THE VASOMOTOR MECHANISM 377 



ment over the trunk of the Body; linen is so good a conductor of 

 heat that it permits any change in the external temperature to 

 act almost at once upon the surface of the Body. After an un- 

 avoidable exposure to cold or wet the thing to be done is of course 

 to restore the cutaneous circulation; for this purpose movement 

 should be persisted in, and a thick dry outer covering put on, until 

 warm and dry underclothing can be obtained. 



For healthy persons a temporary exposure to cold, as a plunge 

 in a bath, is good, since in them the sudden contraction of the 

 cutaneous arteries soon passes off and is succeeded by a dilatation 

 causing a warm healthy glow on the surface. If the bather remain 

 too long in cold water, however, this reaction passes off and is suc- 

 ceeded by a more persistent chilliness of the surface, which may 

 even last all day. The bath should therefore be left before this 

 occurs, but no absolute time can be stated, as the reaction is more 

 marked and lasts longer in strong persons, and in those used to 

 cold bathing, than in others. 



Vasodilator Nerves. We have already noticed, in connection 

 with the control of the vasoconstrictor center, one method by 

 which dilation of arterioles may be secured, namely, by inhibition 

 of the tonic activity of vasoconstrictor fibers. Frequently, how- 

 ever, in the Body this is managed in another way; by efferent 

 vasodilator nerves which inhibit, not the vasoconstrictor center, 

 but the muscles of the blood-vessels directly. The nerves of the 

 skeletal muscles for example contain two sets of efferent fibers : one 

 motor proper and the other vasodilator. When the muscle contracts 

 in a reflex action or under the influence of the will both sets of 

 fibers are excited; so that when the organ is set at work its arteries 

 are simultaneously dilated and more blood flows through it. 

 But if the animal have previously administered to it such a dose 

 of curare as just to throw out of function the true motor-fibers, 

 stimulation of the nerve produces dilation of the arteries with- 

 out a corresponding muscular contraction. Quite a similar thing 

 occurs in the salivary glands. Their cells, which form the saliva, 

 are aroused to activity by special nerve-fibers; but the gland-nerve 

 also contains a quite distinct set of vasodilator fibers which nor- 

 mally cause a simultaneous dilation of the gland-artery, though 

 either can be artificially stimulated by itself and produce its 

 effect alone. 



