8o THE ACTIVE FORCES OF LIVING ORGANISMS 



it is difficult to understand how the nervous system 

 can react upon the digestive and other metaboHc pro- 

 cesses unless it be affected by them. We may, it is 

 true, conceive chemical action as acting directly on 

 the bloodvessels or on the dilator nerves which supply 

 them ; but though some such action probably takes 

 place, it is very doubtful if it be the only factor we 

 have to consider, for motion in general is preceded 

 by sensation or by its equivalent, in this case in the 

 shape of an afferent impulse. Indeed, if we adopt 

 the view that the dilation of the bloodvessels or the 

 relaxation of the mucous membrane is the result of 

 reaction on the part of the vaso-motor and other 

 centres, we must perforce suppose that they have 

 received a stimulus from the seat of activity. This 

 stimulus is a factor which we can by no possibility 

 get rid of if we admit the existence of reaction. In 

 one shape or another it must reach the vaso-motor 

 centre. But if, on the other hand, we suppose the 

 same fibres to act as carriers of both sensory or afferent 

 and motor impulses, or at least as discharging the dual 

 function, we must imagine a dilating vibration to pass 

 from the affected bloodvessel along the dilating fibre 

 until it reaches the nerve-cell at the end of it, which 

 would then take up the peculiar vibratory rhythm, 

 adding force to it and thus reacting. That this course 

 of events does not represent that which happens in 

 some cases will be demonstrated by an example, but 

 that it may occur in others is a possibility to which 

 it is well not entirely to shut one's eyes. 



