COORDINATION 89 



17. Stimulation and coordination by chemical means ; hor- 

 mones. In previous chapters we have dealt chiefly with 

 examples of stimulation of muscle and gland cells by nervous 

 impulses and of the coordination of the work of organs 

 through the central nervous system; but there is another 

 way by which both stimulation and coordination are effected. 

 An irritable cell will respond to other stimuli than nervous 

 impulses ; among these are a sharp blow, sudden heating, 

 make or break of an electric current, and exposure to the 

 action of certain substances. The last is generally spoken 

 of as chemical stimulation, and we shall meet with examples 

 of this in our subsequent study. One will suffice for the 

 present. After the food has undergone a preliminary diges- 

 tion in the stomach by the acid gastric juice, it is passed 

 into the small intestine, where its digestion is completed. 

 The first requisite for this purpose is the secretion of pan- 

 creatic juice, and this is secured as follows: the acid of the 

 stomach contents liberates from the lining cells of the first 

 part of the intestine a substance known as secretin, which 

 enters the blood and chemically excites the cells of the pan- 

 creas to secrete pancreatic juice. By this means the pancre- 

 atic juice is secreted into the intestine at precisely the time 

 that it is needed there ; that is, as each consignment of acid 

 food is discharged from the stomach (see Chap. VIII, p. 113). 

 A substance thus liberated in one organ and stimulating 

 another organ to activity at the time when such activity is 

 needed is known as a hormone (Greek hormao, " I arouse " ). 



The action of secretin evidently presents, in addition to its 

 feature of stimulation, an element of purposeful coordination, 

 since it insures the proper cooperation of the stomach and 

 pancreas in the work of digestion ; and other examples of the 

 same thing might be cited. We have, however, only to refer 

 the student to the case of adrenaline, already described in 

 Chapter VI, for the most striking example of coordination 

 produced by chemical means. 



