viii THE EXCRETION OF URINE 455 



organ of secretion, we need only examine whether the epithelial 

 cells of the convoluted tubules, which undoubtedly represent the 

 secretory part of the uriniferous tubules, undergo changes which 

 can be detected in the microscope in consequence of functional 

 activity, as compared with their cytological structure in the state 

 of functional rest. Owing to the continuous flow of urinary secre- 

 tion, the kidneys and their constituent cells can never, normally, 

 be in a state of absolute rest. Yet they are capable of being, and 

 often are, in a state of relative rest, under all physiological condi- 

 tions in which secretion is more or less reduced, e.g. in a protracted 

 absolute fast, and especially during hibernation, which for winter- 

 sleeping animals undoubtedly represents a periodically recurrent 

 physiological state. 



Many physiologists and histologists have taken up this subject 

 (Nussbaum, Gibbes, Kruse, Lorenz, Tornier, and others) ; here we 

 can only refer to the more recent work of Sauer (1895), Trambusti 

 (1898), and R. and A. Monti (1900). 



Sauer, in Heidenhain's laboratory, minutely described the 

 histological structure of the epithelium in the post - glomerular, 

 convoluted tubules of many mammals under various conditions, 

 and decided that the cells of these tubules are constantly provided 

 with the " brush-border " (orlo a spazzola), so that those authors 

 are right who regard this structure as a constant morphological 

 character, and those others wrong who regard it as an expression 

 of functional activity in the cells which exhibit it. According to 

 Sauer the resting tubules only differ from those which are in 

 secretory activity in having a narrower lumen. 



According to Trambusti, on the contrary, this brush-border of 

 the cells of the post-glomerular tubules is not an integral part of 

 those cells. It is absent in rest, and is a temporary expression of 

 the function of the secreting cells. Below the "brush-border" there 

 is, according to Trambusti, another small striated border, which 

 he considers a constant character of the cell, and to which he 

 ascribes the greatest importance in the elimination of the secretory 

 products. 



As between these two opposite conclusions, the results of 

 R. and A. Monti seem to us decisive, owing to their lucidity and 

 the strictly comparable physiological conditions under which the 

 experiments were carried out. They compared the renal tubules 

 of the marmot after prolonged hibernation (October to the end of 

 February), when the secreting function of the kidney must be 

 reduced to a minimum, with those of marmots that are awake and 

 fully nourished with milk. The difference in appearance of the 

 lumen and epithelial cells of these tubules is shown in Figs. 123 

 and 124. The uriniferous tubules of the waking marmot are 

 always more dilated than those of the hibernating animal, in 

 which the walls almost touch each other, and the lumen is nearly 



