210 SPECIAL HISTOLOGY. 



urine. The hinderance to the flow of the blood gives rise to a 

 considerable lateral pressure in the glomeruli, and a large 

 quantity of blood-plasma is forced through the thin opposing 

 membrane (the walls of the capillaries and the epithelium). 

 Since all the elements of the plasma are not found in the urine, 

 and those that are, present in it totally different proportions to 

 those in which they exist in the blood, it is obvious that the 

 membrane in question does not act simply as a filter, but also, 

 for reasons at present unknown, retains certain substances 

 (protein-compounds, fat), whilst it allows others [urea, &c.) to 

 pass through with peculiar facility. In this way there is 

 formed in the commencements of the tubuli uriniferi, probably 

 a very diluted urine, which afterwards, as it flows towards the 

 pelvis of the kidney, reciprocally acts and is acted upon by the 

 blood circulating around the tubuli uriniferi, receiving addi- 

 tional substances from it (perhaps more urea), but also yielding 

 up certain of its own constituents (water and salts), and thus, 

 at last, becomes true urine. 



As regards the chemical composition of the kidney we know 

 very little. Frerichs (1. c. p. 42) found, in a healthy kidney, 

 16-30 — 18g solid matter, 72 — 73*70 water. Of the former, the 

 fat amounted to 0*63 — 0*lg, although, according to Owen 

 Rees, it may amount to 1'86; the greater portion, however, is 

 probably albumen, with regard to which, Ludwig especially has 

 shown, that it exists in large quantity in the kidney, a fact that, 

 from the micro-chemical characters of the epithelial cells of 

 the tubuli uriniferi, cannot be wondered at. 



[In the higher animals, the secretion of the urine takes place 

 without any formation or dissolution of cells and consequently 

 the normal urine just evacuated contains no morphological 

 elements. It is only occasionally that epithelial cells from the 

 urinary passages, especially from the bladder and urethra, occur 

 in it; but we almost always find mucus from the same 

 localities, forming clouds or a light sediment, occasionally with 

 mucus- corpuscles • and lastly, spermatic filaments after emissions. 

 In inflammations, hemorrhages, exudations, fatty kidney, we 

 find pus-corpuscles, oil-drops, blood-globules, blood, and fibrinous 

 coagula, moulded in the tubuli uriniferi, in the form of cylin- 

 drical casts, and epithelium from the tubuli, isolated or in 



