CHAP. IL] RESPIRATION. 597 



nerves on which we have dwelt above 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 have already said, and indeed 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 

 impulses 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 pro- 

 foundly the character of the respiratory movements. And though 

 these effects may be partly indirect, (the emotion modifying the 

 heart-beat or the tonus of the arteries, and so influencing the 

 flow of blood through the respiratory centre,) they are chiefly 

 due to the direct action 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 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 affects the breathing 

 and most frequently arrests it. The effects of stimuli of various 

 strengths brought to bear on various nerves are very varied. 

 Sometimes the result is an increase of inspiration ; and that 

 either by a quickening of the rhythm or by an increase of the 

 individual breaths or by a combination of the two. Sometimes 

 the result is an inhibition of inspiration accompanied or not by 

 an increase of expiration, and sometimes, as when the stimulation 

 causes a cough, the expiratory results may be out of all proportion 

 to the modifications of inspiration. While in the case of some 

 nerves, for instance, as we have seen, the superior laryngeal, and it 

 is said the splanchnic nerves, the effects are exclusively or at least 

 chiefly inhibitory of inspiration and augmentative of expiration, 

 that is expiratory, and in others perhaps chiefly augmentative 

 of inspiration or inspiratory, in the case of most nerves the effect 

 may be according to circumstances either in the one direction or 

 the other. Perhaps as a rule weak stimuli tend to augment 

 and strong to inhibit inspiration ; but the effects of artificially 

 stimulating sensory nerves are complicated and often confused, 

 because powerful afferent impulses by giving rise to pain may, 

 through impulses generated by the pain itself and descending to 

 the medulla from the brain, act in an indirect as well as in a 

 direct manner; and the prominence of the indirect painful im- 

 pulses will in any experiment depend on the anaesthetic used. 



