56 THE STRUCTURE OF THE FOWL 



ducts are reduced to the main tubes which 

 open into the duodenum. The walls of the 

 larger ducts contain muscular tissue and are 

 lined by columjiar epithelium (Fig. 29). 



Throughout the pancreas, and between its 

 alveoli, are clusters of polyhedral cells forming 

 the cell-islets of Langerhans. These are evid- 

 ently concerned in the production of a secretion 

 which finds its way into the blood-stream and 

 acts as a regulator of metabolism. The 

 pancreas, therefore, has to be regarded as a 

 double organ. In the first place it produces 

 the digestive pancreatic secretion by the 

 activity of the epithelium lining its alveoli. 

 Secondly, the islets of Langerhans elaborate 

 an internal secretion. 



