226 



ESSENTIALS OF PHYSIOLOGY. 



general blood-pressure tracing, and shows the effect of stimulating the 



peripheral end of a divided renal nerve. 



By one or other of these methods it is found that division of the 



nerves passing to the kidney produces an increase of its volume and 



an increased flow of blood 

 through it, whereas stimula- 

 tion of the nerves has the 

 opposite effect. Since in these 

 experiments the general 

 FIG. 87.-Oncometer. arterial pressure remains prac- 



tically unaltered, the changes 



in the rate of blood flow produced by section or stimulation of the 



renal nerves must be brought about by alterations in the calibre of 



the arterioles of the kidney. 



Experiments of this kind show that the arterioles of almost every 



organ in the body are supplied with vaso-constrictor nerves. The tone 



FIG. 88. Tracing of arterial blood pressure (1), and kidney volume (2). 

 Between X and X the 10th thoracic nerve-root was stimulated, 

 causing a decrease in kidney volume. (From Practical Physiology, 

 by Pembrey and others. ) 



of these vessels is controlled by a centre, the vaso-motor centre, lying 

 in the medulla oblongata ; nerve fibres pass from the cells of this 

 centre down the spinal cord, to end in all probability round cells in the 

 lateral horn in the dorsal region and round corresponding cells in the 

 lumbar region. These cells give off small medullated fibres, which 

 leave the spinal cord by the anterior roots and enter the white rami 



