ACTIVE (?) DILATATION OF VESSELS. 44.*> 



nerve, is not peculiar to tlie vessels of the car — it occurs in all 

 probability in every vessel of the body ; and the dilatation 

 fiom irritation is not confined to the capillaries and small 

 arteries — it extends up to the larger branches, so that the artery 

 of the part is not only wider than usual, but it pulsates forcibly. 

 It is indeed so much greater than that produced by division of 

 tlie vaso-motor nerve, that some have supposed it to be due to 

 nerves causing the vessels to dilate actively, and not simply to 

 yield to the pressure of the blood in them. But this hypothesis 

 is unnecessary, for Ludwig has found that irritation of a 

 sensory nerve has a double effect : (1) It causes the vessels of 

 the part to which it is distributed to dilate ; (2) it causes the 

 vessels in other parts of the body to contract, so that the general 

 blood-pressure is raised, and the blood driven into those vessels 

 which are relaxed. The part supplied by the irritated nerve 

 consequently gets its supply of blood doubly increased by the 

 dilatation of its own vessels, and the contraction of those in 

 other parts of the body. So constant is this contraction, that 

 Ludwig has employed the increased blood-pressure which it 

 causes as an indication that sensory impressions have been 

 conveyed to the nervous centres,* and its great importance in 

 regard to the action of counter-irritants will be hereafter ap- 

 parent. I ought to say, however, that the contraction really 

 seems to be universal, and to aftect more or less the vessels of 

 the part whose nerves have been irritated, as well as those of 

 the rest of the body, but this contraction in them is more than 

 counterbalanced by dilatation. Thus it is that the vessels of 

 this part not unf requently contract before the}; dilate, and some- 

 times the dilatation is transient, and is succeeded by contrac- 

 tion. It may seem strange that the irritation conveyed by a 

 sensory nerve to the vaso-motor centre should arrest its action 

 over some vessels and increase it over others ; but this idea 

 only occurs when we forget that what we call the vaso-motor 

 centre is really a collection of ganglionic cells connected with 

 nerves going to different parts of the body, just as a telegraph 

 station may contain numerous instruments, by which messages 

 may be sent to different parts of the country. The adaptation of 



* Ludwig and Dittmar, Lndwlg's Arheiten, 1870, p. 4. 



