SECRETIONS 245 



of two kinds ; some provide secretions which enter 

 the alimentary canal and secure the digestion of 

 the food; others elaborate materials which pass 

 into the blood and exercise an important influence 

 in maintaining that nicely adjusted equilibrium in 

 the living tissues which constitutes health. Neither 

 observation of the intact body during life nor 

 anatomical studies after death could possibly have 

 thrown any light whatever upon the functions of 

 the second class. The pancreas is a gland posses- 

 sing both of the types of function mentioned. 

 Its 'external' secretion is concerned with digestion, 

 its 'internal' secretion with chemical equilibrium 

 in the tissues. In connection with the former an 

 extraordinary instance of correlation among the 

 activities of different organs in the body was 

 brought to light, fourteen years ago, by Bayliss 

 and Starling. The proper progress of digestion 

 requires that the external secretion of the pancreas 

 should be poured upon the food as soon as the 

 latter leaves the stomach for the intestines. How 

 is this sequence of events secured in nature? The 

 stomach provides an acid secretion for carrying 

 out the preliminary stages of digestion, and when 

 its contents pass into the intestine they are under 

 normal circumstances strongly acid. Now the 

 minute cells which line the surface of the intestine 

 are in a peculiar way sensitive to the influence of 

 acids. On contact with them they are immediately 

 provoked to an act of internal secretion, and pour 



