14:8 HANDBOOK OF PHYSIOLOGY. 



which occurs as the vessels become larger (on the side of the arteries) is 

 the appearance of muscular fibres. Thus the nervous system is more 

 powerful in regulating the calibre of the smaller than of the larger 

 arteries. 



It was long ago shown by Claude Bernard that if the cervical sympa- 



FIG. 137. -Plethysmograph. By means of this apparatus, the alteration in volume of the arm, 

 B, which is inclosed in a glass tube. A, filled with fluid, the opening through which it passes being 

 firmly closed by a thick gutta percha band, F, is communicated to the lever, K, and registered 

 by a recording apparatus. The fluid in A communicates with that in B, the upper limit of which is 

 above that in A. The chief alterations hi volume are due to alteration in the mood contained in the 

 arm. When the volume is increased, fluid passes out of the glass cylinder, and the lever, D, also is 

 raised, and when a decrease takes place the fluid returns again from B and A. It will therefore be 

 evident that the apparatus is capable of recording alterations of blood-pressure in the arm. Ap- 

 paratus founded upon the same principle have been used for recording alterations in the volume 

 of the spleen and kidney. 



thetic nerve is divided in a rabbit, the blood-vessels of the corresponding 

 side of the head and neck become dilated. This effect is best seen in 

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

 arteries are seen to become larger. The whole ear is distinctly warmer 

 than the opposite one. This effect is produced by removing the arteries 

 from the influence of the central nervous system, which influence nor- 

 mally passes down the divided nerve; for if the peripheral end of the 

 divided nerve (i. e., that farthest from the brain) be stimulated, the 

 arteries which were before dilated return to their natural size, and the 

 parts regain their primitive condition. And, besides this, if the stimu- 

 lus which is applied is too strong or too long continued, the point of 

 normal constriction is passed, and the vessels become much more con- 

 tracted than normal. The natural condition, which is somewhere about 

 midway between extreme contraction and extreme dilatation, is called 

 the natural tone of an artery, and if this is not maintained, the vessel is 

 said to have lost tone, or if it is exaggerated, the tone is said to be too 

 great. The influence of the nervous system upon the vessels consists in 

 maintaining a natural tone. The effects described as having been pro- 

 duced by section of the cervical sympathetic and by subsequent stimula- 

 tion are not peculiar to that nerve, as it has been found that for every 

 part of the body there exists a nerve the division of which produces the 



