620 PHYSIOLOGY OF RESPIRATION. 



reflex stimulation. The further question arises as to the nature of 

 the automatic stimulus. Inasmuch as the activity of the center 

 controls the gaseous exchanges of the blood, it was natural perhaps 

 for physiologists to look to the gases of the blood for the origin of 

 the internal stimulus. Experiments show beyond question that 

 the condition of the gases in the blood has a direct and marked 

 influence upon the activity of the center. If for any reason the 

 blood supplying the center becomes more venous, the respirations 

 are increased in force or rate or both, and indeed the activity of the 

 center is in a general way increased in proportion to the venosity 

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

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

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

 condition known as apnea. These facts may be accepted as com- 

 pletely 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 through- 

 out life? The three possible views have been defended : (1) That 

 the normal stimulus is a lack of sufficient oxygen (Rosenthal). 

 When sufficient O is supplied the center ceases to act, becomes 

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

 of C0 2 (Traube). When this excretion is quickly removed the cen- 

 ter ceases to act, becomes apneic. (3) It is possible that the two 

 factors may co-operate. The blood that flows through the center 

 may stimulate the cells by virtue of the fact that it does not remove 

 the C0 2 fast enough and does not supply sufficient oxygen. It is 

 difficult, indeed impossible, to arrive at any certain conclusion upon 

 this point. Much evidence has been collected to show that the 

 action of the respiratory center is increased when the tension of the 

 C0 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 suffi- 

 ciently great, acts as a stimulus. Experiments, however, have 

 indicated that the accumulation of the C0 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 quantity of carbon dioxid in the blood. The 

 following results were obtained: 



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

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



