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 the web of a frog's foot be watched under the microscope, 

 any individual small artery will be found to vary considerably in 

 calibre from time to time, being sometimes narrowed and sometimes 

 dilated; and 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 paler. During 



