Structure of the Hepatic Substance.— The substance of the liver is made up of lob ules of more or less equal size, separated by areolar connective tissue, which is a continua tion of the fibrous covering of the organ or capsule of Glisson. In these fibrous septa be tween the lobules are found the larger blood vessels, biliary ducts, lymphatics and nerves. The lobule under a low power of the micro scope is seen to be composed of radiating col umns of liver cells converging to a central point in which is situated a large vein (intra lobular vein). belonging to the system of the inferior vena cava. The columns of liver cells branch and anastomose with each other, and be tween them are spaces occupied by blood capil laries and the primary biliary passages. The liver cells themselves are polyhedral in shape and are so arranged that a capillary space, known as the biliary capillary, is left between them. The capillary blood vessels are simi larly formed by the peculiar juxtaposition of the liver cells, leaving spaces in which the ca pillary blood vessels extend from the terminal branches of the portal vein in the septa at the periphery of the lobules to the central vein al ready mentioned. The latter, collecting the blood from the capillary system, transports it to larger vessels known as sublobular veins, which finally unite and form the hepatic veins and discharge the blood into the inferior vena cava. The hepatic arteries and their terminal branches occupy the septa and supply blood to the connective tissue constituting the septa and to the walls of the portal veins and biliary ducts. The venous blood of this distribution is dis charged into the terminals of the portal vein within the septa.
At the periphery of the hepatic lobules, the epithelial cells of the hepatic columns continue directly into the smaller bile ducts which are situated in the interlobular septa. These pri mary bile ducts unite to form larger ducts and finally a right and left hepatic duct emerging from the transverse fissure unite to form a common bile duct. This is further joined by the cystic duct or duct of the gall-bladder and it terminates in the duodenum in a common ori fice with the pancreatic duct.
The structure of the liver is such that the blood entering through the portal circulation comes in contact with the liver cells on one side while the capillary biliary passages are on the other side of the hepatic cell.
The lymphatic vessels accompany the portal vein and hepatic artery and another system ac companies the branches of the hepatic vein. The capillaries penetrate into the lobules, pass ing between the hepatic cells.
Functions of the The physiologi cal functions of the liver are probably very nu merous and are but imperfectly understood. In the fcetus the liver is an important source of the red blood cells, reversing the process of adult life in which they ate destroyed by the formation of bile. The position of the organ indicates that one of its important functions is the elaboration of materials presented by the blood coming from the gastrointestinal tract, and probably also the purification of this blood.
As far as they are known, the functions of the organ may be divided into the metabolic and biliary.
Metabolic Functions.— It is known that the liver acts upon sugars and other carbohydrate materials to convert them into glycogen which is stored up by the hepatic cells. This accumu
lates during digestion and is utilized in the in tervals, so that the percentage of sugar in the blood is kept at an approximately uniform stand ard. It is known that in certain diseases (for example, diabetes), the function is greatly dis turbed with resulting consequences that affect the general health. The liver shares with the muscles of the body this glycogenic function. It also has the function of removing or neu tralizing certain toxins and of removing cer tain metallic salts.
Another important metabolic function is that of forming urea, the final stage in the meta bolism of albuminous food. Uric acid also is to some extent formed in the liver but is more particularly the product of the lymphatic or gans and tissues.
Biliary Function.—The formation of bile is one of the important attributes of the liver, though possibly this function is less predomi nating than has hitherto been believed. The bile is an albuminous liquid containing bile pig ments (bilirubin, biliverdin), bile salts (glyco cholate and taurocholate of soda), nucleo-albu min, cholesterin, fat and other less important substances including various salts. The bile is secreted more or less intermittently, the amount being greatest about the middle part of the day andincreasing after meals. The daily output is from one to two pints. Its functions are nu merous. Among other properties it aids in the absorption of fatty foods, stimulates intestinal peristalsis and prevents intestinal fermentation, and is the vehicle of excretion from the body of certain waste products, notably hemoglobin. It is known that bacteria and other minute for eign bodies are discharged from the system through this medium.
Authorities are divided as to the amount of the work which the liver performs in the production of sugar. Bernard believes that the glycogen is a reserve of carbohydrates, making the deduction from the fact that the liver is found to contain more glycogen than any other organ and that after death the glycogen is re placed by sugar. Another authority, Pavy, is of the opinion that the liver forms no sugar, but uses it in the form of glycogen. Still another, Seegen, declares that sugar is formed in the liver and not from glycogen but from protein and fat. As the blood normally has about 0.15 per cent of sugar, an amount which is cut in half, if the action of the liver is removed from the circulation, it is considered reasonably good evidence that the sugar in the blood comes either from the liver or from some other prod uct formed in the liver. Also the amount of sugar in the blood does not normally rise above 0.3 per cent without overflowing through the kidneys into the urine, which shows that the amount of sugar that can be destroyed by the liver is limited. The liver thus permits the accumulation and storing of this valuable ma terial which would otherwise be lost. If sugar in the form of a glucose solution is introduced into the blood in such a way as not to pass through the liver, it is immediately discharged via the urine, while if it is so injected as to pass into the liver first, a comparatively large quantity remains there.