826 PHYSIOLOGY OF DIGESTION AND SECRETION. 



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

 the liver, thus indicating that the blood of an animal alter cjgestion 

 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 

 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 was always obtained. 

 It follows from the last experiment that the liver cells are able to 

 convert carbonate of ammonium into urea. The reaction may be 

 -expressed by the equation (NH,),CO,-2H.O = CON.H,. Schon- 

 dorff * in some later work showed 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 subsequently 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 investigators (Hahn, Massen, Nencki, and Pawlow). which are 

 described more fully in the next chapter, it was shown that when the 

 liver is practically destroyed there is a distinct diminution in the 

 urea of the urine. In birds uric acid takes the place of urea as the 

 main nitrogenous excretion of the body, and Minkowski has shown 

 that in them removal of the liver is followed by an important 

 diminution in the amount of uric acid excreted. From experiments 

 such as these it is safe to conclude that urea is formed in the liver 

 and is then given to the blood and excreted by the kidney. In 

 treating of the physiological history of urea an account will be given 

 of the views proposed with regard to the antecedent substance or 

 substances from which the liver produces urea. 



Physiology of the Spleen.— Much has been said and written 

 about the spleen, but we are yet in the dark as to the distinctive 

 function or functions of this organ except so far as the lymph- 

 tissue contained in it is concerned. The few facts that are known 

 may be stated briefly without going into the details of theories that 

 have been offered at one time or another. The older experimenters 

 demonstrated that this organ may be removed from the Ijody with- 

 out serious injury to the animal. An increase in the size of the 

 lymph-gland^ and hyperplasia of the bone-marrow has been stated 

 to occur aftvir extirpation ; but this is denied by others, and, whether 

 true or not, it gives no decisive clue to the normal functions of the 

 .spleen. Observations upon splenectomized dogs indicate that it 

 * Pfluger's "Archiv f. die gesammte Physiologie," 54, 420, 1893. 



