576 THE OUTGO OF ENERGY 



2. Note the rate of flow in the web-vessels in 

 the uninjured limb. Stimulate the sciatic nerve 

 with the single induction current repeated at 

 intervals of five seconds. 



The vessels of the web will dilate. 



The vasoconstrictor and vasodilator fibres also 

 react differently to cold. If the hind limb (cat) 

 be cooled, the stimulation that normally causes 

 vasoconstriction will cause vasodilatation. 



Vasoconstrictor and vasodilator fibres are not 

 always found in the same nerve-trunks ; in the 

 chorda tympani nerve, for example, there are only 

 dilator fibres. 



The central relations of the dilator nerves have 

 not been sufficiently studied to warrant their 

 discussion here. 



Reflex Vasomotor Actions. 1. Note the rate 

 of flow in the vessels of the web in a lightly 

 curarized frog. Stimulate the skin (not too near 

 the bulb or cord) with tetanizing currents. The 

 stimulus must not be repeated often, or fatigue 

 will obscure the result. 



Eeflex constriction of the vessels will take place. 

 The sensory impulse is carried by afferent fibres 

 to the vasomotor centres. 



Repeat the experiment, using in place of the 

 electrical a mechanical stimulus, such as pinching 

 the skin with forceps. 



