3O4 THE CIRCULATION IN THE BLOOD-VESSELS. [CH. xxi. 



now produces such effects as are usually obtained when the 

 cervical sympathetic is irritated ; for instance, dilatation of the 

 pupil, raising of the upper eyelid, and constriction of blood-vessels 

 of the head and neck. (See accompanying diagram, fig. 285.) 



Such experiments as these are important because they teach us 

 that though the action of nerves may be so different in different 

 cases (some being motor, some inhibitory, some secretory, some 

 sensory, &c.), after all what occurs in the nerve trunk itself is 

 always the same ; the difference of action is due to difference 

 either in the origin or distribution of the nerve-fibres. If we go 

 back to our old illustration in which we compared the nerve 



Fig. 286. Arterial blood-pressure tracings showing Traube-Hering waves. (Starling.) 



trunks to telegraph wires, we may be helped in realising this. 

 The destination of a certain group of telegraph wires may be 

 altered, and the alteration may produce different consequences at 

 different places ; the electric change, however, in the wires would 

 be the same in all cases. So the nerve impulse going along a 

 nerve is always the same sort of molecular disturbance ; if it is 

 made as in the experiment just described, to go by a wrong 

 channel, it produces just the same results as though the impulse 

 had reached its destination by the usual channel. 



The Vaso-motor centre can be excited directly, as by 

 induction currents ; the result is an increase of blood-pressure 

 owing to an increase of the contraction of the peripheral arterioles. 



