THE ELIMINATION OF METABOLIC PRODUCTS. 583 



these experiments also are capable of more than one interpretation. 

 Dreser 1 attempted to find out at which place acid-fuchsin was elimi- 

 nated. He injected daily three or four cubic centimeters of acid-fuchsin 

 into the dorsal lymph-sac of frogs. An hour or so afterwards there was 

 no pigment present in the glomeruli nor in the convoluted tube, but only 

 in the lumen of the central part of the tube. If the injection were 

 repeated, the glomeruli remained colorless; but in the convoluted tubes, 

 the red coloration gradually extended into the end of the epithelium 

 which is toward the lumen. Experiments with alizarin-carmine showed 

 that the coloration did not appear in the Tubuli contorti, but only in the 

 distal parts of the tubes. Dresser concluded from these experiments and 

 those with other dyes, that in the convoluted tubes secretion alone takes 

 place, and no reabsorption. We see, therefore, that the decision as to 

 the absorption capacity of the epithelium of the convoluted tubes is entirely 

 dependent upon the interpretation of these microscopical pictures. As 

 long as this problem cannot be decided positively, it is quite impos- 

 sible to establish the function of the different parts of the uriniferous 

 tubes. The same microscopical appearance may be regarded as resulting 

 from an absorption or from an elimination. The entire question as to 

 the special functions of the anatomically different parts of the uriniferous 

 tubes remains absolutely unsettled by the above experiments. 



Let us now see whether our knowledge of the functions of the kidneys 

 is sufficient to give us a clear idea of the formation of urine. In the first 

 place we must state that it has never been found possible to detect posi- 

 tively the presence of secretory nerves in the epithelium of the kidneys. 

 All the nervous influences which have shown an effect upon the elimina- 

 tion of urine can be either directly or indirectly attributed to a change 

 in the innervation of the blood-vessels. The amount of the blood circu- 

 lating is closely related to the amount of urine secreted. This relation is 

 almost self-evident, for the greater the amount of blood passing through the 

 kidneys, the greater will be the opportunity for the cells of the kidney 

 to form their secretion. The blood-pressure also is to be considered. 

 The whole arrangement of the glomeruli is such that the blood must pass 

 the Malpighian body with a relatively high pressure. In this respect the 

 above-mentioned behavior of the efferent vessels is important, which are 

 considerably narrower than the afferents. Ludwig 2 makes use of this 

 fact for the foundation of his much-discussed theory of urine-elimination. 

 He attempted to make his explanation as mechanical as possible, and 

 assumed first of all that there was a filtration through the glomeruli at 



1 H. Dreser: Z. Biol. 21, 41 (1885). Cf. P. Griitzner: Pfliiger's Arch. 24, 441 

 (1881). M. Nussbaum: ibid. 16, 139 (1878); 17, 580 (1879). 



2 Ludwig: Wagner's Handworterbuch der Physiol. 2, 629 (1844); Wiener med. 

 Wochenschr. 14, No. 13, 14 (1864) ; Sitzber. Kais. Akad. Wissensch. 48 (1863). 



