DIGESTION 345 



tinction into body and duct, as in Lieberkiihn's crypts in the intes- 

 tines, or in which one or more of the tubes open into a duct, as in 

 the glands of the fundus of the stomach. Some are branched tubes, 

 several of which may end in a common duct ; such are the glands of 

 the pyloric end of the stomach, and the Brunner's glands in the 

 duodenum. In others the main duct ramifies into a more or less 

 complex system of small channels, into each of the ultimate branches 

 of which one or more (usually several) of the secreting tubules or 

 alveoli open. The salivary glands and the pancreas belong to this 

 class of compound tubular or racemose glands, and so does the liver 

 of such animals as the frog. But in the latter organ the typical 

 arrangement is obscured in the higher vertebrates by the predomi- 

 nance of the portal bloodvessels over the system of bile-channels as 

 a groundwork for the grouping of the cells. 



In every secreting gland there is a vascular plexus outside the 

 cells of the gland-tubes, and a system of collecting channels on 

 their inner surface ; and in a certain sense the cells of every gland 

 are arranged with reference to the bloodvessels on the one hand, and 

 the ducts on the other. But in the ordinary racemose gland the 

 blood-supply is mainly required to feed the secretion ; the cells of 

 the alveoli have either no other function than to secrete, or if they 

 have other functions, they are not such as to entail a great dispro- 

 portion between the size of the cells and the lumen of the channels 

 into which they pour their products. For both reasons the relation 

 of the grouping of the cells to the duct-system is very obvious, to 

 the blood-system very obscure. In the liver the conditions are 

 precisely reversed. We cannot suppose that the manufacture of a 

 quantity of bile less in volume than the secretion of the salivary 

 glands, though doubtless containing far more solids, requires an 

 immense organ like the liver, and a tide of blood like that which 

 passes through the portal vein. And, as we shall see, the liver has 

 other functions, some of them certainly of at least equal importance 

 with the secretion of bile, and one of them evidently requiring from 

 its very nature a bulky organ. Accordingly, both the richness of the 

 blood-supply and the size of the secreting cells are out of proportion 

 to 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 interlobular veins which carry blood to it ; (2) the rich capil- 

 lary 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 sepa- 

 rately into the ducts, form a plexus with each other within the 

 hepatic lobule (see also footnote, 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 and there a crescent-shaped 

 group of small deeply-staining cells (crescents of Gianuzzi) outside 

 the columnar layer (Fig. 141, A", B"), and between it and the basement 

 membrane, while the gland-tubes of the fundus of the stomach have 

 in the same situation a discontinuous layer of large ovoid cells, 



