SEC. 5. CHANGES IN THE CALIBRE OF THE MINUTE 

 ARTERIES. VASO-MOTOR ACTIONS. 



The middle coat of all arteries contains circularly disposed 

 plain muscular fibres. As the arteries become smaller, the mus- 

 cular element becomes more and more prominent as compared 

 with the elastic element, until, in the minute arteries, the middle 

 coat consists entirely of a series of plain muscular fibres wrapped 

 round the elastic internal coat. Nerve-fibres belonging to the 

 sympathetic system are distributed largely to blood-vessels, but 

 their terminations have not as yet been clearly made out. By 

 galvanic, or still better by mechanical 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. 



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

 any individual small artery will be found to vary in calibre, being 

 sometimes narrowed and sometimes dilated. During the narrowing, 

 which is obviously due to a contraction of the muscular coat of the 

 artery, the attached capillary area and the corresponding veins 

 become less filled with blood, and paler. During the stage of 

 dilation, which corresponds to the relaxation of the muscular coat, 

 the same parts are fuller of blood and redder. It is obvious that, 

 the pressure at the entrance into any given artery remaining the 

 same, more blood will enter the artery when relaxation takes place 

 and consequently the resistance offered by the artery is lessened, 

 and less when contraction occurs and the resistance is consequently 

 increased. The blood always flows in the direction of least 

 resistance. 



