116 PHYSIOLOGY FOR DENTAL STUDENTS. 



liver; iii any case, the long chain of carbon atoms of which we 

 have seen fat acid molecule to be composed (see p. 24) becomes 

 oxidized (burnt), not all at once but piece by piece, two carbon 

 atoms being split off at a time. If the fat acid chain originally 

 contained an even number of carbon atoms, the oxidation 

 process may stop short when there are yet four carbon 

 atoms in the chain, thus producing oxybutryic acid 

 (CH 3 CHOHCH 2 COOH). This imperfect metabolism of fat oc- 

 curs in severe cases of diabetes and often causes death. It also 

 occurs in carbohydrate starvation, and indicates, more clearly 

 than any thing else, that even carbohydrates are essential for life. 



Metabolism of Carbohydrates. It will be remembered that 

 these include the starches and the sugars, and that during diges- 

 tion they are all hydrolyzed to dextrose or laevulose, as which 

 they are absorbed into the blood of the portal vein. This ab- 

 sorption is rapid, so that a striking increase in the percentage 

 of sugar occurs in the blood of the portal vein shortly after the 

 food has been taken. Most of this excess of sugar does not imme- 

 diately gain entry to the blood of the systemic circulation, how- 

 ever, because it is retained by the liver. For this purpose the 

 liver cells convert the sugar into the starch-like substance, glyco- 

 'gen, which becomes deposited in their protoplasm as irregular 

 colloidal masses, which stain with iodine and carmine. The liver 

 does not manage in this way to remove all of the excess of sugar 

 from the portal blood, so that, even in a healthy animal, there 

 is a distinct postprandial increase of sugar, or hyperglycaemia, as 

 it is called, in the systemic blood. If too much sugar passes the 

 liver it causes so marked a postprandial hyperglycffimia that 

 some sugar escapes into the urine, thus causing glycosuria, which 

 is one of the early symptoms of diabetes, and whose occurrence 

 furnishes us with a warning that less carbohydrates should be 

 given in the food. If the warning be heeded, the severer form 

 of the disease will very probably be staved off. 



The glycogen deposited in the liver stays there until the per- 

 centage of sugar in the systemic blood begins to fall below its 

 proper level (which in man is about 0.1 per cent), when it 

 becomes reconverted into sugar, which is added to the blood. 



