THE DISCOVERY OF THE VASO-MOTOR NERVES 243 



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. Since the general blood pressure 

 is fairly constant the size of the local vessels controls the supply of blood to 

 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 to 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 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 mus- 

 cular 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 different parts of the body is also co-ordinated by the same regu- 

 lative and controlling 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 

 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, 

 the amount of normal constriction is passed and the vessels become much 

 more contracted than before. The natural condition, which is midway 

 between extreme contraction and extreme dilatation, is called the natural 



