1142 



PHYSIOLOGY 



up to the vagus centre, which would stop the movement of inspiration. 

 The movement of expiration would then begin, and the collapse of the lungs 

 thereby produced would itself send impulses up the vagi which would tend 

 to excite an inspiratory movement. Both inspiration and expiration would 

 therefore be shortened, and the successive movements would follow one 

 another at a shorter interval than if the vagi were not functional. In this 

 way, under normal circumstances, the rhythm of the respiratory centre 

 must be determined reflexly through the agency of the vagi, while the chief 

 factor in determining the total pulmonary ventilation is, as we have seen, 

 the carbon dioxide tension of the blood. 



RT Lung. 



L T Lung. 



FIG. 526. Diagram to illustrate Head's experiment on the effect of collapse of the 

 lung. B.O, respiratory centre; R.v, L.V, right and left vagi. 



In the foregoing account we have spoken of the expiratory and inspiratory effects 

 of the vagus as if they were of equal importance. It seems probable however that the 

 inhibitory or expiratory impulses started by the inspiratory movement, the only or 

 the more active part of normal respiration, play a more prominent part in the regulation 

 of respiration than do the inspiratory impulses ; and one observer (Gad) goes so far as 

 to deny altogether the existence of two kinds of respiratory fibres in the vagus. Accord- 

 ing to Gad, the vagus, as regards the respiratory centre, is a purely inhibitory nerve. 

 Hence the primary effect of dividing both vagi is an increased inspiratory tone. This 

 view at first seems paradoxical, in that it explains the final slowing of respiration after 

 section of the vagi as due to the cutting off of previous inhibitory impulses. But inhi- 

 bition in all tissues has a twofold effect. Although the immediate effect is diminution 

 of activity, yet the diminished disintegration necessarily associated with lowered 

 activity means an increase of the anabolic at the expense of the catabolic processes of 

 the tissues. In this way we explained the diminished excitability occurring in a nerve 

 at the anode of a constant current, and it will be remembered that the secondary result 

 of anelectroronus was increased irritability and consequent excitation at break of the 

 constant current. The same sort of process must occur in the respiratory centre. A 

 continued restraint of its rhythmic activity must lead to a heaping up of its irritable 

 material, so that the final result is a state of hyperexcitability in which the centre, so to 

 speak, boils over on the slightest provocation. 



In this condition a cutting off of the inhibitory impulses must at first increase the 



