DIGESTION. 265 



spond to a lobule, while between them and the secreting tubes or 

 are longer or shorter intermediary ducts. The larger ducts possess a 

 very distinct lumen and a membrana propria lined with columnar epi- 

 thelium, 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 

 arc branched and convoluted tubes, with a membrana propria lined with 

 a single layer of columnar cells. They have no distinct lumen, its place 

 being taken by fusiform or branched cells. Heidenhain has observed 

 that the alveoli 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, 



Fro. 195. Section of the pancreas of a dog during digestion, a, alveoli lined with cells, the outer 

 zone of which is well stained with haematoxylin ; d, intermediary duct lined with squamous epithelium. 

 X 350. (Klein and Noble Smith.) 



and a 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 the protoplasm of the cells, from material supplied 

 to it by the blood. The granules are thought to be not the ferment 

 itself, but material from which, under certain conditions, the ferments of 

 the gland are made, and therefore called Zymogen. 



Pancreatic Secretion. The secretion of the pancreas has been ob- 

 tained for purposes of experiment from the lower animals, especially the 

 dog, by opening the abdomen and exposing the duct of the gland, which 

 is then made to communicate with the exterior . A pancreatic fistula is 

 thus established. 



