VASOCONSTRICTOR CENTER 235 



instrument designed to measure the variations in the volume of an organ. 

 If the finger, the whole hand, the spleen, or the kidney be placed in such an 

 instrument and the proper steps be taken to record the volume changes, it 

 will be found that the volume of the enclosed organ is constantly changing 

 with every variation of the blood pressure. If the nerves to the organ are 

 stimulated by the usual rapidly interrupted induction current, for example 

 the splanchnics to the kidney, then there is a decrease in the volume of the 

 organ. This decrease takes place even when there is a simultaneous in- 

 crease of the arterial blood pressure, a result that can be explained only on 

 the assumption of vascular decrease in the organ. The decrease in the flow 

 of blood to the specific organ can be induced only by a great decrease in the 

 size of the arterioles produced by contractions of the circular muscles of 

 their walls. 



FIG. 203. Plethysmogram of the Hind Limb of a Cat, showing Vaso-constriction upon 

 Stimulating the Sciatic 64 times per second. To be read from right to left. (Bo\vditch 

 and Warren.) 



Vaso-motor Tone. Vaso-constrictor changes are constantly occur- 

 ring in the blood vessels of the organs of the body, a fact that has been 

 abundantly demonstrated by the plethysmographic experiments just men- 

 tioned. Direct inspection of the ear of an albino rabbit will show that the 

 arteries, and veins as well, are now full and large and red, and the interspaces 

 filled with blood, and now pale and constricted, and the interspaces relatively 

 bloodless. If the cervical sympathetic is cut as in Bernard's experiment, 

 then the ear vessels remain dilated, that is they lose their tone. This 

 shows that the condition is dependent primarily on the constant discharges 

 of nerve impulses from the nervous system. It is said that the vessels 

 regain their tone after a time when the nerves are cut. The regained power 

 may be ascribed to the muscle fibers themselves. 



Vaso-constrictor Center. When the tonic influence exerted by the 

 nerve fibers on the arterioles is traced back into the central nervous system, 

 it is found to be associated with the activity of certain groups of nerve cells, 

 or centers, which are called the I'aso-constrictor centers. This determination 



