THE NERVOUS REGULATION OF THE BLOODVESSELS 173 



(6) For the separation of the effects of stimulation of vaso-constrictor 

 and vaso-dilator fibres when they are mingled together, as is the case 

 in many nerves, advantage is taken of certain differences between them. 

 For example, the vaso-constrictors lose their excitability sooner than 

 the vaso-dilators when cut off from the nerve-cells to which they belong. 

 So that if a nerve is divided, and some days allowed to elapse before 

 stimulation, only the dilators will be excited. The vaso-dilators are 

 more sensitive to weak stimuli repeated at long intervals than to strong 

 and frequent stimuli, and the opposite is true of the constrictors. When' 

 a nerve containing both kinds of fibres is heated, the excitability of 

 the vaso-constrictors is increased in a greater degree than that of the 

 dilators; when the nerve is cooled, the dilators preserve their excita- 

 bility at a temperature at which the constrictors have ceased to respond 

 to stimulation (Fig. 79). 



The Chief Vaso-Motor Nerves. The first discovery of vaso-motor 

 nerves was made in the cervical sympathetic. When this nerve is 



Fi g> 79. Plethysmograms: Hind-Limb of Cat (after Bowditch and Warren). To be 

 read from right to left. On the left hand is shown the effect of slow stimulation 

 of the sciatic (i per second); on the right hand the effect of rapid stimulation 

 (64 per second). In the first case the limb swelled owing to excitation of the 

 vaso-dilators; in the second, it shrank through excitation of the vaso-constrictors. 



cut, the corresponding side of the head, and especially the ear, 

 become greatly injected owing to the dilatation of the vessels. This 

 experiment can be very readily performed on the rabbit, and the 

 changes are most easily followed in an albino. The ear on the side 

 of the cut nerve is redder and hotter than the other; the main 

 arteries and veins are swollen with blood, and many vessels formerly 

 invisible come into view. The slow rhythmical changes of calibre, 

 which in the normal rabbit are very characteristically seen in the 

 middle artery of the ear, disappear for a time after section of the 

 sympathetic, although they ultimately again become visible (Prac- 

 tical Exercises, p. 215). 



Stimulation of the cephalic end of the cut sympathetic causes a 

 marked constriction of the vessels and a fall of temperature on the 

 same side of the head. From these facts we know that the cervical 



