THE CONTROL OF THE RESPIRATION 333 



imals and, in the case of man, in different individuals. Thus, when a 

 man is made suddenly to breathe into compressed air, the apnea often 

 lasts for about half a minute, the pause being then broken by a deep ex- 

 piration followed by a further pause, then again an expiration, and so 

 on with progressively shorter pauses. Disregarding for the present 

 any influences which changes in the composition of the air in the lungs 

 or of the gases in the blood might have in producing the apnea, we may 

 consider the possibility that it is the result of afferent fibers in the 

 vagus. This is an old view, but the most recent experimental evidence 

 does not lend support to it. It was shown by Boothby and Berry, 14 for 

 example, that a similar apnea, though indeed of shorter duration, could 

 be produced in dogs in which the pulmonary branches of both vagus 

 nerves had been severed two months previously. The apnea is, there- 

 fore, not a reflex of the vagus, and must be interpreted as due to nerv- 

 ous impulses passing to the respiratory center from some other part of 

 the nervous system, perhaps from centers higher up, or to stimuli trans- 

 mitted to the respiratory center possibly through afferent fibers in the 

 respiratory muscles. 



The formerly very popular theory that respiration is controlled au- 

 tomatically by alternate distention and collapse of the alveoli, acting 

 through the afferent fibers of the vagus nerve on the respiratory center 

 in such a way as to bring the opposite act with each expiration and 

 inspiration, must, therefore, be abandoned. But we can not deny that 

 the vagus plays a most important role in the control of the function of 

 the respiratory center, for apart from the effect which we have seen to 

 follow the severence of continuity of the nerve, there is the important 

 observation of Alcock and others 13 that when nonpolarizable electrodes 

 are placed on the vagus nerve and connected with a galvanometer, a 

 current of action occurs toward the end of each inspiration in quiet 

 breathing; and when the respirations are forced, a current of action 

 appears during both inspiration and expiration. Another reason for 

 believing that the vagi have some important function to perform in con- 

 nection with the control of respiration is the fact, observed by F. H. Scott, 16 

 that in an intact animal, when atmospheres containing increasing percent- 

 ages of carbon dioxide are respired, the respirations become both deeper 

 and quicker, whereas in one whose vagi have been cut the carbon diox- 

 ide causes only a deepening of the respirations. From this result it 

 would appear that the vagi exert an influence on the rate of the respira- 

 tions but not on their depth, this effect, as we shall see later, being de- 

 pendent primarily on changes in the composition of the blood supplying 

 the respiratory center. It is probable that both controlling agencies act 

 together, the one serving to maintain the center in a proper state of 



