174 HUMAN PHYSIOLOGY. 



The bile-channels empty into the interlobular ducts, which meas- 

 ure about ^olT of an inch in diameter and are composed of a thin, 

 homogeneous membrane lined by flattened epithelial cells. 



As the interlobular bile-ducts unite to form large trunks, they 

 receive an external coat of fibrous tissue, which strengthens their 

 walls ; they finally unite to form one large duct (the hepatic duct), 

 which joins the cystic duct; the union of the two forms the ductus 

 communis choledochus, which is about three inches in length, the 

 size of a goose-quill, and opens into the duodenum. 



The gall-bladder is a pear-shaped sac, about four inches in length, 

 situated in a fossa on the under surface of the liver. It is a reser- 

 voir for the bile, and is capable of holding about one ounce and a 

 half of fluid. It is composed of three coats : 



1. Serous, a reflection of the peritoneum. 



2. Fibrous and muscular. 



3. Mucous. 



Functions of the Liver. The liver is a complex organ having a 

 variety of relations to the general processes of the body. While 

 its physiologic actions are not yet wholly understood, it may be said 

 that it 



1. Secretes bile. 



2. Forms glycogen. 



3. 'Assists in the formation of urea and allied products. 



The Secretion of Bile. The characteristic constituents of the 

 bile do not preexist in the blood, but are formed in the interior 

 of the liver cells of materials derived from the venous and arterial 

 blood. The hepatic cells, absorbing these materials, elaborate them 

 into bile-elements, and in so doing undergo histologic changes similar 

 to those exhibited by other secretory glands. The bile once formed, 

 it passes into the mouths of the bile capillaries, near the periphery 

 of the lobules. Under the influence of the vis a tergo of the new- 

 formed bile it flows from the smaller into the large bile-ducts, and 

 finally empties into the intestine, or is regurgitated into the gall- 

 bladder, where it is stored up until it is required for the digestive 

 process in the small intestine. The study of the secretion of bile 

 by means of biliary fistulae reveals the fact that the secretion is 

 continuous and not intermittent ; that the hepatic cells are constantly 

 pouring bile into the ducts, which convey it into the gall-bladder. 

 As this fluid is required only during intestinal digestion, it is only 



