692 PHYSIOLOGY OF RESPIRATION. 



the ventricle depends upon the previous systole of the auricle. 

 That an active expiration is not caused reflexly by the mechanical 

 expansion of the lungs seems to be demonstrated by the fact that 

 the most forcible voluntary inspiration is followed by a passive, 

 not an active expiration. The relations of the inspiratory and 

 expiratory centers under the various conditions of artificial stim- 

 ulation are very complex, and although it is possible to represent 

 these relations more or less completely by schemata of some sort 

 it does not seem advisable at present to seriously consider such 

 hypotheses. 



The Nature of the Automatic Stimulus to the Respiratory 

 Center. — We have accepted the view that the respiratory (inspira- 

 tory) center is essentially automatic, although very sensitive to 

 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, 

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

 gether, 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? Much evidence has been collected to show 

 that the action of the respiratory center is increased when the ten- 

 sion of the CO2 in the blood is raised without altering that of the 

 oxygen, and that a similar result is obtained, but much less readily, 

 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 may act as a stimulus. Experiments, how- 

 ever, have indicated that the accumulation of the CO2 is much 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 



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

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



