300 



THE DIGESTIVE ORGANS. 



functional activity of the gland the granules gradually disappear, 

 while the fluid secretion simultaneously makes its appearance in the 

 lumen, although the granules have as yet never been observed in 

 the lumen itself. After secretion the cell grows again until it 

 reaches its original size, only again to begin the formation of zymo- 

 gen granules. Whether the cells of the gland are destroyed or not 

 during secretion is still a matter of uncertainty, but does not seem 

 probable. 



An intermediate tubule similar to those of the salivary glands 

 connects with each alveolus, and then passes over into a short in- 

 traltifiular duct. This is lined, as in the salivary glands, with 

 columnar epithelial cells, which are not, however (at least in 

 man), striated at their basal ends. The intralobular ducts merge 





Centro-acinal 



cell. 



Intermediary 

 duct. 



Intralobular _ 

 duct. 



Alveolus. 



iL Intermediary 



duct. 



Fig. 241. From section through human pancreas ; X about 200 (sublimate). 



into excretory ducts, which finally empty into the pancreatic duct. 

 The epithelium of the excretory ducts is simple columnar in type. 

 Goblet cells are seen only in the pancreatic duct. 



In the secreting alveoli small protoplasmic, polygonal, and even 

 stellate cells are often seen, the so-called centro-acinal cells, or cells 

 of Langcrhans. The significance of these structures is not fully 

 understood. Langerhans himself supposed that they belonged to 

 the walls of the excretory ducts. This interpretation seems war- 

 ranted by the fact that it has been found that the secreting cells of 

 the alveoli are directly joined to the low cells of the intermediate 

 tubules. When the alveoli lie closely packed together, the ad- 

 joining intermediate tubules fuse and are reduced to one or, at 

 most, a few cells. As a result a condition is seen within the 

 alveolar complexus, especially when the excretory ducts are in a 

 collapsed state, closely resembling the structures seen by Langer- 



