CIRCULATION. 487 



Many investigators have studied vaso- motor phenomena by means of the 

 plethysmograph, an apparatus invented by Mosso for recording the changes in 

 the volume of the extremities. The member, the vaso-motor nerves of which 

 are to be studied, is placed within a cylinder filled with water, from which a 

 tube leads to a recording tambour. 1 An increase in the volume of the member, 

 such as would be brought about by the expansion of its vessels, causes a corre- 

 sponding volume of water to enter the tambour tube, thus raising the pressure 

 in the tambour and forcing its lever to rise. A constriction of the vessels, on 

 the contrary, causes the recording lever to fall. 



In addition to these general methods, special devices have been employed 

 in the researches into the vaso-motor nerves of the brain. 



In considering the observations made with these various methods it will 

 be advisable to begin with the differences between the two kinds of vaso-motor 

 nerves. 



Differences between Vasoconstrictor and Vaso-dilator Nerves. The 

 differences between vaso-constrictor and vaso-dilator nerves are particularly 

 interesting for the reason that both vaso-constrictor and vaso-dilator fibres are 

 often found in one and the same anatomical nerve. The sciatic nerve is a 

 good example of this. By taking advantage of these differences the investi- 

 gator may determine whether one or both kinds of fibres are present in any 

 anatomical nerve ; whereas, without this knowledge, the effects produced by 

 the stimulation of the one might be wholly masked by the effects produced by 

 the stimulation of the other. 



The vaso-constrictors are less easily excited than the vaso-dilators. The 

 simultaneous and equal stimulation of the dilator and constrictor nerves going 

 to the submaxillary gland causes vaso-constriction, dilatation appearing after 

 the stimulation ceases, for the after-effect of excitation is of shorter duration 

 with the constrictors than with the dilators. 2 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. 3 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. 4 

 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 



1 An improved method of recording is given by Bowditch and Warren, 1886, p. 420. 



2 Anrep and Cybulski, 1884. 



3 Lepine, 1876, p. 26 ; Howell, Budgett, and Leonard, 1894, p. 306. 



4 Ostroumoff, 1876, p. 232; Bowditch and Warren, 1886, p. 436; Bradford, 1889, p. 390. 



