ELEMENTARY PHYSIOLOGY 



(p. 34) that the small arteries may be directly affected 

 by the nervous system, whicli controls the state of con- 

 traction of their walls, and regulates their calibre, and thus 

 governs the supply of blood to each part of the body 

 according to its varying needs. Tiie control of the ner- 

 vous system over the circulation in particular spots is of 

 such paramount importance that we must now deal with 

 this also in some detail. 



A phenomenon with which every one is more or less 

 familiar, either as experienced on themselves or observed 

 on other persons, is that known as blushing. Now 

 blushing is a purely local modification of the circulation, 

 and it will be instructive to consider how a blush is 

 brought about. An emotion, sometimes pleasurable, 

 sometimes painful, takes possession of the mind ; there- 

 upon a hot flush is felt, the skin grows reil, and according 

 to the intensity of the emotion these changes are confined 

 to the cheeks only, or extend to the " roots of the hair," 

 or " all over." 



What is the cause of these changes ? The blood is a red 

 and a hot fluid ; the skin I'eddens and grows hot, because 

 its vessels contain an increased cjuantity of this red and 

 hot fluid ; and its vessels contain more, because the small 

 arteries suddenly dilate, the natural moderate contraction 

 of their muscles being superseded l)y a state of relaxation ; 

 and this relaxation comes on because the action of the 

 nervous system which previously kept the muscles in a 

 state of moderate contraction is, for the time, sus- 

 pended. 



On the other hand, in many peo])le, extreme terror or 

 rage causes the skin to grow cold, and the face to appear 

 pale and pinched. Under these circumstances, in fact, 

 the supply of blood to the skin is greatly diminished, in 

 conseipience of an increased contraction of the muscles of 

 the small arteries whereby these become unduly narrowed 

 or constricted, and thus allow only a small quantity of 

 blood to pass through them ; and this increased con- 



