72 Physiology. 



and sending branches to the heart and lungs in the chest, 

 and to the liver, stomach, and other organs in the abdomen. 

 In many places these nerves form a thick network called 

 a plexus. One very large plexus on the dorsal surface of 

 the stomach is called the solar plexus. (See Fig. 44.) 



Regulation of the Size of the Arteries. In the last chap- 

 ter we learned that in the walls of the arteries are muscle 



fibers having a ring-like 



Spinal Cord 



arrangement. When these 



muscle fibers shorten they 



i ^i 



make the artery narrower, 

 and less blood can flow 

 through it. When the 

 muscle fibers relax, they 



Fig. 45. ideal Cross-section of the Nervous lengthen : the artery be- 



System. (After Landois and Stirling.) ' 



comes wider, and more 



blood flows through it. Now these muscle fibers are .under 

 the control of the sympathetic nerves. The sympathetic 

 nerves, therefore, regulate the amount of blood that goes 

 to every organ. 



Blushing. The sudden reddening of the face means 

 that more blood is flowing through the skin of .the face. 

 The arteries by which blood reaches the face have quickly 

 widened, and this is because the muscle fibers in the walls 

 of the arteries have suddenly relaxed. To go still further 

 back in the explanation, some emotion has started nerve 

 currents which travel along the fibers of the sympathetic 

 nerves and caused the arteries to widen. 



Sudden Pallor. On the other hand, if the muscle fibers 

 in the walls of the arteries suddenly shorten, the face will 

 turn pale, because less blood flows in the skin of the face. 

 Such a change, as before, is due to the nerve currents 



