380 HANDBOOK OF PHYSIOLOGY. 



alveoli are longer or shorter intermediary ducts. The larger ducts 

 possess a very distinct lumen and a rnembrana propria lined \vith 

 columnar epithelium, the cells of which are longitudinally striated, but 

 are shorter than those found in the ducts of the salivary glands. In the 

 intralobular ducts the epithelium is short and the lumen is smaller. 

 The intermediary ducts opening into the alveoli possess a distinct lumen, 

 with a membrana propria lined with a single layer of flattened elongated 

 cells. The alveoli are branched and convoluted tubes, with a membraua 

 propria lined with a single layer of columnar cells. They have a 

 distinct lumen, though spindle-shaped cells are often seen in the 

 centre of the acini. Heidenhain has observed that the alveolar 

 cells in the pancreas of a fasting dog consist of two zones, an inner or 

 central zone which is finely granular, and which stains feebly, and a 



Fig. 265. Section of the pancreas of armadillo, showing the two kinds of gland-structure. (V. I>. 



Harris.) 



smaller parietal zone of finely striated protoplasm which stains easily. 

 The nucleus is partly in one, partly in the other zone. During digestion, 

 it is found that the outer zone increases in size, and the central zone 

 diminishes; the cell itself becoming smaller from the discharge of the 

 secretion. At the end of digestion the first condition again appears, the 

 inner zone enlarging at the expense of the outer. It appears that the 

 granules are formed by and stored up in the protoplasm of the cells, from 

 material supplied to it by the blood. The granules are thought to 

 consist of material from which, under certain conditions, the ferments 

 of the gland are developed, and which is therefore called Zymogen. In 

 addition to the ordinary alveoli of the pancreas there are found distri- 

 buted irregularly in the gland other collections of cells of a different 

 character. They are considerably smaller, their protoplasm is more 

 granular, and is less easily stained with haematoxylin, and their nuclei 

 are small and deeply staining, being situated also more toward the 

 centre of the cells. The collections of cells vary in size and shape, and 



