582 LECTURE XXV. 



the tubes. 1 To be sure, there are a number of experiments which do not 

 agree with such an assumption. 2 A reabsorption has been observed 

 only under conditions which cannot be regarded as normal. The frog is 

 a particularly suitable subject for such experiments. In this animal the 

 kidneys receive their blood from two sources, the renal artery, and the 

 renal portal vein. The first provides the Malpighian body with ma- 

 terial, while the latter leads directly to the uriniferous tubule. If the 

 renal artery is ligated, the secretion of urine stops completely. On 

 the other hand, if the flow of blood through the renal portal vein is 

 stopped, then urine continues to be secreted. If the uriniferous tubules 

 have the function of taking up water and solid constituents from the 

 products secreted by Bowman's capsules to give them up to the blood 

 again, then it would be expected that after ligating the blood-vessels 

 supplying the uriniferous tubule, that there would be an increased 

 elimination of urine. This was, however, not the case. On the con- 

 trary, the amount of urine diminished. We must regard this question 

 as unsettled. It is indeed conceivable that an absorption takes place 

 only under certain conditions. We must at this place mention how 

 extremely difficult it is to obtain a clear judgment when there is any 

 meddling with the normal functions of the kidneys. We never know 

 exactly what the primary cause of a phenomenon is, and what takes place 

 only secondarily. Above all, we have to consider the important influence 

 of the blood-supply upon the formation of the urine. It is, for example, 

 perfectly possible that in the above case the diminished secretion of the 

 urine may have been caused by an obstruction in the flow of the blood to 

 the glomeruli. We shall come to this question again when we discuss 

 the question of reabsorption. 



In order to investigate the functions of the separate divisions of the 

 uriniferous tube, substances have been introduced into the circulation 

 which can be easily detected in microscopical preparations. Thus 

 Heidenhain 3 found that after the injection of sodium sulphindigotate 

 into the blood, it reappeared in the epithelial cells of the uriniferous 

 tubules. He concluded from this discovery that these cells have the 

 function of adding certain specific constituents of the urine to the secre- 

 tion which it receives from the capsules of Bowman. This is not neces- 

 sarily true, for the microscopical pictures might equally well have 

 been caused by a reabsorption of the dye from the uriniferous tubule. 

 Carmine has also been used for such experiments, 4 and the results of 



1 H. Ribbert: Virchow's Arch. 93, 169 (1883). W. M. Sobieranski: Arch, exper. 

 Path. Pharm. 35, 144 (1895). 



2 A. Gurwitsch: Pfliiger's Arch. 91, 71 (1902). A. P. Beddard: J. Physiol. 28, 20 

 (1901). 



3 R. Heidenhain: Arch, mikro. Anat. 10, 1 (1874). Cf . Pfliiger's Arch. 9, 1 (1874). 



4 Cf. Adolf Schmidt: Pfliiger's Arch. 48, 34 (1891). 



