168 



BLOOD-PRESSURE IN THE ARTERIES. 



agents concerned in this process. When they relax, the arterial blood- 

 pressure falls, and when they contract, it rises. These actions of 

 muscular fibres are controlled and regulated by the action of the vaso- 

 motor nerves (vol. ii.) 



(e.) Collateral Vessels. The arterial pressure within a given area of 

 the vascular system must rise or fall according as the neighbouring 

 areas are diminished, whether by the application of pressure, or a 

 ligature, or are rendered impervious, or as these areas dilate. 

 The application of cold or warmth to limited areas of the body 

 increasing or diminishing the atmospheric pressure on a part the 

 paralysis or stimulation of certain vaso-motor areas (vol. ii.), all pro- 

 duce remarkable variations in the blood-pressure. [The effect of 

 dilatation of a large vascular area on the arterial pressure is well 

 shown by what happens when the blood-vessels of the abdomen are 

 dilated. If the central end of the superior cardiac nerve of a rabbit be 

 stimulated, after a few seconds the blood-vessels of the abdomen dilate, 

 and gradually there is a steady fall of the blood-pressure in the 

 systemic arteries. Fig. 76 is a blood-pressure tracing showing the 

 height of the blood-pressure before stimulation, a. The stimulation 

 was continued from a to b, and after a certain latent period, there is 

 a steady fall of the blood-pressure. The nerve which causes this reflex 



Fig. 76. 



Kymographic tracing showing the effect on the blood-pressure of stimulation of the central end of the 

 depressor nerve in the rabbit. Stimulation began at a, and ended at b ; o x, the abscissa. 



