THE LIVER. 151 



of epithelium. In the human liver this tubular structure is dis- 

 guised by the facts that the tubules anastomose with each other, and 

 that their lumina are very minute and bounded by only two cells 

 when seen in cross-section. So inconspicuous are these lumina that 

 a casual glance at a section of a liver would not reveal the fact that 

 it was a glandular organ. 



The interstitial tissue of the liver consists of a few sparsely 

 distributed fibres continuous with those of Glisson's capsule. 



The intricate structure of the liver prepares us for the fact that 

 its function is an extremely complex one. It is a secreting gland, 

 elaborating the bile and discharging it into the duodenum. But 

 the bile has more than one purpose. It aids in the digestion and 

 absorption of food, and it also contains excrementitious matters 

 destined to leave the body through the alimentary tract. Even the 

 secretory function of the liver, therefore, serves a double purpose : 

 the supply of substances useful to the organism and the elimina- 

 tion of products that would be detrimental if retained. 



But the function of the liver is not confined to the elaboration 

 of the bile. It also acts as a reservoir for the storage of nourish- 

 ment, which can be drawn upon as needed by the organism. This 

 is the meaning of the glycogen and fat which have infiltrated the 

 cells. 



The food-materials that are absorbed from the digestive tract pass 

 into the system through two channels : the lymphatic and the portal 

 circulations. The latter carries them to the liver, where some of 

 the fat, probably after desaponification, is taken up by the epithelial 

 cells, which also appropriate a portion of the sugar in the portal 

 blood, transforming it into glycogen and holding it in that form 

 until a relative deficiency of glucose in the blood reveals its need 

 by the system. 



The blood comes into such close relations with the epithelial cells 

 of the liver that an interchange of soluble substances between them 

 appears to be about as easy a matter as the interchange of gases 

 between the blood and the air in the lungs ; and, as in the latter 

 case, this interchange is mutual : some matter passing from the 

 blood to the liver-cells and some from the cells to the blood. In 

 the lung there is a gaseous regeneration of the blood ; in the liver, 

 a renovation as to certain of its soluble constituents. 



The Gall-bladder. The bile is secreted continuously by the liver, 

 for it is an excrement ; but it is discharged intermittently into the 



