CIRCULATION. 197 



with the constrictors than with the dilators. Warming increases and cooling 

 diminishes the excitability of the vaso-constrictors to a greater degree than is 

 the case with the vaso-dilators. Thus if the hind limb of an animal be 

 warmed, the stimulation of the sciatic nerve will cause vaso-constriction ; 

 while if it be cooled the same stimulation will cause vaso-dilatation. 1 Vaso- 

 constrictors are more sensitive to rapidly repeated induction shocks (tetaniza- 

 tion) and less sensitive to single induction shocks than are vaso-dilators. Thus 

 if the sciatic nerve is stimulated with induction shocks of the same strength, it 

 will be found that a rapid repetition of the stimuli will give vaso-constriction, 

 while with single shocks at intervals of five seconds vaso-dilatation is the result. 

 Vaso-constrictors degenerate more rapidly than vaso-dilators after separation 

 from their cells of origin. The stimulation of the peripheral end of the frog's 

 sciatic nerve immediately after section causes constriction. Several days later 

 the same stimulation causes vaso-dilatation, the constrictor nerves having already 

 degenerated (see Fig. 44, B). The maximum effect of stimulation is more 

 quickly reached with the vaso-constrictor than with the vaso-dilator nerves. 

 There is also a difference in the latent period, or interval between stimulation 



A B 



Fig. 44.— Curves obtained by enclosing the hind limb of a cat in the plethysmograph and stimu- 

 lating the peripheral end of the cut sciatic nerve (Bowditch and Warren, 1886, p. 447). The curves read 



from right to left. In each case the vertical lines show the duration of the stimulus — namely, fifteen 

 induction shucks per second during twenty seconds. Curve A shows the contraction of the vessels pro- 

 duced by the excitation of the freshly-divided nerve; curve B, the dilatation produced by an equal 

 excitation of the nerve of the opposite side four days after section, the vaso-constrictor nerves having 

 degenerated more rapidly than the vaso-dilators. 



and response. Bowditch and Warren have found the latent period of the 

 vaso-constrictor fibres iu the sciatic to be about 1.5 seconds, while that of the 

 vaso-dilators is 3.5 seconds. Finally, the two sorts of nerves have been said 

 to differ in the manner in which they are distributed. The vaso-constrictor 

 nerves leave the cord as medullated fibres, enter the sympathetic chain of gan- 

 glia and end in terminal branches probably in contact with a sympathetic 

 ganglion-cell. The constrictor impulse is forwarded to the vessel by a 

 process of this cell, either directly or by means <>f -till other sympathetic 

 ganglion-cells. The vaso-dilator fibre, on the contrary, was thought to run 

 directly from the cord to the blood-vessel ; but recent investigations make it 

 probable that all spinal vaso-motor fibres end in sympathetic ganglia. 



Origin and Course. — The vaso-motor nerves the general properties of 

 which have just been studied are axis-cylinder processes of sympathetic gan- 

 glion-cells. They follow, fir a time at least, the course of the corresponding 

 'Howell, Budgett, and Leonard: Journal of Physiology, L894, \\i. p. 298. 



