350 ESSENTIALS OF PHYSIOLOGY. 



reversible ferment. We may perhaps regard glycogen as a sort of 

 current account, which fluctuates from day to day, whereas the store of 

 fat in the body represents a more permanent reserve, or capital account, 

 which can be called upon in times of stress. 



Although the main source of glycogen is carbohydrate food, it can 

 also be formed to some extent from protein, since, when an animal is 

 starved until its liver is presumably free from glycogen, and is then 

 killed shortly after a large meal of protein, some glycogen is found in 

 its liver. There is no evidence that glycogen can be formed from fat. 



Glycogen is found most abundantly in the liver, but it occurs 

 in muscles, being especially plentiful in f ratal muscles, and it is also 

 present in the white blood corpuscles. 



FATE OF SUGAR. 



The carbohydrate taken into the body ultimately undergoes one of 

 two changes. Some of it, more particularly in herbivora, is converted 

 into fat ; the remainder passes from the blood to the tissues, where it 

 is oxidised and used as a source of energy. There is direct evidence 

 that sugar is made use of by the tissues. Using the heart-lung pre- 

 paration (p. 199), Starling found that the normal heart used up sugar 

 at the rate of about 4 milligrams per gram of heart per hour. 

 Further indirect evidence to the same effect is furnished by the fact 

 that the glycogen disappears most rapidly from the liver when the 

 functional activity of the tissues is greatest. Thus severe muscular 

 exercise or the convulsions induced by strychnine lead to the rapid 

 disappearance of glycogen from the liver. Neither the conditions which 

 determine the taking up of sugar by the tissues from the blood, nor 

 the intermediate stages in its oxidation are fully known, but it is 

 probable that lactic acid is an intermediate product in the conversion 

 of sugar into carbonic acid and water. 



Some light is thrown on the conditions which influence the setting 

 free of sugar from the liver and its further oxidation in the tissues by 

 certain abnormal conditions in which sugar appears in the urine, and 

 which are known as glycosuria. 



Glycosuria. The urine normally contains slightly less than O'l per 

 cent, of dextrose, and does not reduce an alkaline solution of copper 

 sulphate ; the term glycosuria is only used when the urine contains 

 dextrose in sufficient amount definitely to reduce such a solution. 

 This may occur in a variety of circumstances. If dextrose is injected 

 into the circulation or under the skin, the percentage in the blood 

 rises (hyperglycsemia), and the sugar is at once excreted by the kidneys. 

 A similar condition, known as (1) Alimentary Glycosuria, is observed 



