1072 EXCRETION 



this process as outlined by Ludwig and more recently by Cushny,i 

 also takes place under normal conditions. In general, it may be said 

 that this point has not been satisfactorily proven; at least, the evi- 

 dence so far presented does not point toward a reabsorption of suffi- 

 cient magnitude to account for the complete concentration of the 

 freshly formed watery urine. 



The factor of reabsorption has been emphasized in more recent 

 years by Brodie and Callis,^ and especially by Cushny. Possibly the 

 strongest point against this contention is that the amount of water 

 which would have to be reabsorbed from the uriniferous tubules, ap- 

 proximates the enormous value of 70 liters per day, but Cushny believes 

 that this is not a convincing criticism, inasmuch as the secretion 

 for each tubule would even then be only about 0.014 c.c. in the course 

 of one hour. Ribbert^ has approached this problem by removing 

 as extensive a portion of the tubules as possible, the contention being 

 that if reabsorption actually takes place, a much more fluid urine should 

 then be obtained. While this was actually the case, these results and 

 their interpretation in favor of the absorption theory have been ad- 

 versely criticized by Boyd^ and H. Meyer. ^ The latter in particular 

 lays stress upon the fact that the character of the urine after partial 

 removal of the medullary substance more closely approaches that of 

 an albumin-free filtrate. Furthermore, Gurwitsch^ has pointed out 

 that the ligation of the renal portal system in frogs diminishes the 

 quantity of the urine, as compared with that secreted by the normal 

 organ on the opposite side. Consequently, if the tubules actually did 

 absorb a large portion of the water of the newly formed urine, the 

 abolition of their function should really give rise to a more copious 

 and watery urine. As has just been stated, this is not the case. In 

 addition, it must, of course, be evident that a process of secretion 

 invariably necessitates two solutions, namely, the blood and the secre- 

 tory product separated by an animal membrane, and Magnus, Soll- 

 mann and others have shown repeatedly that any interchange between 

 these cannot be effected without the participation of the dissolved 

 substances. For this reason, a slight reabsorption may be essential 

 at times to equalize osmotic conditions, but not at all for the singular 

 purpose of removing only the water. 



"Modem" Theory of the Secretion of Urine. — The so-called 

 "modern" theory of Cushny embodies the principles of urinary secre- 

 tion as outlined by Ludwig, and in addition, a reabsorption of the water 

 and inorganic constituents of the newly formed urine. The latter 

 is effected by a vital activity on the part of the epithelium of the tub- 



1 The Secretion of Urine, London, 1917; also: Addis and Sheoky, Am. Jour, of 

 Physiol., xliii, 1917, 363. 



2 Jour, of Physiol., xxxiv, 1906, 224. 

 5 Virchow's Archiv, xciii, 1883, 169. 

 * Jour, of Physiol., xxviii, 1902, 76. 



^ Marb. Sitzungsber., 1902. 



« Pfliiger's Archiv, xci, 1902, 71. 



