176 HUMAN PHYSIOLOGY. 



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 dextrose by the agency of a fer- 

 ment. Glycogen does not undergo oxidation in the blood ; this 

 process 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 colorless, and soluble 

 in water ; by boiling with dilute acids, or subjected to the action of 

 an animal ferment, it is easily converted into dextrose. When an 

 excess of sugar is generated by the liver, dextrose 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 gly- 

 cosuria. 



Formation of Urea. The liver is now regarded by many physiolo- 

 gists as the principal organ concerned in urea formation. The liver 

 normally contains a certain amount of urea ; and if blood be passed 

 through the excised liver of an animal which has been in full di- 

 gestion when killed, a large amount of urea i obtained. The 

 clinical evidence proves that in destructive diseases of the liver 

 substance there is at once a falling-off in urea elimination. Various 

 drugs which stimulate liver action increase the amount of urea in 

 the urine. 



Influence of the Nerve System. The nervous system directly 

 controls the functional activity of the liver, and more especially its 

 glycogenic function. It was discovered by Bernard that puncture 

 of the medulla oblongata is followed by so enormous a production 

 of sugar that it is at once excreted by the kidneys, giving rise to 

 diabetic or saccharine urine. This part of the medulla is, however, 

 the vaso-motor center for the blood-vessels of the liver. Destruction 

 of this center, or injury to the vaso-motor nerves emanating from 

 it in any part of their course, is followed at once by dilatation of the 

 hepatic blood-vessels, slowing of the blood-current, a profound dis- 

 turbance of the normal relation existing between the blood and 

 liver-cells, and a production of sugar. Many of the hepatic vaso- 

 motor nerves may be traced down the cord as far as the lumbar 

 region, while others leave the cord high up in the neck and enter 



