THE CONTROL OF THE RESPIRATION 349 



Such a method is obviously much more physiological than one in which 

 the air-tube is suddenly clamped at the end of inspiration and the lungs 

 left in a distended condition. When the air under pressure is 

 breathed there is usually a cessation of breathing, called apnea. 

 The extent to which it occurs varies very considerably in different an- 

 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 stimulation of 

 the termination of afferent vagus fibers in the walls of the alveoli. 

 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 through 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. 



It has usually been taught that section of the vagus nerves of both 

 sides results in death from pneumonia within a few days. E. A. Schafer 

 has shown that this is not the case but that animals (cats) can be kept 

 alive practically indefinitely if precautions are taken to prevent the 

 obstruction of the larynx which supervenes, through paralysis of the 

 laryngeal muscles, when the inferior laryngeal nerves are cut. The 

 obstruction causes asphyxia followed by congestion and edema of the 

 lungs. If the larynx be cauterized so as to prevent obstruction double 

 vagotomy even in the neck has no greater influence on breathing than 

 perhaps a slight and often transitory slowing. 53 



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 excite the opposite act following each inspiration and 

 expiration, must, therefore, be abandoned. But we cannot 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 severance of continuity of the nerve, there is the important 



