SKIN. 79 



tests, even when the blood of the portal vein does not contain a trace of 

 sugar. 



Origin and Destination of Glycogen. Glycogen appears to be 

 formed de novo in the liver cells, from materials derived from the food, 

 whether the diet be animal or vegetable, though a larger per cent, is 

 formed when the animal is fed on starchy and saccharine, than when fed on 

 animal food. The glucose, which is one of the products of digestion, is 

 absorbed by the blood vessels, and carried directly into the liver; as it does 

 not appear in the urine, as it would if injected at once into the general 

 circulation, it is probable that it is detained in the liver, dehydrated and 

 stored up as glycogen. The change is shown by the following formula : 

 Glucose. Water. Glycogen. 



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



The glycogen thus formed is stored up in the hepatic cells for the future 

 requirements of the system. When it is carried from the liver it is again 

 transformed into glucose by the agency of a ferment. Glycogen does not 

 undergo oxidation in the Hood ; this takes place in the tissues, particularly 

 in the muscles, where it generates heat and contributes to the development 

 of muscular force. 



Glycogen, when obtained from the liver, is an amorphous, starch-like 

 substance, of a white color, tasteless and odorless, and soluble in water; by 

 boiling with dilute acids, or subjected to the action of an animal ferment, 

 it is easily converted into glucose. When an excess of sugar is generated 

 by the liver, it can be found, not only in the blood of the hepatic vein, 

 but also in other portions of the body; under these circumstances it is 

 eliminated by the kidneys, appearing in the urine, constituting the condition 

 of glycosuria. 



The Nervous System influences the production of the glycogenic 

 matter ; irritation of the medulla oblongata, between the auditory and pneu- 

 mogastric nerves, is followed by an increase in the production of sugar, 

 and its appearance in the urine, which, however, is only temporary. 



SKIN. 



The Skin, the external investment of the body, is a most complex and 

 important structure, serving (l) as a protective covering ; (2) an organ for 

 tactile sensibility ; (3) an organ for the elimination of excrementitious 

 matters. 



The Amount of Skin investing the body of a man of average size is 



