THE CIRCULATION. 195 



Second. The constrictor nerves of the chest and abdomen 

 take their origin in the medulla, also, then through the 

 spinal cord they reach the sympathetic ganglia of the chest 

 and abdomen by means of the communicating branches, 

 and from these ganglia they proceed to the visceral organs. 

 The constrictors going to the organs in the abdominal 

 region go mainly through the large splanchnic nerves and 

 the solar plexus from which they spread in all directions. 

 As there are so many and such large arteries in the ab- 

 domen this splanchnic nerve is probably the most important 

 constrictor nerve in the body. Third. Constrictor nerves 

 going to the trunk and the extremities arise in the medulla, 

 also, pass down the cord and through the communicating 

 branches reach the sympathetic ganglia. From these they re- 

 join the spinal nerves and with them are distributed at the 

 periphery. It will thus be seen that all the constrictor nerves 

 arise in the medulla, but that before reaching their destina- 

 tion they pass through sympathetic ganglia. For this 

 reason they are often called sympathetic nerves. 



The Vaso- Constrictor Center. 



These nerves are governed by a center which lies in the 

 medulla where they originate. This center is automatic, 

 and is in constant excitation, and so, as stated before, there 

 is a continued contraction in all the arteries supplied. But 

 this vaso-constrictor center may be inhibited in part, that 

 is, it may be prevented from keeping the arteries con- 

 tracted. Such an inhibition would, of course, result in a 

 dilatation of the arteries affected. In this way the blood sup- 

 ply of the visceral organs, especially, is largely controlled. 

 What, now, are the influences that inhibit this center and 

 so cause a dilatation? 



First: Psychic influences inhibit it. The explanation 

 of blushing lies in the fact that psychic influences (embar- 

 rassment, shame, etc.), reach that part of the center con- 

 trolling the arteries of the face and inhibit it, and the 



