BIGGEST AND BUSIEST FACTORY 201 



scopic units, of which the liver is made up, were originally 

 placed in the wall of the bowel and were concerned in — 

 not the production of digestive juices, but the elaboration 

 of body-fuel absorbed from the chyme. Instead of being 

 placed all along the bowel these units have been collected 

 in one mass — the liver (see fig. 25, p. 92). In the liver 

 they are able to deal with elements absorbed from the food 

 while on their way to the general circulation of the body. 

 All the blood from the alimentary tract is gathered by the 

 portal vein and poured into a spongework of capillary 

 vessels which permeates the liver. The living units there 

 deal with the products of digestion before the portal blood 

 joins that of the general circulation. The liver is a great 

 chemical laboratory in which the products absorbed from 

 the bowel are turned into substances which are ready for 

 consumption by the tissues of the body. The reagents 

 employed are ferments or enzymes ; all are manufactured 

 in the living units of the liver. The liver serves not 

 only as a laboratory but also as a storehouse. The 

 derivatives of starches and sugars — carbohydrates — are 

 stored within it in the form of glycogen. When the body 

 tissues are in need of fuel the liver changes the glycogen 

 to blood-sugar and sets free the amount required in the 

 circulating blood. The liver also deals with the products 

 derived from nitrogenous foods. It is during their pre- 

 paration that urea is formed as a by-product. That, too, 

 is thrown into the blood stream, from which it is extracted 

 by the kidney and then discharged from the body. In 

 the course of the many chemical operations carried on 

 in the liver there are other by-products which are not 

 returned to the blood, at least when the liver is acting 

 normally, but are collected and discharged into the 

 duodenum by a system of ducts as bile or gall (fig. 41). 

 A special reservoir — the gall-bladder — is attached to the 

 main bile duct, but in all stages of digestion it is full 

 (fig. 40). Nor do we ever find it empty ; its real use is an 

 unsolved puzzle. The bile has no direct digestive action 

 on the chyme, but it has an important influence on the fats 

 contained in it, breaking them up into an emulsion and 



