THE NERVOUS CONTROL OF THE BLOOD VESSELS 1041 



When we endeavour to trace these limb dilator fibres back to the cord, 

 we find no trace of their passage through the sympathetic system. It was 

 shown by Strieker and Morat that dilatation of the vessels of the hind limb 

 can be produced by stimulating the posterior roots of the nerves going to the 

 limb, i. e. far below the point of origin from the cord of the constrictor fibres 

 to the same part of the body. Since it has been definitely shown by embryo- 

 logists and histologists that in higher mammals all the fibres making up the 

 posterior roots have their origin in the cells of the posterior root ganglion, 

 this observation was widely discredited, until it was confirmed by Bayliss 



per sec. 



4 per sec. 



16 per sec. 

 64 per sec. 



FIG. 474. Effect on the volume of the hind limbs of the cat of stimulating the sciatic 

 nerve with induction shocks at different rates. It will be noticed that with one 

 shock per second there is hardly any constriction, but considerable dilatation, 

 whereas with 64 shocks per second the only effect produced is vaso-constriction. 

 Curves to be read from right to left. (BowDiTCH and WARREN.) 



for all manner of stimuli. Stimulation of the posterior roots, either before 

 or after they have passed through the ganglia, causes dilatation of the vessels 

 in the area of the supply of the roots, whatever be the nature of the stimulus 

 employed, whether electrical, chemical, or mechanical (Fig. 475). This effect 

 is not destroyed by previous section of the posterior roots on the proximal 

 side of the ganglia, showing that the fibres by means of which the dilatation 

 is produced have the same origin and course as the ordinary sensory nerves 

 to the limbs. Since the vaso-dilator impulses pass along these nerves in a 

 direction opposite to that taken by the normal sensory impulses, Bayliss 

 has designated them as antidromic impulses. So far this phenomenon of 

 a nerve fibre functioning (not merely conducting) in both directions is 

 almost without analogy in our knowledge of the other nerve functions of 

 66 



