246 THE CIRCULATION OF THE BLOOD 



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. 



a b 



FIG. 203. Plethysmograms of the Hind Leg of the Cat, showing a, vaso-contriction 

 on stimulating the sciatic at the rate of 15 stimuli per second for twenty seconds. In b, 

 the dilation of the blood vessels of the opposite leg of the same animal is shown on 

 stimulating the sciatic which had been cut four days previously. The vaso-con- 

 strictor nerves were degenerated and the vaso-dilators still active. (Bowditch 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 inter- 

 spaces 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 con- 

 stant 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 vaso-constrictor centers. This determi- 

 nation is made in part by the method of sectioning. A lesion of the 

 cerebro-spinal axis below the corpora quadrigemina is followed by partial 

 or complete general dilatation of the blood vessels and a great fall of blood- 

 pressure. This is due to the isolation of the vaso-constrictor center, which 

 lies in the floor of the fourth ventricle, a few millimeters caudal to the 

 corpora quadrigemina, and extends longitudinally over an area of about 

 three millimeters. Owsjannikow has shown that the center is composed of 



