CHEMISTRY OF DIGESTION AND NUTRITION. 271 



In a starving animal glycogen will finally disappear, except perhaps in traces, 

 but this disappearance will occur 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 will take 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 will increase rapidly in these muscles as compared with those of 

 the other leg, that have been contracting meantime and using up their glycogen. 

 Formation of Urea in the Liver. The nitrogen contained in the proteid 

 material of our food is finally eliminated, after the metabolism of the proteid 

 is completed, mainly in the form of urea. As will be explained in another 

 part of this section, it has been definitively proved that the urea is not formed in 

 the kidneys, the organs which 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 now gone so far as to demonstrate that it arises 

 chiefly in the liver, hence the property of forming urea must be added to the 

 other important functions of the liver-cell. Schroder l performed a number of 

 experiments in which the liver was taken from a freshly-killed dog and irri- 

 gated through its blood-vessels by 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 accompanied 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 which the liver can convert to urea. It is to be 

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

 blood from the well-fed animals showed no increase in urea after being circu- 

 lated through an isolated kidney or muscle. As further proof of the urea- 

 forming power of the liver Schroder found that if ammonium carbonate was 

 added to the blood circulating through the liver to that from the fasting as 

 well as from the well-nourished animal a very decided increase in the urea 

 always followed. It follows from the last experiment that the liver-cells are 

 able to convert carbonate of ammonia into urea. The reaction may be ex- 

 pressed by the equation (NH 4 ) 2 CO 3 2H 2 O = CON 2 H 4 . Schondorff 2 in some 

 recent work has shown that if the blood of a fasting dog is irrigated through 

 the hind legs of a well-nourished animal, no increase in urea in the blood can 

 be detected ; but if the blood, after irrigation through the hind legs, is subse- 

 quently passed through the liver, a marked increase in urea results. Obviously, 

 the blood in this experiment derives something from the tissues of the leg 

 which the tissues themselves cannot convert to urea, but which the liver-cells 

 can. Finally, in some remarkable experiments upon dogs made by four in- 

 vestigators (Hahn, Massen, Nencki, and Pawlow), which will be described 



1 Archivfiir experimentelle Pathohgie und Phaimakologie, vols. xv. and xix., 1882 and 1885. 



2 Pfliiger's Archivfur die gesammte Physiologie, 1893, vol. liv. p. 420. 



