THE CIRCULATION OF THE BLOOD 381 



Slow stimulation of the peripheral ends of various spinal nerves is followed 

 by dilatation of the blood-vessels in the areas to which they are distributed. 



From these and many other facts of a similar character it is probable 

 that the blood-vessels of some of the organs at least are under the control 

 of two antagonistic classes of nerve-fibers, one augmenting the degree of 

 their contraction (the vaso-constrictors) , the other diminishing it through 

 inhibition (the vaso-inhibitors) . Through the cooperative antagonism of 

 these two classes of nerves the caliber of the blood-vessels and thereby the 

 volume of the blood is accurately adapted to the needs of each organ both 

 during rest and during activity. It is also to the alternate activity of these 

 nerves that the variations occurring from time to time in the volume of 

 organs are to be attributed. 



A general vaso-dilatator center has never been located and there are 

 many reasons for thinking that such a center has no anatomic existence. 

 There are, however, special or local vaso-dilatator centers in the medulla 

 oblongata and in various regions of the spinal cord especially in the sacral 

 region. 



Physiologic Properties of Vaso -motor Nerves. The vaso-constrictor 

 and the vaso-dilatator nerve-fibers differ somewhat in their physiologic 

 properties, as shown by the results of experiment. Thus, when a mixed 

 nerve, i.e., one containing both classes of fibers e.g., the sciatic is stimu- 

 lated with frequently repeated induced currents, the constrictor effect is 



A 



FIG. 179. PLETHYSMOGRAMS OF THE HIND-LEG OF THE CAT FOLLOWING STIMULATION OF 

 THE SCIATIC NERVE. In A the rate of stimulation was sixteen per second, in B one per secor 

 for fifteen seconds. To be read from right to left. (Boroditch and Warren.) 



the more pronounced, the dilatator effect being wanting or prevented; when 

 stimulated with slowly repeated induced currents, the dilatator effect is 

 the more pronounced. These different effects are strikingly shown i 

 Fig. 179, A and B. 



In the experiment of which these tracings are the result the leg of a cat 

 enclosed in a plethysmograph and the variations in volume due to diktat* 

 or contraction of the vessels, following stimulation of the sciatic nerve 

 recorded by means of tambour and lever on a slowly revolving cylinder. 

 A the fall of the curve indicates a diminution of volume, from contract! 

 blood-vessels following a rate of stimulation of the sciatic nerve of i 

 second for fifteen seconds. In B the rise of the curve indicates an n 

 in volume from dilatation of the vessels following a rate of stimulation of i 



