232 THE CIRCULATION OF THE BLOOD 



which occurs as the vessels become smaller, is the disappearance of 

 muscular fibers. 



The office of the muscular coat is to adjust the size of the arterioles and, 

 therefore, the flow of the blood. It is to regulate the quantity of blood to be 

 received by each part or organ, and to adjust this quantity to the requirements 

 of each, according to various circumstances, but chiefly according to the de- 

 gree of activity which each organ at different times exhibits. The amount of 

 work done by each organ of the body constantly varies, and the variations 

 often quickly succeed each other, so that, as in the muscles for example, 

 within the same hour a part may be now very active and now quite inactive. 

 In all its active exercise of function, such an organ requires a larger supply of 

 blood than is sufficient for it during the times when it is comparatively 

 inactive. 



It is evident that the heart cannot regulate the blood-supply to each part 

 of the body at different periods independently of the other parts. Neither 

 could this be regulated by any general and uniform contraction of the arteries. 

 But it may be regulated by the power which the arteries of each part have, 

 through their muscular tissue, of contracting or relaxing so as to diminish 

 or increase their size. In this way the supply of blood is varied according 

 to the requirements of the particular part of the body to which the vessels 

 are distributed. Thus, while the ventricles of the heart determine the total 

 quantity of blood to be sent onward at each contraction, and the force of 

 its propulsion, and while the large and merely elastic arteries distribute the 

 blood and equalize its stream, the smaller arteries by means of their muscu- 

 lar tissue regulate and determine the proportion of the whole quantity of 

 blood which shall be distributed to each particular organ. 



The variation of the size of arterioles and, therefore, of the resistance 

 to the flow of the blood in them is secured by the contractions of the muscular 

 tissue, but the muscles are regulated in their contractions by the nervous 

 system. The muscular tissue in the blood vessels of the organs of the dif- 

 ferent parts of the body is also co-ordinated by the same regulative and con- 

 trolling influence of the nervous system. 



The Discovery of the Vaso-motor Nerves. More than half a cen- 

 tury ago (1851) it was shown by Claude Bernard that if the cervical sym- 

 pathetic nerve is divided, the blood vessels of the corresponding side of the 

 head and neck become dilated. This effect is best observed in the ear, 

 which if held up to the light is seen to become redder and the arteries to 

 become larger. The whole ear is distinctly warmer than the opposite one. 

 This effect is produced by removing the arteries from the tonic influence of 

 the central nervous system, which influence normally passes along the course 

 of the divided nerve. 



If the peripheral end of the divided nerve be stimulated in its course 

 toward the organ, i.e., that farthest from the brain, the arteries which were 



