ANTIDROMIC IMPULSES 87 



one set of fibres capable of conveying impulses in both 

 directions ? 



When the skin in any part is irritated, the underlying 

 vessels are dilated, as is well known. This might be regarded 

 as a simple reflex action were it not for the fact that the 

 effect occurs even after section of the nerve-trunk. But 

 when the peripheral part of the nerve has degenerated, the 

 effect is abolished. Here, then, is a mechanism which 

 clearly involves the nerve-trunk, but neither the posterior 

 root ganglion nor the spinal cord. The effect can only be 

 explained by assuming that each of the fibres in the 

 posterior root divides^ into two branches, one supplying 

 the skin, the other the vessels lying beneath. When the 

 cutaneous nerve-ending is stimulated the disturbance is 

 propagated not only centrally but throughout the whole 

 fibre, and an inhibition of the tonus of the blood-vessel 

 results. This is therefore termed an axon-reflex. (Fig. 20, 

 p. 88). 



It would seem therefore that the posterior root contains 

 not two kinds of fibres but one kind, which usually convey 

 impulses in both directions. The impulses passing towards 

 the periphery are termed antidromic. 



Is there a nervous centre which on stimulation produces 

 vaso -dilatation ? Such a centre has been stated to exist 

 in the fourth ventricle, distinct from the vaso-constrictor 

 centre, but this observation is not confirmed. 



We may now summarise the position with regard to the 

 nervous control of the blood-vessels. Nearly all the 

 arterioles of the body are under the control of nerves which 

 have a constrictor effect upon them. These nerves belong 

 to the sympathetic system. Some arterioles, particularly 

 those of the somatic system, are in addition suppUed with 

 nerves which have an inhibitory effect. These nerves are 

 identical with the sensory nerves {see Fig. 20). In such 

 vessels the tonus of the muscular coat is determined by the 

 relative strength of these two antagonistic impulses, and 

 it so happens that vaso-constrictor influences are usually 



