



