180 



THE DIGESTIVE SYSTEM 



the connective tissue of the capsule, tissue of the same sort as the 

 latter continues inward, accompanying the vessels, and di\ides 

 the li\'er into se^'eral lobes, which in turn are subdivided into numer- 

 ous lobules. (Fig. 120.) The liver lobule is the unit of hepatic 

 structure in mammals, each lobule being roughly a five- or six-sided 

 polyhedral prism in form. Each lobule is composed of anastomosing 

 cords of liver cells that radiate outward from the center which is 

 occupied by a central vein. The indi\'idual cords have two rows 

 of cells whose boundaries are normally clearly defined. The indi- 

 vidual cells are polyhedral in form, with a central nucleus, or 

 possibly two or more nuclei, each wdth one or more nucleoli. Gran- 

 ules of glycogen (animal starch) may be demonstrated in the 



Fig. 120. — Photograph of several lobules in the liver of the pig. 



cytoplasm. Fat droj^lets appear to be normally present and ^•ary 

 inversely in amount with the glycogen. Pigment granules may also 

 be present. Considering the variety of functions in which the 

 hepatic cells participate, it is surprising that there is only one type 

 of cell. Instead of the cells pouring their secretions into a lumen 

 which the free ends of the cells adjoin, as is the case in other exo- 

 crine glands, the hepatic cords have a network of small canaliculi, 

 the bile capillaries, ruimiug between adjacent faces of cells forming 

 the cords. 



To understand the C()mi)()sition of the liver it is imi)ortant to 

 understand the distribution of the blood vessels to a lobule. Blood 

 enters the liver from two sources, the hepatic artery and the i)ortal 

 vein, and leaves through the hepatic veins which emi)ty into the 

 vena cava. The ail'erent vessels, the he])atic artery and i)()rtal 



