196 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



vessels distal to the artery. The restrictions are, that the variations in the 

 lateral pressure in the artery are indicative of changes in the size of the distal 

 vessels only when the general blood-pressure remains unaltered, or alters in a 

 direction opposite to the change in the artery investigated. An example will 

 make this plain. Dastre and Morat, in order to demonstrate the presence of 

 vaso-motor fibres for the hind limb in the sciatic nerve, connected a manometer 

 with the central end of* the left femoral artery, and a second manometer with 

 the peripheral end of the right femoral artery, distal to the origin of the pro- 

 funda femori-. The anastomoses between the principal branches of the fem- 

 oral artery are so numerous and so large that the circulation in the limb can 

 be maintained by the profunda femoris alone. Dastre and Morat could there- 

 fore compare the general blood -pressure with the blood-pressnre in the right 

 hind limb. On stimulating the peripheral end of the right sciatic nerve, the 

 blood-pressure rose in the arteries of the limb, but remained stationary in the 

 arteries of the trunk, connected with the first manometer through the central 

 end of the left femoral artery. The rise of blood-pressnre in the operated 

 limb, while the blood-pressnre in the rest of the body remained unchanged, 

 proved that the vessels in the operated limb were constricted. 



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

 plcthysmograph, 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 rilled with water, from which a 

 tube leads to a recording tambour. 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 Vaso-constrictor 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 ' 



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



gator may determine whether one or both kinds of fibres are present in an}' 

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

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

 the stimulatioD of the other. 



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

 Hinnltaneous and equal stimulation of the dilator and constrictor nerves going 

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

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



