VASO-DILATOR NERVES. 257 



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, j 



I Vaso-Dilator Nerves. / We have already seen, in the case 

 orthe stomach, one method by which a locally increased 

 blood-supplv may be brought about in an organ while it is 

 at work. Usually, however, in the Body this is managed 

 in another way; by vjy^dil^ or 



paralyze, not the vaso-motor centre, but the intrinsic nerv- 

 ous supply of the blood-vessels. The nerves of the skeletal 

 muscles for example contain two sets of fibres: one motor 

 proper and the other vaso-dilator. When the muscle con- 

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

 sets of fibres are excited; so that when the organ is set at 

 work its arteries are simultaneously dilated and more blood 

 flows through it. Quite a similar thing occurs in the sali- 

 vary glands. Their cells, which form the saliva, are aroused 

 to activity by special nerve-fibres; but the gland nerve also 

 contains vaso-dilator fibres which simultaneously cause a 

 dilatation of the gland artery. Through such arrange- 

 ments the distribution of the blood in the Body at any 

 moment is governed: so that working parts shall have 

 abundance and other parts less, while at the same time the 

 general arterial pressure remains the same on the average; 

 since the expansion of a few small local branches but little 

 influences the total peripheral resistance in the vascular sys- 

 tem. Moreover, commonly when one set of organs is at 

 work with its vessels dilated, others are at rest with their 

 arteries comparatively contracted, and so a general average 

 blood-pressure is maintained. Few persons, for example, 

 feel inclined to do brain work after a heavy meal: for then 

 a great part of the blood of the whole Body is led off into 

 the dilated vessels of the digestive organs, and the brain 

 gets a smaller supply. On the other hand, when the brain 

 is at work its vessels are dilated and often the whole head 

 flushed: and so excitement or hard thought after a meal 

 is very apt to produce an attack of indigestion, by diverting 



