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Henry Gray (1825–1861).  Anatomy of the Human Body.  1918.
 
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 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 centroacinar cells of Langerhans. These are prolongations of the terminal ducts. The true secreting cells which line 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 membrane, and an inner granular one next the lumen. In hardened specimens the outer zone stains deeply with various dyes, whereas the inner zone stains slightly. During activity the granular zone gradually diminishes in size, and when exhausted is only seen as a small area next to the lumen. During the resting stages it gradually increases until it forms nearly three-fourths of the cell. In some of the secreting cells of the pancreas is a spherical mass, staining more easily than the rest of the cell; this is termed the paranucleus, and is believed 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 carbohydrate metabolism, but numerous researches have so far failed to elucidate their real function.


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



FIG. 1105– Section of pancreas of dog. X 250. (See enlarged image)

  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.
 
3. 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 (b) the genital organs, which are concerned with the process of reproduction.

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