CIRCULATION OF THE BLOOD. 153 



tion to the other elements; in fact, in passing from capillary vessels, made 

 up as we have seen of endothelial cells with a ground substance, the first 

 change 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 has been shown that if the cervical sympathetic nerve be divided in 

 a rabbit, the blood-vessels of the corresponding side become dilated. The 

 effect is best seen 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 dis- 

 tinctly warmer than the opposite one. This effect is produced by remov- 

 ing the arteries from the influence of the central nervous system, which 



FIG. 138. Plethysmograph. By means of this apparatus, the alteration in volume of the arm, 

 E, which is enclosed 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, D, 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 in volume are due to alteration in the blood 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 to A. It will therefore be evident 

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

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

 and kidney. 



influence usually 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 

 stimulus which is applied be 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 be not maintained, the vessel is said 

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

 The influence of the nervous system upon the vessels consists in maintain- 

 ing a natural tone. The effects described as having been produced by 

 section of the cervical sympathetic and by subsequent stimulation are not 

 peculiar to that nerve, as it has been found that for every part of the 



