696 PHYSIOLOGY OF RESPIRATION. 



of the blood. On the other hand, if the blood supplying the center 

 is more arterialized than normal, by active ventilation of the lungs, 

 for instance, the center acts more feebly or may fail to act altogether, 

 giving the condition known as apnea. These facts may be accepted 

 as completely demonstrated, but they do not go far enough. When 

 we speak of the arterial blood being more venous than normal we 

 mean that it contains less oxygen and more carbon dioxid than 

 normal arterial blood. Which of these conditions serves to stimulate 

 the center, and which may be regarded as the constant stimulus 

 throughout life? The three possible views have been defended: 

 (1) That the normal stimulus is a lack of sufficient oxygen (Rosen- 

 thai). When sufficient O is supplied the center ceases to act, 

 becomes apneic. (2) That the normal stimulus is the presence of 

 an excess of CO 2 (Traube). When this excretion is quickly re- 

 moved the center ceases to act, becomes apneic. (3) It is possible 

 that the two factors may co-operate. Much evidence has been 

 collected to show that the action of the respiratory center is in- 

 creased when the tension of the CO 2 in the blood is raised without 

 altering that of the oxygen, and that a similar result is obtained 

 if the tension of oxygen is greatly diminished without any change 

 in that of the carbon dioxid, so that it must be admitted that a 

 change in either factor, if sufficiently great, acts as a stimulus. 

 Experiments, however, have indicated that the accumulation of 

 the CO 2 is the more efficient stimulus of the two.* Zuntz reports 

 the following interesting experiments, in which the extent of the 

 respiratory movements was measured by the amount of air 

 breathed in a minute. In one series the amount of oxygen in the 

 air breathed was reduced. This change did not affect the quan- 

 tity of carbon dioxid in the blood. The following results were 

 obtained : 



Normal air volume breathed per minute = 7,325 to 9,000 c.c. 



Air with 10 to 11.5 per 



cent, oxygen " " " = 8,166 to 9,428 " 



Air with 8 to 10 per 



cent, oxygen = 9,093 to 12,810 " 



A reduction of one-half of the oxygen in the air breathed had little 

 effect upon the respirations. From our present standpoint, how- 

 ever, the important thing is not the amount of oxygen in the air, 

 but the amount in the blood. Paul Bert's experiments! upon 

 living animals indicate that when the oxygen of the air is reduced 

 by a half the amount of oxygen in the blood is diminished by about 

 one-third. Assuming this to be correct, it is evident that a very 



*See Zuntz, "Archiv f. Physiologic," 1897, 379. See also Friedlander 

 and Herter, "Zeit. f. physiol. Chemie," 2, 99, and 3, 19. 

 t Bert, "La pression barometrique," 1878, 691. 



