SEC. 6. CHANGES IN THE CALIBRE OF THE MINUTE 

 ARTERIES. VASO-MOTOR ACTIONS. 



163. We have seen ( 108) that all arteries contain plain 

 muscular fibres, for the most part circularly disposed, and most 

 abundant in, or sometimes almost entirely confined to, the middle 

 coat. We have further seen that as the arteries become smaller, 

 the muscular element as a rule becomes more and more prominent 

 as compared with the other elements, until, in the minute arteries, 

 the middle coat consists almost entirely of a series of plain mus- 

 cular fibres wrapped round the internal coat. Nerve fibres, of 

 whose nature and course we shall presently speak, are distributed 

 largely to the arteries, and appear to end chiefly in fine plexuses 

 round the muscular fibres, but their exact terminations have not 

 as yet been clearly made out. By mechanical, electrical, or other 

 stimulation, this muscular coat may, in the living artery, be made 

 to contract. During this contraction, which has the slow character 

 belonging to the contractions of all plain muscle, the calibre of the 

 vessel is diminished. The veins also as we have seen possess 

 muscular elements, but these vary in amount and distribution 

 very much more in the veins than in the arteries. Most veins 

 however are contractile, and may vary in calibre according to the 

 condition of their muscular elements. Veins are also supplied 

 with nerves. It will be of advantage however to consider 

 separately the little we know concerning the changes in the 

 veins and to confine ourselves at present to the changes in the 

 arteries. 



If any individual small artery in the web of a frog's foot be 

 watched under the microscope, it will be found to vary considerably 

 in calibre from time to time, being sometimes narrowed and 

 sometimes dilated ; arid these changes may take place without 

 any obvious changes either in the heart beat or in the general 

 circulation ; they are clearly changes of the artery itself. During 

 the narrowing, which is obviously due to a contraction of the 

 muscular coat of the artery, the capillaries fed by the artery and 

 the veins into which these lead become less filled with blood, and 



