The liver is bounded by a connective tissue capsule which extends into its substance as highly branched septae. The afferent blood vessels and lymphatics follow this connective tissue highway throughout the liver. Efferent vessels traverse a route separate from connective tissue scaffolding. Notice how the capsule extends as a septum into the liver about one-third of the way from left, immediately below a large capsular blood vessel. The connective tissue septae invaginating from the capsule delineate hepatic lobules, the structural unit of the liver.
Central veins are quite prominent and provide an easy means of orientation in sections of liver.
Examination of a triad in cross section should reveal a bile duct and branches of the hepatic artery and hepatic portal vein . Due to plane of section, one can often observe more than one of each of these structures in a given portal tract or absence of one or more structures. Lymphatic vessels are also present, but are tough to see in standard paraffin sections, which is probably why it is not called a portal tetrad. See if you can picture the lobule, then move your mouse pointer around the image to confirm it.