SECRETION. 447 



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 main hepatic duct: viz., a mucous, a muscle, 

 and a fibrous coat. 



Influence of the Nerve System. 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 splanch- 

 nics and the vagi. Many of the nerves which enter the liver are vaso-motor 

 in function; as to whether others are secretor in character is yet a subject 

 of investigation. It has been asserted that nerve filaments have been demon- 

 strated 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). 



3. The formation of urea. 



4. The conjugation of products of protein putrefaction. 



The Elaboration of Bile. The physical properties and chemical com- 

 position of the bile have already been considered (page 191) . The character- 

 istic salts of the bile, sodium glycocholate and taurocholate, do not pre-exist 

 in the blood, and therefore must be formed by the liver cells out of materials 

 derived from the blood of the intralobular capillaries. The antecedents of 

 the bile salts, glycocoll and taurin, are crystallizable nitrogenized compounds, 

 and known chemically as amido-acetic and amido-ethylsulphonic acids. 

 Their chemic composition indicates that they are derivatives of the proteins, 

 though the intermediate stages in their production are unknown. The 

 origin of the cholalic acid with which they are combined is equally obscure. 

 The bile salts as they are found in the bile are produced however in the liver 

 cells by metabolic activity. 



The primary coloring-matter of the bile, bilirubin, has been shown to 

 be a derivative of hematin, a product of the disintegration of hemoglobin. 

 It is supposed that the liver cells bring about this change by combining water 

 with hematin, with the abstraction of iron. The product thus formed is bili- 

 rubin, which is excreted, while the iron is for the most part retained in the 

 liver cells. 



Cholesterin is a waste product derived largely from the nerve-tissue. 

 It is brought to the liver and simply excreted by the cells. The remaining 

 constituents of the bile, water and inorganic salts, are secreted here in the 

 same way as in all other glands. 



When once formed, the liver cells discharge these various compounds 

 into the channels by which they are surrounded; they then pass into the open 



