484 REGULATION OF RESPIRATORY CENTRE. [Book ii. 



know from daily experience, of all the apsychical nervous cen- 

 tres, 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 im- 

 pulses direct to the diaphragm and other respiratory muscles), 

 we find that emotions and painful sensations alter profoundly 

 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 stimula- 

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

 proportion to the modifications of inspiration. 



§ 299. The complicated nature of the respiratory centre is 

 further shewn by the fact that it appears to consist of two lat- 

 eral halves which normally work in unison and yet may be made 

 to work independently. If the spinal bulb be carefully divided 

 in the middle line respiration may continue to go on in quite a 

 normal fashion. If, however, one vagus be then divided, the 

 respiratory movements, both costal and diaphragmatic, on the 

 side of the body on which division of the vagus has taken place, 

 become slower than those on the other side, so that the two 

 sides are no longer synchronous ; and a stimulus confined to 

 one vagus affects the respiratory movements of that side of the 

 body only. So also a section of a lateral half of the cord below 

 the bulb stops the respiratory movements on that side alone. 



§ 300. Besides these nervous influences, however, there is 

 another circumstance which perhaps above all others affects the 

 respiratory centre, and that is the condition of the blood in 

 respect to its respiratory changes ; the more venous (less arte- 



