DISCOVERY OF THE VASO-MOTOR NERVES 211 



large and merely elastic arteries distribute the blood and equalize its stream, 

 the smaller arteries by means of their muscular tissue regulate and deter- 

 mine 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 muscular tissue, but the 

 muscles are regulated in their contraction by the nervous system. The 

 muscular tissue in the blood-vessels of the different organs of the body is also 

 coordinated by the same regulative and controlling influence of the nervous 

 system. 



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

 century ago it was shown by Claude Bernard that if the cervical sympathetic 

 nerve is divided, the blood-vessels of the corresponding side of the head and 



FIG. 201. Small Artery and Vein of the Frog's Web. A, Under normal conditions; B, upon 

 stimulation of the sciatic nerve; A -, artery; V, vein. In this experiment the vein also showed 

 well-marked vaso- constriction. (New figure by Greene.) 



neck become dilated. This effect is best seen in the ear, which if held up to 

 the light is seen to beceme redder, and the arteries to become larger. The 

 whole ear is distinctly warmer than the opposite one. This effect is pro- 

 duced by removing the arteries from the influence of the central nervous sys- 

 tem, 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 

 before dilated return to their natural size, and the parts regain their former 

 condition. And, besides, if the stimulus is very strong or very long continued, 



