758 PHYSIOLOGY OF DIGESTION AND SECRETION. 



of glycogen namely, that it is a temporary reserve supply of 

 carbohydrate material that may be rapidly converted to sugar and 

 oxidized with the liberation of energy it is found that the supply 

 of glycogen is greatly affected by conditions calling for increased 

 metabolism in the body. Muscular exercise quickly exhausts the 

 supply of muscle and liver glycogen, provided it is not renewed 

 by new food. Observations on isolated muscles have shown 

 definitely that the local supply of glycogen is diminished when the 

 muscle is made to contract (see p. 60). In a starving animal 

 glycogen finally disappears, except perhaps in traces, but this 

 disappearance occurs much sooner if the animal is made to use its 

 muscles at the same time. It has been shown also by Morat and 

 Dufourt that if a muscle has been made to contract vigorously 

 it takes up much more sugar from an artificial supply of blood sent 

 through it than a similar muscle which has been resting ; on the 

 other hand, it has been found that if the nerve of one leg is cut 

 so as to paralyze the muscles of that side of the body, the amount 

 of glycogen is greater in these muscles than in those of the other 

 leg that have been contracting meantime and using up their gly- 

 cogen. The further history of glycogen is considered in the section 

 on Nutrition. 



Formation of Urea in the Liver. The nitrogen contained in 

 the protein material of our food is finally eliminated, mainly in the 

 form of urea. It has been definitely proved that the urea is not 

 formed in the kidneys, the organs that eliminate it. It has long been 

 considered a matter of the greatest importance to ascertain in what 

 organ or tissues urea is formed. Investigations have gone so far as 

 to demonstrate that it arises in part at least in the liver; hence the 

 property of forming urea must be added to the other important func- 

 tions of the liver cell. Schroder * performed a number of experi- 

 ments in which the liver was taken from a freshly killed dog and 

 irrigated through its blood-vessels with a supply of blood obtained 

 from another dog. If the supply of blood was taken from a fasting 

 animal, then circulating it through the isolated liver was not followed 

 by any increase in the amount of urea contained in it. If, on the 

 contrary, the blood was obtained from a well-fed dog, the amount 

 of urea contained in it was distinctly increased by passing it through 

 the liver, thus indicating that the blood of an animal after digestion 

 contains something that the liver can convert to urea. It is to be 

 noted, moreover, that this power is not possessed by all the organs, 

 since blood from well-fed animals showed no increase in urea after 

 being circulated through an isolated kidney or muscle. As further 

 proof of the urea-forming power of the liver Schroder found that 



* " Archiv f. experimentelle Pathologie und Pharmakologie," 15, 364, 

 1882, and 19, 373, 1885. 



