KATABOLISM. 



44I 



the contrary, it consists mainly of certain peculiar, very 

 solid, nucleated cells called liver cells (Fig. 305). 



Four Systems of Tubes. — An eliminating organ 

 or gland has usually three systems of tubes or vessels : 



(1) The artery and its branches, carrying blood to the 

 work. (2) The vein and its branches, carrying back the 

 blood when the work is done ; these two connect 

 through the capillaries. (3) A system of excretory or 

 secretory tubes, carrying away the product of manufac- 

 ture. But the liver has four systems of tubes ramifying 

 through its mass of liver cells: (1) The hepatic artery, 



(2) the hepatic vein, (3) the portal vein, and (4) the biliary 

 ducts. The one which 



is peculiar is the portal 

 vein. We have already 

 (page 328) drawn atten- 

 tion to the peculiar- 

 ity of this vein. It is 

 the vein corresponding 

 to the mesenteric and 

 splenic arteries. But, in- 

 stead of emptying its 

 blood, like all normal 

 veins, into the vena cava, 

 close at hand, it goes to 

 the liver, to be ramified 



by capillaries through its substance, and only after doing 

 so carries its blood to the hepatic vein and thence to the 

 vena cava and the general circulation. So that instead 

 of two systems of vascular pipes, arteries, and veins, 

 connecting with one another through the capillaries, we 

 have three systems of such pipes — viz., hepatic artery, 

 portal vein, and hepatic vein — all connected continu- 

 ously by capillary circulation, so that water injected 

 into any one of these trunks will flow out of the other 



Fig. 305. — Microscopic structure of the 

 liver : lc y liver cells ; epc, epithelial 

 cells ; id, biliary duct. 



