738 PHYSIOLOGY OF DIGESTION AND SECRETION. 



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 can not 

 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 ex- 

 cretion 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. 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 body without 

 serious injury to the animal. An increase in the size of the lymph- 

 glands and of the bone-marrow has been stated to occur after ex- 

 tirpation; but this is denied by others, and, whether true or not, it 

 gives but little clue to the normal functions of the spleen. Some 

 observers* find that the removal of the spleen causes a marked 

 diminution in the number of red corpuscles and the quantity of 

 hemoglobin. They infer, therefore, that the spleen is normally 

 concerned in some way in the formation of red corpuscles. Others, 

 however, report with equal positiveness that removal of the spleen 

 has no effect upon the number of red corpuscles or upon the power of 

 the animal to regenerate its corpuscles after hemorrhage^ The 

 most definite facts known about the spleen are in connection with its 

 movements. It has been shown that there is a slow expansion and 

 contraction 'of the organ synchronous with the digestion periods. 

 After a meal the spleen begins to increase in size, reaching a maximum 

 at about the fifth hour, and then slowly returns to its previous size. 

 This movement, the meaning of which is not known, is probably due 

 to a slow vasodilatation, together, perhaps, with a relaxation of the 

 tonic contraction of the musculature of the trabecular. In addition 



*Laudenbach, " Centralblatt fur Physiologie, " 9, 1, 1895. 



t Paton, Gulland, and Fowler, " Journal of Physiology, " 28, 83, 1902. 



