THE NERVOUS SYSTEM 253 



extent that a dilation in one large area seems often to be 

 offset by a compensatory constriction in another. We 

 count on this reciprocal relation in our medical and hy- 

 gienic practice. For instance, we think that if we can en- 

 courage blood-flow at the surface we shall reduce it in the 

 deeper parts. This is often an object, since congestions 

 are important features of many disorders. We antici- 

 pate this balanced reaction when we make use of hot ap- 

 plications and when we give alcohol for a cold. 



No one is likely to overestimate the services rendered 

 by the vasomotor system. It has both a general and a 

 local action. In the first sense it maintains a certain 

 average state of contration in the arteries the arterial 

 tone which keeps up to a desirable level the pressure in 

 the arterial trunks. It is this pressure which guarantees 

 the prompt and sufficient supply of blood wherever the 

 paths are opened. The principle is the same that is ob- 

 served in the mains distributing water through the streets 

 of a city. The pressure must be great enough to keep up 

 the supply on the high ground as well as the low. Leakage 

 or wasteful use of water will lower the pressure and the 

 failure will be noted first on the hills. An unusual dila- 

 tion of many blood-vessels, such as probably accompan- 

 ies an attack of indigestion where the abdominal organs are 

 engorged, may lessen the pressure and the volume of blood 

 passing through the brain with the consequence that there 

 is faintness. Partial relief is found in lying down because 

 the factor of gravity is eliminated and the head given an 

 equal chance with the rest of the body. 



We notice the failure of the vasomotor system to do its 

 duty in cases like the above. We ought also to appreciate 

 how remarkable it is that the adjustments usually occur 

 with such smoothness and success. It is really a wonder- 

 ful thing that we can rear up the elongated human form 

 from a horizontal to a vertical position without entirely 

 deranging the circulatory mechanism. That the blood 

 does not distend the vessels below the heart and forsake 

 those above must be due in a great measure to vasomotor 



