THE SECRETION OF THE DIGESTIVE JUICES 375 



the calibre of the ultimate channels that carry the secretion away. 

 The so-called bile-capillaries, which represent the lumen of the secreting 

 tubules, are mere grooves in the surface of adjoining cells; and the 

 architectural lines on which the liver lobule is built are: (i) the inter- 

 lobular veins which carry blood to it; (2) the rich capillary network 

 which separates its cells and feeds them; and (3) the central intra- 

 lobular vein which drains it. Thus a network of cells lying in the 

 meshes of a network of blood-capillaries takes the place of a regular 

 dendritic arrangement of ducts and tubules; and in accordance with 

 this the bile-capillaries, instead of opening separately into the ducts, 

 form a plexus with each other within the hepatic lobule (see also foot- 

 note, p. 14). 



The ducts and secreting tubules of all glands are lined by cells of 

 columnar epithelial type, but the type is most closely preserved in the 

 ducts. In none of the digestive glands is there more than a single 

 complete layer of secreting cells. But the alveoli of the mucous 

 salivary glands show here aiul there a crescent-shaped group of small 

 deeply-staining cells (crescents of Gianuzzi) outside the columnar layer 

 (Fig. 158, A", B"), and between it and the basement membrane, while 

 the gland-tubes of the f undus of the stomach have in the same situation 

 a discontinuous layer of largo ovoid cells, termed parietal from their 

 position, oxyntic (or acid -secreting) from their supposed function 

 (Figs. 155-157). Access to the lumen of the glands is provided for 

 these deeply-placed parietal cells and for the cells of the crescents by 

 fine branching channels which enter and surround the cells. The 

 serous salivary glands, the pyloric glands of the stomach, and the 

 Lieberkiihn's crypts, have but a single layer of epithelium; and since 

 there is no hepatic cell which is not in contact with at least one bile 

 capillary, the liver may be regarded as having no more. The same is 

 true of the pancreatic alveoli, except that in the centre of many of the 

 acini a few spindle-shaped cells (centro-acinar cells), apparently con- 

 tinued from the lining of the smallest ducts, may be seen. Remarkable 

 histological changes, evidently connected with changes in functional 

 activity, have been noticed in most of the digestive glands. In dis- 

 cussing these, it will be best to omit for the present any detailed reference 

 to the liver, since, although there are histological marks of secretive 

 activity in this gland as well as in others, and of the same general 

 character, they are accompanied, and to some extent overlaid, by the 

 microscopic evidences of other functions (p. 534). The serous salivary 

 glands and the pancreas can be taken together; so can the glands of the 

 various regions of the stomach; the mucous salivary glands must 

 be considered separately. 



Changes in the Pancreas and Parotid during Secretion. The cells 

 of the alveoli of the pancreas or parotid during rest, as can be seen 

 by examining thin lobules of the former between the folds of the 

 mesentery in the living rabbit, or fresh teased preparations of the 

 latter, are filled with fine granules to such an extent as to obscure 

 the nucleus. In the parotid the whole cell is granular, in the 

 pancreas there is still a narrow clear zone at the outer edge of the 

 cell which contains few granules or none; in both, the divisions 

 between the cells are very indistinct, and the lumen of the alveolus 

 cannot be made out. During activity the granules seem to be 

 carried from the outer portion of the cell towards the lumen, and 



