CHAP, ii.] RESPIRATION. 627 



taking place. We have also seen that when both vagus nerves 

 are divided the respiration is slower and deeper, but is otherwise 

 regular. If however after removal of the brain above the bulb 

 both vagus nerves are divided, if the respiratory centre be cut off 

 at one and the same time from impulses passing down from the 

 higher parts of the brain, and from impulses ascending the vagus 

 nerves, the result is that the respirations take on the form of a 

 series of long-continued inspiratory spasms. It would seem as if 

 there were a tendency in the respiratory centre to go off into 

 tetanic inspiratory explosions, that this tendency is held in check 

 by impulses from the brain when the vagus nerves are divided, 

 and by impulses along the vagus nerves when the brain is 

 removed, but meets with no adequate checks when impulses from 

 both sources are cut off at the same time. 



367. Hypotheses have been put forward to explain the 

 changes in the respiratory centre which lead to the rhythmic dis- 

 charge of inspiratory and expiratory impulses, and the further 

 changes which result from the advent of augmenting and inhibitory 

 impulses ; but these as yet remain mere hypotheses and it would 

 not be profitable to discuss them here. We may add that though 

 the analogy of the cardiac nervous mechanism, in which we can 

 anatomically distinguish between augmentor and inhibitory fibres, 

 justifies us in speaking of augmentor and inhibitory respiratory 

 fibres as existing in the vagus nerve, we are not as yet able to 

 distinguish them by anatomical methods. We may further add 

 that so exquisitely sensitive is the respiratory centre to these 

 afferent impulses, that stimuli too slight to produce any appreciable 

 effect when applied to afferent nerves connected with an ordinary 

 centre, such as a spinal reflex centre, may produce marked effects 

 on the respiratory centre. For instance, the feeble electric current 

 which is developed when the cut end of a divided vagus is 

 replaced in the wound, the circuit between the cut end and the 

 longitudinal surface of the nerve being closed through the blood 

 or lymph of the wound, is often sufficient to develope inhibitory 

 impulses. Again, when the connection of the respiratory centre 

 with the lungs through the vagus nerves is abolished, not by 

 section of the nerves but by freezing both nerves at some part of 

 the course of each nerve (an operation which, while completely 

 blocking the passage of impulses along the nerve-fibres, does not 

 itself act as a stimulus) the effect on the respiratory movements 

 is much more in the direction of increasing and prolonging the 

 inspiratory act than of slowing the rhythm. Hence it would 

 appear that what we have previously described as the result of 

 dividing both vagus nerves, is partly due to the blocking of 

 natural impulses' and partly to the section of the nerves, and 

 possibly to electric currents, developed as suggested above, acting 

 as stimuli, and thus giving rise to artificial impulses. 



368. The double or alternate respiratory action of the vagus 



