THE CIRCULATION OF THE BLOOD 



385 



submaxillary gland becomes bright red in color. Its tissues being unable 

 to consume all the oxygen, the blood emerges in the veins almost arterial 

 in color. Stimulation of Jacobson's nerve has the same effect on the blood- 

 vessels of the parotid gland. Stimulation of the branches of the sacral 

 nerves which collectively constitute the nervus erigens is followed by a dilata- 

 tion of the blood-vessels of the sexual organs. 



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 vase-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. 



A B 



FIG. 177. 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 second 

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



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 

 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 in 

 Fig. 177, A and B. 



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

 enclosed in a plethysmograph and the variations in volume due to dilatation 

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



2S 



