NERVOUS CONTROL OF THE BLOOD-VESSELS 1123 

 which, however, passes off after two or three days. If now the peri- 

 pheral end of the divided nerve is stimulated, dilatation of the vessels 

 is produced (Fig. 469). 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 

 stimulating its peripheral end with induction shocks repeated at slow 



i per sec. 



4 per sec. 



16 per sec. 

 54 per sec. 



FIG. 470. 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.) 



intervals one to four per second. The effects of different rates of 

 stimulation on the limb-nerves of the cat are shown in Fig. 470. 



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 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 tb 



