HYDROCARBONACEOUS SUBSTANCES. 61 



of biliary salts, and some albuminous matters. The glucose and biliary 

 salts are removed by washing the precipitate with alcohol. The remain- 

 der is then boiled for a quarter of an hour with a concentrated solution 

 of potassium hydrate, which dissolves the albuminous matters, but does 

 not affect glycogen. After nitration it is again dissolved in water, the 

 traces of alkali removed by the addition of a little acetic acid, and the 

 glycogen re-precipitated by alcohol in excess. It is then dried and may 

 be kept in the form of a white pulverulent mass, which retains its prop- 

 erties for an indefinite time. 



In watery solution it exhibits the characteristic properties of an amy- 

 laceous substance, being converted into sugar by all agencies which 

 have a similar effect on starch, namely, by boiling with a dilute mineral 

 acid, and, at a moderately warm temperature, by the contact of saliva, 

 the pancreatic or intestinal juices, or the serum of blood. If allowed to 

 remain in the liver after death, or brought in contact with its tissue 

 after removal, a portion is transformed into glucose by the albuminous 

 matters of the hepatic substance. 



The quantity of glycogen in the liver varies, with the kind of food 

 used, from about 7 to 17 per cent. It is more abundant with vegetable 

 than with animal food, and is most abundant of all under a diet of carbo- 

 hydrates. It increases after digestion, and diminishes with fasting, 

 disappearing altogether after an abstinence of four or five days. It 

 will then reappear very rapidly after a meal of starchy or saccharine 

 matters. From these facts it is apparent that glucose, when taken as 

 food, or absorbed from the alimentary canal, is deposited in the liver 

 under the form of glycogen. The change which takes place is a dehy- 

 dration, as follows : 



GLUCOSE. WATER. GLYCOGEX. 

 C 6 H 12 6 -H 2 = C 6 H 10 5 . 



While in this condition the glycogen forms part of the substance of 

 the liver, and is probably, a material of reserve, to be afterward con- 

 sumed in some other part of the body. It appears to be gradually 

 reconverted into glucose in the intervals of digestion, and to disappear 

 under this form from the hepatic tissue. 



Glycogen presents accordingly, in every respect, a strong analogy with 

 vegetable starch. Its abundant presence in the embryonic organs, from 

 which it disappears when they have acquired their growth, is like the 

 deposit of starch in a seed, to be used up in the act of germination. And 

 in the adult animal it is probable that a large portion, if not all, of the 

 carbo-hydrates taken as food pass through the glycogenic condition be- 

 fore they are finally employed in the nutrition of the body. 



Fats, 



The fats form a well marked group of organic bodies which are widely 

 diffused both in the vegetable and the animal kingdom. They are dis- 

 tinguished from the carbo-hydrates, first, by the fact that they do not 

 contain hydrogen and oxygen in the proportion to form water, the 



