618 INFLUENCE OF VAGUS NERVES. [BOOK n. 



an emotion, or by a dash of cold water on the skin, or in a hundred 

 other ways ; but the fact that the action of the centre may be thus 

 modified from without, is no proof that the continuance of its 

 activity is dependent on extrinsic causes. 



In attempting to decide this question we naturally turn to the 

 pneumogastric as being the nerve most likely to serve as the 

 channel of afferent impulses setting in action the respiratory 

 centre. If both vagus nerves be divided, respiration still con- 

 tinues, though in a modified form. This proves distinctly that 

 afferent impulses ascending those nerves are not the efficient 

 cause of the respiratory movements. We have seen that when 

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

 movements still continue. This proves that the respiratory 

 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 impulses 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 respi- 

 ratory centre are not simply afferent impulses reaching the centre 

 along afferent 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 im- 

 pulses, reaching that centre at the time of their being generated. 

 The action of the centre is automatic, not simply reflex. 



364. 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 bulb from the left ventricle, 

 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 



