THE VASCULAR MECHANISM. 



220 



can be readily recorded by enclosing the distal part of the limb in a 

 plethysmograph of suitable shape, which is connected with a tambour 

 and a recording lever. 



Bayliss has shown that the vaso-dilator fibres to the limbs leave the 

 spinal cord by the posterjoxj'oots, and that stimulation of the peripheral 



Blood 

 pressure. 



FIG. 89. Stimulation of peripheral end of 7th lumbar posterior root. 



(Bayliss.) 



end of the posterior roots causes marked dilatation of the arterioles 

 (fig. 89). The posterior root fibres starting in the skin and deep 

 tissues normally carry impulses from these structures to the spinal 

 cord and brain. When stimulation of the peripheral end of the 



FIG. 90. Scheme to show path of antidromic impulses (axon reflex). 



posterior root causes dilatation of the arterioles, the impulses must 

 pass towards the periphery, that is, in the opposite direction to that 

 taken by the impulses from the skin. For this reason the impulses 

 running towards the periphery and causing vaso-dilatation have been 

 called antidromic. In normal circumstances a stimulus applied to the 

 skin at A (fig. 90) will give rise to an impulse s passing along the 



