Chap. IX.] NUTRITION AND METABOLISM. 181 



veins the lobules cluster, being aggregated round, and, to some 

 extent, supported on minute intra-lobular venules. The lobules 

 are divided from each other by connective tissue, in which there 

 run : (1) the inter-lobular venules, which are the ultimate 

 branches of the portal system; and (2) the inter-lobular arterioles, 

 the ultimate branches of the hepatic artery. Thus each lobule 

 has a double blood-supply, the blood from each source being 

 collected by the intra-lobular venule. In addition to the blood- 

 capillaries within the lobule there are also bile canaliculi. The 

 hepatic cells, of which the lobule is composed, are polygonal in 

 form, and are so arranged as to leave minute channels (the bile- 

 canaliculi) between them. At the margins of the lobules these 

 canaliculi are connected with minute tubes, the inter-lobular 

 bile ducts, which form networks in the inter-lobular connective 

 tissue, and eventually, by continued fusion one with another, 

 pass into the larger bile ducts, by which the product may either 

 be delivered directly to the alimentary canal, or may pass into 

 the storage reservoir, the gall-bladder. Thus the duct of this 

 great gland does not break up into a number of blind and 

 dilated tubes, but into a network of canaliculi. It is not a 

 racemose, but a reticulated gland. 



The Gland Products. The product of the salivary glands is 

 saliva. It is a thin, watery, slightly alkaline fluid, which is seen 

 under the microscope. to contain, besides flattened epithelial cells 

 from the mucous membrane of the mouth, a number of smaller 

 rounded corpuscles which may show amoeboid movements. The 

 salivary fluid is especially secreted by the parotid and the sub- 

 maxillary, the sub-lingual secreting a mucous fluid. An important 

 constituent of the salivary fluid is a ferment called ptyalin. It 

 is one of those curious chemical bodies, a very small quantity of 

 which may induce extensive chemical changes in other sub- 

 stances. The chemical change induced by ptyalin is the con- 

 version of starch ((C 6 H 10 6 ) n ) first into its isomer dextrin (a 

 body of similar chemical composition, but with widely different 

 properties), and then into sugar (glucose, C 6 H 12 6 + H 2 0, or in 

 some cases maltose, C^H^On + HaO). When we learn or 

 remember that starch mucilage will not diffuse through a mem- 



