CHAP, ii.] EESPIEATIOK 475 



cause of the respiratory movements. We have seen that when 

 the spinal cord is divided below the spinal bulb, the facial and 

 laryngeal movements still continue. This proves that the respir- 

 atory centre is still in action, though its activity is unable to 

 manifest itself in any thoracic movement. But when the cord 

 is thus divided, the respiratory centre is cut off from all sensory 

 impulses, save those which may pass into it from the cranial 

 nerves of sensory function; and that these sensory cranial 

 nerves are not specially concerned in developing the activity of 

 the respiratory centre is shewn by the fact that the division of 

 these cranial nerves by themselves, when the bulb and spinal 

 cord are left intact, does not do away with the continuance of 

 respiration. One cranial nerve, as we shall see, is especially 

 concerned in respiration, viz. the vagus nerve; but if after 

 removal of the brain above the bulb both vagus nerves be 

 divided, respiration still goes on; indeed the respiratory im- 

 pulses proceeding from the centre are, though in a peculiar 

 way, exaggerated. Hence though we cannot put the matter 

 to an experimental test by dividing every sensory nerve in the 

 body, while leaving the motor nerves of respiration intact, such 

 an operation being practically impossible, we may infer that 

 the respiratory impulses proceeding from the respiratory centre 

 are not simply afferent impulses reaching the centre along affer- 

 ent nerves and transformed by reflex action in that centre. 

 They evidently start de novo from the centre itself, however 

 much their characters may be affected by afferent impulses, 

 reaching that centre at the time of their being generated. The 

 action of the centre is automatic, not simply reflex. 



295. We find, on inquiry, that the activity of the centre 

 is profoundly influenced by two classes of events. These, as 

 we might expect, are on the one hand events producing changes 

 in the quality of the blood distributed to the spinal bulb through 

 the arteries, especially as regards its gases, that is to say, events 

 modifying the interchange taking place in the lungs ; and on 

 the other, hand nervous impulses, started in various ways and 

 reaching the centre along various nerves or nervous tracts. It 

 will be convenient to consider the latter first. 



Afferent nervous impulses may affect the centre in many 

 various ways. The whole act of breathing or of taking a 

 breath is a double act consisting of an inspiration and an expira- 

 tion, and nervous impulses may especially affect the one or the 

 other. One mode of breathing may differ from another in the 

 depth of the individual breath, in the volume of air taken in 

 and given out ; and nervous impulses may increase or may 

 diminish the depth of a breath, the volume of air respired. 

 One mode of breathing again differs from another in the rapidity 

 with which one breath succeeds another, that is, in the rate of 

 rhythm ; and nervous impulses may slow or may quicken the 



