THE VASOMOTOR NERVES. 535 



nections entirely rigid the smaller, quicker changes in volume caused by the 

 heart beat are also recorded. A volume pulse is obtained resembling in its 

 general form the pressure pulse given by the sphygmograph. When used 

 for this purpose the instrument is described as a hydrosphygmo graph. 



Records taken of the volume of the hand, foot, brain, or any 

 other organ show that in addition to the changes caused by the 

 heart beat and by the respiratory movements, there are other more 

 irregular variations that are continually occurring the cause of which 

 is to be found in the variations in the amount of blood in the organ. 

 Day and night these changes in volume take place, and they are 

 referable to the activity of the vasomotor system. Vasoconstriction 

 or vasodilatation in the organ itself cause what may be called 



Fig. 221. Detailed drawing of the glass plethysmograph with the arrangement of rub- 

 ber glove to prevent leaking without compressing the veins. 2, The glove with its gauntlet 

 reflected over the end of the glass cylinder ; 1 and 3, supporting pieces of stout rubber tub- 

 ing ; D and E, sections of outer and inner rings of hard rubber to fasten the reflected rubber 

 tubing and reduce the opening for the arm. 



an active change in volume. But vasoconstriction or vasodilata- 

 tion in other organs may cause a perceptible change, of a passive 

 kind, in the volume of the organ under observation. For, since 

 the amount of blood remains the same, a change in any one organ 

 must affect more or less the volume that is, the blood contents 

 of all other organs. 



General Distribution and Course of the Vasoconstrictor 

 Nerve Fibers. These fibers belong to the autonomic system, and 

 consist, therefore, of a preganglionic fiber arising in the central 

 nervous system and a postganglionic fiber arising from the cell of 

 some sympathetic ganglion. The general arrangement of the auto- 

 nomic system (p. 231) should be reviewed in this connection. It 

 has been shown by experiments of the kind described under the last 

 heading that vasoconstrictor fibers are present in numerous nerve 



