1218 PHYSIOLOGY 



these organs in respiration. Let us consider, for instance, what will 

 happen if the influence of the two vagi could be suddenly thrown in 

 after these nerves have been divided. (This experiment can, in fact, 

 be realised more or less completely if the functional division of the vagi 

 be effected by cooling or by ether narcosis.) The animal would be 

 breathing slowly and deeply. If at the beginning of an inspiration 

 the vagi become functional the expansion of the lungs caused by the 

 inspiratory movement would send inhibitory impulses 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. 



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. According 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 inhibition 

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

 activity, yet the diminished disintegration necessarily associated with diminished 

 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 anelectrotonus was increased irritability and con- 

 sequent 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 activity of the centre, leading to the increased inspiratory tonus already 

 described. But, unchecked by any reining impulses, the centre enters upon a 

 career of spendthrift activity. Each inspiratory contraction is maximal, but the 

 centre, exhausted by the effort, has to wait a considerable time before it can 

 accumulate sufficient energy for the next ; hence the final result of section of 

 both vagi is deepening and slowing of respiration. 



Although Gad has rendered great service in emphasising the importance of 



