PANCREATIC JUICE. This is a colorless fluid, secreted in a lobular gland which is situated behind the stomach, and runs transversely from the spleen across the vertebral column to the duodenum. The most important constituent of the pancreatic juice is an organic substance, termed _pancreatin_. THE BILE. The blood which is collected by the veins of the stomach, pancreas, spleen, and intestines, is discharged into a large trunk called the portal vein, which enters the liver. This organ also receives arterial blood from a vessel called the _hepatic artery_, which is given off from the aorta below the diaphragm. If the branches of the portal vein and hepatic artery be traced into the substance of the liver, they will be found to accompany one another, and to subdivide, becoming smaller and smaller. Finally, the portal vein and hepatic artery will be found to terminate in capillaries which permeate the smallest perceptible subdivisions of the liver substance, which are polygonal masses of not more than one-tenth of an inch in diameter, called the _lobules_. Every lobule rests upon one of the ramifications of a great vessel termed the _hepatic vein_, which empties into the inferior vena cava. There is also a vessel termed the _hepatic duct_ leading from the liver, the minute subdivisions of which penetrate every portion of the substance of that organ. Connected with the hepatic duct, is the duct of a large oval sac, called the _gall-bladder_. Each lobule of the liver is composed of minute cellular bodies known as the _hepatic cells_. It is supposed that in these cells the blood is deprived of certain materials which are converted into bile. This secretion is a glutinous fluid, varying in color from a dark golden brown to a bright yellow, has a specific gravity ranging from 1018 to 1036, and a slightly alkaline reaction. When agitated, it has a frothy appearance. Physiologists have experienced much difficulty in studying the character of this secretion from the instability of its constituents when subjected to chemical examination. [Illustration: Fig. 52. Section of the Liver, showing the ramifications of the portal vein. 1. Twig of portal vein. 2, 2', 2", 2"'. Interlobular vein. 3, 3', 3", Lobules.]