CHAP, iv.] METABOLIC PROCESSES OF THE BODY. 707 



lobular portal veinlets, are disposed in a network with meshes 

 elongated in a radial direction, and converge at the centre of the 

 lobule to form the intralobular veinlet falling into the sublobular 

 (hepatic) vein, and secondly of radiating columns of cells filling up 

 the radiating meshes of this vascular network. Hence in a 

 section of a hardened and prepared uninjected liver, in which 

 the blood vessels are largely emptied, the areas of the sections 

 of lobules are indicated by the radially converging columns of 

 cells, and (according to the animal employed) are more or less 

 distinctly marked out by the septa of connective tissue, in which 

 may be seen here and there the lumina of the larger interlobular 

 veins. In lobules, in which the section has passed through the 

 middle of the lobule, the lumen of the central intralobular vein 

 will also be visible ; but often the section will cut a lobule so 

 superficially as to miss the intralobular vein altogether ; and it is 

 only when the section happens to pass through the middle of the 

 lobule in the plane of the long axis of the flask, that the origin of 

 the intralobular vein in the middle of the body of the flask and 

 its course along the neck to the sublobular vein is displayed, 



447. If the section be extensive enough there may be seen 

 here and there sections of the portal vein, hepatic artery and bile- 

 duct running in Glisson's capsule. Sections of the branches of 

 the hepatic vein formed by the union of sublobular veins may also 

 be seen. These may be recognised by the absence or by the 

 extreme scantiness of any connective tissue wrapping to the vein, 

 even in the case of the larger branches. The wall of the vein 

 too is very thin and consists of hardly more than the tunica 

 intima resting on a thin connective tissue basis, muscular fibres 

 being so very scanty that the tunica media may be said to be 

 absent. 



The walls of the portal vein on the contrary are thick and 

 muscular ; the trunk is more abundantly supplied with muscular 

 fibres than any other vein in the body ; and the branches within 

 the liver are, in diminishing degree, thick and muscular. This is 

 intelligible when it is remembered that the blood is distributed 

 into capillaries from the portal vein as from an artery ; and indeed 

 it has been maintained that the portal vein is subject to rhythmic 

 contractions of its walls, as if to assist in the passage onward of 

 the blood. Neither in the trunk nor in the branches are any 

 valves present, and these are also absent from the branches of the 

 hepatic vein. 



The branches of the hepatic artery are very much smaller than 

 the branches of the portal vein, and even much smaller than the 

 branches of the bile-duct in company with which they run. As 

 they proceed in their course they supply the walls of the portal 

 veins and of the bile-ducts and the substance of Glisson's capsule, 

 and eventually discharge their blood into the portal veinlets. It 

 has been maintained that some of the finer branches run directly 



452 



