998 



PHYSIOLOGY 



or three days. If now the peripheral end of the divided nerve is stimulated, 

 dilatation of the vessels is produced (Fig. 464). Apparently the constrictor 

 fibres degenerate before the dilator fibres, so that at a certain period after the 

 nerve section only the latter respond to stimulation. On the other hand, 

 it is often possible in the freshly cut nerve to obtain dilatation by stimulat- 

 ing its peripheral end with induction shocks repeated at slow intervals 



i per sec. 



4 per sec. 



16 per sec. 

 64 per sec. 



FIG. 465. Effect on the volume of the hind limbs of the cat stimulating of 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. (BowDircH and WARREN.) 



one to four per second. The effects of different rates of stimulation on the 

 limb-nerves of the cat are shown in Fig. 465. 



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 embryologists 

 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 observa- 

 tion was widely discredited, until it was confirmed by Bayliss 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, 



