THE CIRCULATION OF THE BLOOD 



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 

 tone of an artery. If this is not maintained, the vessel is said to have lost 



FIG. 202. Arm Plethysmograph. Apparatus for measuring the change in volume in the 

 arm due to variation in the blood supply. The arm is enclosed in a glass cylinder which is com- 

 pletely filled with fluid, the opening through which the arm is inserted being closed by a rubber 

 sleeve, A . The cavity of the glass cylinder communicates through the tube, F, G, with the test tube 

 M, which is supported in the jar, P. Any variation in volume in the arm will cause water to flow 

 out or into the test tube, M, which is lowered as the tube fills, and raised as it empties. The rise 

 and fall of the test tube, M, is communicated over the pulley, L, to the writing-pen, N, which re- 

 cords the movements on the smoked cylinder. Kymograph not shown. (Mosso.) 



tone, or, if it is exaggerated, the tone is said to be too great. The effects 

 described as having been produced by section of the cervical sympathetic 

 and by subsequent stimulation are not peculiar to that nerve and the vessels 

 to which it is distributed. 



It has been found that for every part of the body, except the brain, there 

 exists a nerve the division of which produces the same effects, viz., dilatation 

 of the vessels. Such may be cited as the case with the sciatic, the splanch- 

 nic nerves, and the nerves of the brachial plexus; when these are divided, 

 dilatation of the blood-vessels in the parts supplied by them takes place. 

 It appears, therefore, that nerves exist which have a distinct control over the 

 vascular supply of every part of the body. These are called vaso-motor or 

 vaso-constrictor nerves. But the arterioles are also under the influence of 



