THE NERVOUS SYSTEM 251 



the favoring of organs which need it, is the work of the 

 vasomotor system. The walls of the microscopic vessels, 

 those which adjoin the capillaries in particular, are provided 

 with muscular elements of the same general order as those 

 which produce the movements of the digestive tract. 

 These contractile elements are connected with the terminal 

 branches of certain nerves. Hence the anatomic study of 

 the tissues involved, even when unsupported by physio- 

 logic experiments, makes clear the possibility that the 

 centers when acting reflexly or otherwise may influence 

 the diameter of the blood-vessels and the volume of the 

 circulation in any or all regions of the body. 



Physiology reinforces anatomy at this point. For sixty 

 years it has been certainly known that nervous regulation 

 of the blood-flow is a fact. Much earlier than this the 

 power was assumed to exist, "Ubi stimulus, ibi affluxus," 

 they said, meaning that where there is activity there is an 

 increase of blood. The paling and the flushing of the skin, 

 occurring either in response to external changes of tempera- 

 ture or emotional conditions, are strongly suggestive of a 

 central command of the vessels. This has been taken for 

 granted in our own treatment of the matter of the main- 

 tenance of a normal body temperature. Much as the heart 

 is reached by impulses which inhibit it and by others which 

 spur it to a greater expenditure of energy, the small arteries 

 and veins are subject to antagonistic influences. We speak 

 of a vasoconstrictor effect when we mean a reinforcement 

 of the existing tone, and we use the word vasodilator in 

 the opposite sense, that is, to characterize changes in 

 which the degree of contraction is lessened, with the result 

 that the blood finds its way in greater quantity through the 

 affected vessels. 



The difference between the cardiac and the vasomotor 

 factors in the control of the circulation can be made plain 

 by means of an illustration borrowed from the laundry. 

 Suppose that a supply-pipe runs along above a row of 

 tubs and bears a faucet for each. Suppose, also, that there 

 is a stop-cock on the pipe before it reaches the tubs. Fi- 



