EXTERNAL SECRETIONS 489 



walls, receive from the cells some of the products of their secretor activity, 

 and hence are comparable to the lumen of the alveoli of other secreting 

 glands. At the periphery of the lobules the bile capillaries communicate 

 with larger channels which are the beginnings of the hepatic or bile-ducts 

 lying in the interlobular spaces. The interlobular bile-ducts possess a dis- 

 tinct wall lined by flattened epithelium. There is, however, no distinct line 

 of demarcation between the cells of the interlobular ducts and the secreting 

 cells of the liver proper, as the two blend insensibly, the one into the 

 other. As the hepatic ducts increase in size they gradually acquire the 

 structure characteristic of the two main hepatic ducts: viz., a mucous, a 

 muscle, and a fibrous coat. 



FIG. 217. TRANSVERSE SECTION OF A SINGLE HEPATIC LOBULE, i. Intralobular vein, cut 

 across. 2, 2, 2, 2. Afferent branches of the intralobular vein. 3, 3, 3, 3, 3, 3, 3, 3, 3. Interlobular 

 branches of the portal vein, with its capillary branches, forming the lobular plexus, extending to 

 the radicles of the intralobular vein. (Sappey.) 



The Nerve Supply. Experimental investigations have demonstrated 

 that the liver is supplied with nerves derived from the central nerve system. 

 The route of these nerves is probably by way of the vagi and of the splanch- 

 nics as far as the semilunar ganglion, around the cells of which the terminal 

 fibers arborize. From the cells of this ganglion post-ganglionic fibers arise 

 which pass by way oi the hepatic plexus along the course of the hepatic 

 artery and portal vein. Many of the nerves which enter the liver are vaso- 

 motor in function; as to whether others are secretor in character is yet a sub- 

 ject of investigation. It has been asserted that nerve filaments have been 

 demonstrated running between the cells and even penetrating their substance. 

 This fact would indicate that the metabolic processes of the liver are under 

 the control of the central nerve system. 



Functions of the Liver. The anatomic and histologic peculiarities of 

 the liver would indicate that it has a variety of relations to the general pro- 

 cesses of the body. Experimental investigation has brought some of these 

 relations to light. Though its physiologic actions are not yet wholly under- 

 stood, it may be said that it is engaged in : 



1. The elaboration and excretion of bile. 



2. The production of starch (glycogen) and sugar (glucose). 



