Absorption of the carbohydrate food reduced to the condition of monosaccharoses takes place by the portal capillaries, and in this way being carried by the blood stream to the liver, it is there converted into glycogen. There is hardly any room for doubt as to the various transformations of the carbohydrates up to this point, but our troubles begin here, for we are confronted with two theories as to their further disposal. Before dealing with these in detail, however, it will be profitable to make a few preliminary observations. Chemical analysis demonstrates that blood contains a reducing sugar, but on account of the extreme difficulty of the process, there is no agreement as to its precise quantity. It is generally stated to be from .05 to .2 per cent., and any increase upon the upper limit is held to constitute hyperglycemia: its natural consequence, glycosuria, invariably following. The kidneys are impervious to the passage of a certain percentage of sugar, and it is maintained by some that any glucose above .1 per cent, is excreted in the urine. If this is admitted, then the term hyperglycemia ought to be applied to blood containing .2 per cent. of glucose, and estimating its total volume as 5,000 c.c, quite 5 grams of sugar above the normal would be present. Taking the higher limit as normal, the whole volume of blood would only account for 10 grams or thereby of sugar.

During fasting, portal blood contains the same quantity of sugar as systemic blood, but during absorption from the intestine the percentage is much higher, viz., from .2 to .4 per cent. The liver contains about 10 per cent. of glycogen, and as it weighs about 1,500 grams, that would represent an absorption of 150 grams of sugar from the blood. The amount of glycogen in the muscles is about 1 per cent., and in the aggregate the total content of all the muscular system would be about another 150 grams. This alone would account for something like 300 grams of glycogen, but as it also occurs in the pancreas, the small digestive glands, the lungs, brain, skin, etc, the total amount in the body must be somewhat greater than this.

Although glycogen is found in the white blood corpuscles, yet none has been discovered in the blood plasma, and as in normal circumstances the percentage of glucose in the systemic blood is much lower than that in the portal system, the absorbed carbohydrate food is in the liver converted into glycogen, where it is stored up in the extra-nuclear portions of the cells. Chemically speaking, the transformation of a monosaccharose into glycogen is a simple process, involving only anhydration, but it is really a little more complex than this, as more than two molecules of a monosaccharose (C6HI2O5) must enter into combination, with the loss of one molecule of water for every molecule of monosaccharose, to form glycogen, (C6H40O5)n.

All carbohydrates yielding dextrin and laevulose contribute to the formation of glycogen. Lactose does not, because in the adult there is no digestive ferment capable of hydrolys-ing it. Subcutaneous injections of cane sugar and lactose do not increase glycogen formation, nor can they be reduced in the blood. Hence there can be no inverting enzymes there capable of forming dextrose and laevulose from them, and they are excreted unchanged in the urine. Maltose, on the other hand, when injected into the blood increases glycogen formation, because it is inverted into dextrose. It may be concluded, therefore, that dextrose, laevulose, and possibly galactose, all sugars fermentable by yeast, are the only sugars from which glycogen can be formed. The monosaccharoses, therefore, are absorbed as such, maintain their identity in the portal circulation, and on reaching the liver are changed into glycogen. It may be noted that dextrose is an aldose, i.e., contains the radicle CHO, and laevulose a ketose, with the radicle CO, and as they are incapable of being converted into each other, in all probability they are transformed into glycogen by a different kind of process.

Bernard's Theory

The classical view of the glycogenic function of the liver originated by Bernard maintains that glycogen is a reserve store of carbohydrate material for the future needs of the body analogous to the storing-up of starch in the vegetable world. Bernard sustained the doctrine that all the sugar in the portal blood beyond .2 per cent, becomes glycogen, and whenever the content of systemic blood tends to fall below .2 per cent. of glucose it is at once reinforced by a special conversion of hepatic glycogen into sugar, a process which we now know is effected through the medium of an enzyme designated glycogenase. He contended that the percentage of sugar in the blood of the hepatic vein is always greater than that in the portal vein, even when there is no absorption of food from the bowel, and that the blood going to the muscles contained more sugar than that returning from the muscles. He noted that immediately after death a maximum of glycogen and a minimum of glucose were to be found in the liver, whereas several hours thereafter a maximum of glucose and a minimum of glycogen were to be obtained. He considered, therefore, that glycogen leaves the liver as glucose and is conveyed by the blood to the muscles where it is utilised.