THE CIRCULATION OF THE BLOOD. 331 



seconds. In B the rise of the curve indicates an increase in volume 

 from dilatation of the vessels following a rate of stimulation of i per 

 second for fifteen seconds (Bowditch and Warren). With different 

 rates of stimulation somewhat different results are obtained. 



After division of a mixed nerve the vaso-constrictors degenerate 

 and lose their influence over the blood-vessel in from four to five 

 days, the vaso-dilatators in from seven to ten days, as shown by the 

 response to electrical stimulation. 



When a nerve is cooled, the vaso-constrictors lose their irritability 

 before the vaso-dilatators. 



Vaso- motor Centers. The nerve-cells throughout the spinal 

 cord from which the vaso-motor nerves take their origin may be 

 regarded as nerve-centers which through their related nerve-fibers 

 exert from time to time either a constrictor or a dilatator influence 



l^hiitflmaalhrti^nijlJL 



" ' "Fr"r'lnff'nprt"i ^^TmLMMtf| 



B 



FIG. 156. PLETHYSMOGRAMS OF THE HIND-LEG or 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. 



over the blood-vessels, ^though both the vaso-constrictor and vaso- 

 dilatator centers are in a state of continuous activity, the former 

 decidedly preponderates, as shown by the maintenance of a tonic 

 contraction of the blood- vessels.^ The activity of both centers may 

 be increased or decreased, augmented or inhibited, by nerve impulses 

 reflected to them from the periphery through afferent nerves or 

 through nerve-fibers descending the cord from higher levels of the 

 nervous system. Experiment has shown that when a definite region 

 of the medulla oblongata is punctured or in anywise destroyed there 

 is an immediate dilatation of the blood-vessels throughout the body 

 and a fall of blood-pressure below one-half or one-third of the normal 

 value. This region has a width of one and a half millimeters and 

 extends longitudinally for a distance of four or five millimeters, 



