118 THE NERVOUS SYSTEM AND ITS COXSKKVATK )\ 



vided \vitli such a double supply, the heart may lie made 

 to beat faster through the spurring which it receives from 

 its accelerators, or a similar quickening of its rate may 

 represent merely a lessening of the habitual vagus inhibi- 

 tion. A slowing heart may be exhibiting the results of 

 cessation of the accelerator prodding, or the ca-e may be 

 one of intensified inhibition. On the whole, it is held 

 that the vagus mechanism is the more important of the 

 two. 



Vasomotor Control. A prominent department of the 

 autonomic system is that devoted to the regulation of 

 local blood-flow by bringing about changes in the diameter 

 of the small arteries and veins. The blood-vessels of micro- 

 scopic size are relatively rich in muscular dements. The 

 majority of these contractile cells are placed with their 

 long axes at right angles to the course of the blood, and 

 the natural result of their contraction is to diminish the 

 bore of the vessels and to reduce the flow through them. 

 As the situation is now pictured, it is supposed that the 

 larger arteries and veins shrink and swell as the pressure 

 of the blood within falls or rises, but that the very slender 

 vessels of both classes contract and relax by virtue of 

 the living tissue contained in their walls and in response 

 to impulses sent from the central nervous system. Nerve- 

 fibers conveying impulses to the muscular coats of the 

 small blood-vessel- are called vasomotor libers. Like 

 those which end in the heart, they are of two classes. 



The fir>t kind of vasomotor fibers to be observed at 

 work were those which we now call the vasoconstrictors. 

 Hy this term we de>ignate fiber- l>y means of which a nar- 

 rowing of the vessels can be secured. They have cmi- 

 nections such that when they are stimulated the contrac- 

 tile elements in the walls of the arteries or veins to which 

 i hey extend are shortened. The "tone" of tin' vessels 

 in qiie-linn is said to be increased. A diminished blood- 

 flow through the parts reached by the arterie* or drained 

 by the veins is to be expected. Yasoconst rict ion is the 

 usual effect when the peripheral portion of a nerve chosen 



