1204 



SPLANCHNOLOGY 





and connects together the various lobules of which it is composed. Each lobule, like the lobules 

 of the salivary glands, consists of one of the ultimate ramifications of the main duct, ending in a 

 number of cecal pouches or alveoli, which are tubular and somewhat convoluted. The minute 

 ducts connected with the alveoli are narrow and lined with flattened cells. The alveoli are 



Liver 



Liver 



Alveolus 



Cell-islet 



Right suprarenal gland 

 **&/// 1 / / / 1 II I ltlJll> >> ' 



Left suprarenal gland 

 FIG. 1104. Section through same region as in Fig. 1103, at end of third month. (Toldt.) 



almost completely filled with secreting cells, so that scarcely any lumen is visible. In some 

 animals spindle-shaped cells occupy the center of the alveolus and are known as the centro- 

 acinar cells of Langerhans. These are prolongations of the terminal ducts. The true secreting 

 cells which lire the wall of the alveolus are very characteristic. They are columnar in shape 



and present two zones : an outer one, clear 

 and finely striated next the basement mem- 

 brane, and an inner granular one next the 

 lumen. In hardened specimens the outer 

 zone stains deeply with various dyes, where- 

 as the inner zone stains slightly. During 

 activity the granular zone gradually dimin- 

 ishes in size, and when exhausted is only 

 seen as a small area next to the lumen. 

 During the resting stages it gradually in- 

 creases until it forms nearly three-fourths 

 of the cell. In some of the secreting cells 

 of the pancreas is a spherical mass, stain- 

 ing more easily than the rest of the cell; 

 this is termed the paranucleus, and is be- 

 lieved to be an extension from the nucleus. 

 The connective tissue between the alveoli 

 presents in certain parts collections of cells, 

 which are termed interalveolar cell islets 

 (islands of Langerhans}. The cells of these 

 stain lightly with hematoxylin or carmine, 

 and are more or less polyhedral in shape, 

 forming a net-work in which ramify many 



capillaries. There are two main types of cell in the islets, distinguished as A-cells and B-cells 

 according to the special staining reactions of the granules they contain. The cell islets have 

 been supposed to produce the internal secretion of the pancreas which is necessary for carbo- 

 hydrate metabolism, but numerous researches have so far failed to elucidate their real function. 

 The walls of the pancreatic duct are thin, consisting of two coats, an external fibrous and an 

 internal mucous; the latter is smooth, and furnished near its termination with a few scattered 

 follicles. 



Vessels and Nerves. The arteries of the pancreas are derived from the lienal, and the 

 pancreaticoduodenal branches of the hepatic and superior mesenteric. Its veins open into the 

 lienal and superior mesenteric veins. Its lymphatics are described on page 711. Its nerves are 

 filaments from the lienal plexus. 



THE UROGENITAL APPARATUS (APPARATUS UROGENITALIS ; 

 UROGENITAL ORGANS). 



The urogenital apparatus consists of (a) the urinary organs for the secretion 

 and discharge of the urine, and (6) the genital organs, which are concerned with 

 the process of reproduction. 



Fio. 1105. Section of pancreas of dog. X 250. 



