INNERVATION OF VESSELS 51 



by the sympathetic nervous system by means of the vaso-mo- 

 tor nerves. These are composed of the vaso-constrictors 

 which cause the vessels to contract, and the vaso-dilators 

 which cause them to dilate. The entire physiological distribu- 

 tion of blood is regulated by the vaso-motor system of nerves. 

 It is by their means that the blood is increased to any part 

 of the body where physiological activity is going on, as 



FIG. 26. 



A, vein with valves open. B, with valves closed; stream of blood passing off by 

 lateral channel. (Kirkes after Dalton.) 



when the gastro-intestinal tract is active during digestion, 

 when a muscle is in motion, or a gland in activity. Paraly- 

 sis of (the vaso-constrictors causes blushing, paralysis of the 

 dilators causes pallor as from fright. Outside influences 

 will cause the constrictors to act, as cold ; while alcohol will 

 cause the dilators to act and paralyzes the constrictors. 



The chief vaso-motor center is in the medulla oblongata, 

 while subordinate centers exist in the cord. The vaso-motor 

 fibers reaching the vessels proceed from ganglia in the sym- 



