THE REGULATION OF THE CIRCULATION 273 



which do not share in the constriction. It is just as 

 though one of two faucets over a sink should be turned 

 off; there would be a reinforcement of the discharge 

 from the one that was left open. 



The act of falling asleep is psychologically the reverse 

 of fixing the attention; it is dissipation instead of con- 

 centration. It can be shown by means of the plethys- 

 mograph that it is also the reverse in its physical signs. 

 Individuals have schooled themselves to sleep with the 

 arm in the plethysmograph and the records obtained 

 show that about the time consciousness is lost, or trans- 

 formed to that of the world of dreams, there is a slack- 

 ening of tone in the vessels, an increase of volume in 

 the limb. If what happens in the arm is typical of 

 what takes place elsewhere there must be a lowering 

 of pressure in the main arteries at such a time. Then, 

 if the vessels of the brain do not greatly change their 

 caliber the flow through them will be much reduced. 

 When sleep was under discussion in Chapter XII it 

 was stated that diminished blood-flow might account 

 for its onset. 



Of course the act of waking is a resumption of atten- 

 tion. It is known to be attended by a constriction of 

 the vessels in the part observed. Thus all that we 

 know of the adjustments in favor of the brain is easily 

 coordinated and understood. The reciprocal element 

 in vasomotor changes is apparent in other cases besides 

 this. We assume that a dilation of the surface vessels 

 will relieve internal congestion. This is the prfncfple 

 which justifies hot applications. An obvious feature of 

 the reaciSfch to alcohol is the flushing of the skin, and 

 here too we suppose that there is a corresponding with- 

 drawal of blood from the deeper parts. 



The Afferent Side. So far we have spoken of the 

 cardiac and vasomotor nerves as pathways leading 

 from the central gray matter to the organs of the cir- 

 culation. Little has been said to bring out the fact that 

 they are used in reflex fashion. But the general truth 



18 



