1092 PHYSIOLOGY 



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 became 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. 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 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 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 

 activity of the centre, leading to the increased inspiratory tonus already described. 

 But unchecked by any reign : ng 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 respira- 

 tion. 



Although Gad has rendered great service in emphasising the importance of the in- 

 hibitory or expiratory impulses which ascend the vagi there is no doubt that he went too 

 far in denying the existence of inspiratory fibres in the vagus. This is shown by the 

 following experiment of Head. According to Gad's view, collapse of both lungs implies 

 simply a removal of the normal inhibitory impulses ascending the vagi, and is therefore 

 equivalent to division of these two nerves. If in the rabbit the left vagus be divided, a tube 

 can be introduced into the left bronchus, and artificial respiration can be performed by 

 alternate inflation and collapse of the left lung, without in any way affecting the respira- 



