Section 61. the Liver Is the Most Complicated Gland in the Body

(Figure X.). The bile duct (b.d.) branches again and again, and ends at last in the final pits, the lobuli (lb.), which are lined with secretory epithelium, and tightly packed, and squeeze each other into polygonal forms. The blood supply from which the bile would appear to be mainly extracted, is brought by the portal vein, but this blood is altogether unfit for the nutrition of the liver tissue; for this latter purpose a branch of the coeliac artery, the hepatic serves. Hence in the tissue of the liver we have, branching and interweaving among the lobuli, the small branches of the bile duct (b.d.), which carries away the bile formed, the portal vein (p.v.), the hepatic artery (h.a.), and the hepatic vein (h.v.). (Compare Section 45.) Figure X.b shows a lobule; the portal vein and the artery ramify round the lobules-- are inter-lobular, that is (inter, between); the hepatic vein begins in the middle of the lobules (intra-lobular), and receives their blood. (Compare X.a.) Besides its function in the manufacture of the excretory, digestive, and auxiliary bile, the liver performs other duties. It appears to act as an inspector of the assimilation material brought in by the portal vein. The villi, for instance, will absorb arsenic, but this is arrested and thrown down in the liver. A third function is the formation of what would seem to be a store of carbo-hydrate, glycogen, mainly it would appear, from the sugar in the portal vein, though also, very probably, from nitrogenous material, though this may occur only under exceptional conditions. Finally, the nitrogenous katastates, formed in the working of muscle and nerve, and returned by them to the blood for excretion, are not at that stage in the form of urea. Whatever form they assume, they undergo a further metabolism into urea before leaving the body, and the presence of considerable quantities of this latter substance in the liver suggests this as a fourth function of this organ-- the elaboration of urea.