1038 ABSORPTION 



are usually united into compounds, forming (a) the mineral or inorganic 

 constituents, and (6) the organic constituents of our body. Physiolog- 

 ical chemistry concerns itself chiefly with the latter which present 

 themselves as carbohydrates, fats and proteins. So far, however, 

 chemical analyses have not succeeded in establishing anything further 

 regarding the "life history" of these substances than what might be 

 termed a balance sheet between their ingo and outgo. This need not 

 surprise us, because even the simplest determinations frequently 

 necessitate difficult analytical procedures. In general, it may be said 

 that our knowledge regarding the sum total of the changes which the 

 foodstuffs undergo in our body (metabolism) has been derived from 

 determinations of: 



(a) the quantity and quality of food ingested, 



(6) the quantity and quality of the material excreted, 



(c) the weight of the animal before and after the experiment, and 



(d) the energy evolved by the animal in the form of work and heat, 

 while in the calorimeter. 



THE METABOLISM OF THE CARBOHYDRATES 



The Formation of Glycogen. The animal derives its carbohydrates 

 in the main from vegetable carbohydrates which upon digestion 

 yield three monosaccharides, namely, glucose, fructose and galactose. 

 About 500 grm. of carbohydrate are ordinarily ingested in the course 

 of a day. Our body, however, is normally unable to synthetize this 

 foodstuff, differing in this regard very sharply from the plants, which 

 are able with the help of the chlorophyll to form a simple carbo- 

 hydrate, probably formic aldehyde, from carbon dioxid and water. 

 By condensation this simple substance is then changed into sugar, and 

 eventually into starch. Since the aforesaid sugars are easily inter- 

 convertible, the tissues may form whatever type of sugar they need. 

 This is true, for example, of lactose, a constituent of the secretion of 

 the mammary glands, and of the galactosides of nervous tissue. Since 

 lactose is a compound of glucose and galactose, it requires only a very 

 slight intermolecular rearrangement to produce this substance. In 

 other words, 'there is sufficient evidence at hand to show that one type 

 of sugar may be transformed into another either by the cells of all 

 the tissues or only by those of certain tissues. 



It has been ascertained by Cl. Bernard (1853) that the sugar ab- 

 sorbed is not passed directly into the circulation, because the amount 

 of reducing sugar present in the blood retains the almost constant 

 value of 0.1 to 0.15 per cent, even at the height of digestion. In 

 between the successive periods of absorption the percentage of this 

 substance in the blood of the portal vein is about the same as that of 

 the blood in the systemic channels, whereas during absorption the 



