SEC. 6. CHANGES IN THE CALIBRE OF THE MINUTE 

 ARTERIES. VASO-MOTOR ACTIONS. 



§ 141. All arteries contain plain muscular fibres, for the most 

 part circularly disposed, and most abundant in, or sometimes al- 

 most entirely confined to, the middle coat. Further 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 muscular 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 the*r 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. 



I f 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; 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 



