348 THE RESPIRATION 



nature of this control and the remarkable sensitivity of the center towards 

 it have been thoroughly established. We shall return to this important 

 subject later. Meanwhile we shall proceed to examine the manner in 

 which the center is affected by sensory impulses transmitted to it. 



THE REFLEX CONTROL OF THE RESPIRATORY CENTER 



The afferent nerve fibers going to the respiratory center may conven- 

 iently be divided into two groups: those coming from the respiratory or- 

 gans and those coming from other parts of the body. 



Afferent Impulses from the Respiratory Organs 



If the vagus nerves are cut or their continuity severed by freezing 

 a portion of them, the respiratory movements become markedly slower. 

 Evidently, the vagus nerves in some way hurry up the respiratory 

 movements. Again, if the central end of either vagus is stimulated 

 with the ordinary interrupted faradic current, a profound effect on 

 the respiratory movements is usually observed. This effect is how- 

 ever not strictly predictable. Usually there is a quickening of respira- 

 tion, and if the stimulus is a strong one, there may be a standstill of the 

 thorax in the inspiratory position. On the other hand, if the central 

 end of the nerve is stimulated with other types of stimuli, as by slow, 

 weak faradic shocks or by the stimulus produced by the closure of an 

 ascending voltaic current, the effect may be to prolong expiration rather 

 \than excite inspiration. Such results would seem to indicate that the 

 Vagus contains two kinds of afferent fibers to fke respiratory center, one 

 kind stimulating inspiration, the other, stimulating expiration. 



Supposing that both these kinds of fibers exist, the next question is, how 

 do they become excited at their terminations in the lungs ? The most nat- 

 ural assumption is that the mechanical distention and collapse of the 

 alveoli which occurs with each respiratory act, serves as the stimulus 

 an hypothesis to which support is offered by the observation that, when 

 air is blown into the lungs so as to distend the alveoli, the animal im- 

 mediately makes a forced expiratory movement, whereas when the air 

 is sucked out, the thorax assumes the inspiratory position. 



Of the many methods that have been employed to produce disten- 

 tion of the alveoli, the best is undoubtedly that recently employed by 

 Haldane 13 and Boothby. 14 The person or animal is made to respire through 

 a tube in which is inserted a three-way stopcock, which communicates 

 either with the outside air or with a side-tube leading to a spirometer 

 or bag containing air under slight pressure, so that when the stopcock 

 is turned breathing takes place against a definite positive pressure. 



