THE SECRETION OF URINE 1071 



oxygenated saline solution greatly diminishes the flow of urine. 

 Contrariwise, the perfusion of this organ with oxygenated salt solution 

 increases its quantity. 



Fifthly, attention should be called to the fact that the cells lining 

 the distal urinary tubule, possess all the essentials of secretory cells. 

 Thus, vesicles may be seen to form within their cytoplasm, the con- 

 tents of which are later on discharged into the lumen of the tubule.* 

 Besides, Bowman has observed crystals of uric acid within the cells 

 of the convoluted tubules of birds. Lastly, it is a well-known fact 

 that the secretion of urine may be stimulated by means of various 

 agents to which the name of diuretics has been given, and which in 

 accordance with their stimulating action upon the cells themselves, 

 may be placed in the same class with the lymphagogues, cholagogues 

 and lactagogues. Their action may be tested most advantageously by 

 perfusing the renal portal system of the frog with oxygenated salt 

 solution to which either caffeine, urea, phloridzin, or sodium sulphate 

 has been added. All these agents incite a copious secretion of urine 

 as well as a very striking increase in the oxygen consumption of this 

 organ. Very similar results may be obtained in mammals, but it is 

 to be noted that the urine secreted under the influence of these secreto- 

 gogues, is not at all like the blood plasma in its composition and also 

 varies with the character of the diuretic employed. The vital activity 

 of the renal cells is elucidated further by the fact that the sugar 

 and proteins of the blood are normally retained in the body, whereas 

 peptone and egg albumin, when injected into the circulation, are 

 eliminated almost immediately. Moreover, the kidney possesses 

 the power of abstracting urea from the blood, but does not excrete 

 significant amounts of sugar, and this in spite of the fact that the latter 

 substance is present in much larger quantities than the former. 



Absorption from the Tubules. — While the preceding experiments 

 fully disprove the pure filtration theory of urinary secretion, there is 

 still another point embodied in Lud wig's theory which has given rise 

 to much discussion. Reference is now had to the absorption of water 

 from the urinary tubule to render the urine more concentrated than 

 when first secreted. This reabsorption, it is claimed by Ludwig, is 

 effected through the blood as well as through the lymph. In the first 

 place, it must be admitted that the constituents of the urine may be 

 made to pass in the reverse direction, as can be done by blocking the 

 ureter and allowing the pressure in the tubules to rise well above that 

 prevailing in the renal capillaries. Moreover, when substances, 

 such as potassium iodid, are at this time injected into the pelvis of 

 the kidney, they soon find their way into the blood where they may 

 be recognized chemically. The only question to be decided is whether 



1 Gurwitch, Pfliiger's Archiv, xci, 1902, 71; Courmont and Andre, Jour, de 

 Physiol, et path, gen., vii, 1905, 255; and Hiiber and Konigsberg, Pfliiger's Archiv, 

 cviii, 1905, 323. 



