PANCREAS 



293 



terminate in clusters of alveoli, which often present clover-leaf forms. 

 The centro-alveolar cells have been interpreted as due to the invagination 

 of these ducts into the alveoli, but apparently they do not develop in this 

 way; they are formed as an inner stratum of a two-layered epithelium. 

 The secretory capillaries of the alveoli are shown in Fig. 290. They ex- 

 tend between the centro-alveolar cells to the pancreatic cells, and may be 

 prolonged between the latter, but they do not reach the basement 

 membrane. 



The intercalated duels pass into excretory ducts lined with cuboidal 

 epithelium, without the intervention of secretory ducts such as are found 

 in the salivary glands. The plan of the pancreatic ducts is shown in 

 Fig. 291. The main pancreatic and accessory pancreatic ducts are com- 

 posed of simple columnar epithelium surrounded by a connective tissue 



Intercellular 

 secretory ,' 

 capillary. 



FIG. 290. A, FROM A SECTION OF THE PANCREAS OF AN AD,ULT MAN. X 320. B, AN INTERPRETATION OF 



THE RIGHT LOWER PORTION OF A. 



layer, outside of which is a zone of circular smooth muscle fibers. The 

 latter are gathered into sphincters at the major and minor duodenal 

 papillae, where the ducts open. Occasional goblet cells and small glands 

 resembling mucous glands have been found in the mucosa of the large 

 ducts. 



The blood and lymphatic vessels and nerves of the pancreas resemble 

 those of the salivary glands. The capillaries have notably wide meshes 

 so that considerable portions of the alveoli are not in contact with them. 

 The nerves end around the blood vessels, ducts and pancreatic cells. 

 They are chiefly non-medullated sympathetic fibers from the cceliac 

 plexus, associated with scattered nerve cells found within the pancreas. 

 Lamellar corpuscles occur in the connective tissue. 



The pancreatic islands are usually not to be found in human embryos 

 under 50 mm. in length. Thus they develop only after the pancreatic 

 glands have come together and attained considerable size. They arise as 

 outgrowths from the smaller ducts, with which they may retain a solid 

 stalk- like connection, or they may become wholly detached. According 

 to Bensley, detached islands in the guinea-pig are infrequent. In the 



