176 CHEMICAL CORRELATION 



injected into a vein and distributed throughout the body, 

 it draws together the walls of the tiny blood-vessels and 

 so causes a marked rise in blood-pressure. At first this 

 action on the blood-vessels was thought to be the special 

 function of adrenalin, but it was very soon shown that 

 what adrenalin really does is to stimulate the tiny nerves 

 which terminate in the smooth muscles in the walls of the 

 blood-vessels, and so cause these smooth muscles to con- 

 tract. The action of adrenalin in closing small wounds 

 and in raising blood-pressure is thus incidental to its real 

 function of rousing smooth muscles to activity. An im- 

 portant fact about this action is that the amount of 

 adrenalin normally present in the blood is not enough to 

 bring this about. There must be an increase above that 

 which is ordinarily present if the blood-pressure is to be 

 raised. 



The Regulation of Adrenalin Secretion. — The 

 fact just mentioned has caused the question to be asked, 

 whether there is any condition in which the adrenal glands 

 become more active than usual and pour out into the 

 blood large enough amounts of adrenalin to cause the 

 smooth muscles of the blood-vessels to be excited. Such 

 a situation has been shown to arise in connection with 

 the unfavorable emotions described in the last chapter, 

 where it was seen that in time of stress the body rallies 

 its powers to meet an emergency. The possible connec- 

 tion of adrenalin with this emergency reaction came to 

 light when it was found that not only are the smooth 

 muscles of the blood-vessels stimulated, but all the other 

 effects listed on page 16G are brought to pass through the 

 action of adrenalin in the body. Thus, supposing that 

 the adrenal glands pour out increased quantities of 

 adrenalin in times of emergency, there is a chemical rein- 

 forcement of the effects produced by the nervous system, 

 with the presumable advantage of increasing one's ability 

 to cope with the situation. There is a great deal of evi- 

 dence to show that the adrenals are under the control of 



