CHAP, ii.] RESPIRATION. 359 



to the latter should be augmentative to the former. Indeed the 

 whole matter becomes too complicated to be discussed any further 

 here ; and we have introduced the view not because we regard it as 

 an adequate explanation of the phenomena, but because it affords a 

 useful graphic conception of the molecular activity of these and 

 other automatic nervous centres. We may be at present content with 

 the knowledge that, as far as the vagus is concerned, the respiratory 

 centre as a whole may be influenced by augmentative or inspiratory 

 impulses which run chiefly in the trunk of the nerve and by inhibi- 

 tory or expiratory impulses which run certainly in the superior 

 laryngeal, apparently also in the recurrent laryngeal, and to a certain 

 extent in the trunk also ; in the latter case, however, their presence 

 is manifested under certain conditions only. And while, from the 

 results of simple section of the main trunk, it is clear that the ac- 

 celerating influences are continually at work, it is not so clear that 

 the inhibitory influences are always in action, since section even of 

 both superior laryngeals does not necessarily quicken respiration. 



This double or alternate respiratory action of the vagi may be 

 taken as in a general way illustrative of the manner in which other 

 afferent nerves and various parts of the cerebrum are enabled to 

 influence respiration. As we know from daily experience, of all 

 the apsychical nervous centres, the respiratory centre is the one 

 which is most frequently and most deeply affected by nervous im- 

 pulses from various quarters. Besides the changes brought about 

 by the will (and when we breathe voluntarily we probably make 

 use to some extent of the normal nervous machinery of respiration, 

 working through this, rather than sending independent volitional 

 impulses direct to the diaphragm and other respiratory muscles), 

 we find that emotions, and painful sensations alter profoundly 

 the character of the respiratory movements. Sometimes the 

 breathing thereby becomes quicker and flatter, sometimes it is 

 deepened as well as hurried; at other times it may be slowed 

 or for a w r hile stopped altogether, while occasionally expiratory 

 efforts are made prominent. And though these effects may 

 be partly indirect, the emotion modifying the heart-beat, and 

 so influencing the flow of blood through the respiratory centre, they 

 are chiefly due to the direct actions of nervous impulses reaching 

 that centre from higher parts of the brain. So also impulses from 

 almost every sentient surface, or passing along almost every 

 sensory nerve, may modify respiration in one direction or another, 

 the slighter feebler impulses tending apparently to quicken, and 

 the stronger larger impulses to arrest or inhibit the respiratory 

 discharges. The influence in this way of stimuli applied to the 

 skin is well known to all; but perhaps next to the vagus the 

 nerve most closely connected with the respiratory centre is the 

 fifth nerve, branches of which guard the nasal respiratory channels ; 

 the slightest stimulation of the nostrils at once affecting the 

 breathing and most frequently arresting it. Thus the working of 



