SECRETION OF THE URINE. 399 



cation, appears plausible, or at least worthy of mention. Paying 

 attention to the fact that where vascular filtration i. e., the pas- 

 sage of liquid under pressure through the capillary wall occurs 

 elsewhere in the body it is not only water and salts, but plasma 

 that passes out of the vessels into the interstices of the tissues, we 

 may then assume that the fluid part of the blood, as such, and 

 not merely its watery part, escapes at the glomerulus. That is 

 to say, the solid ingredients of the urine in a diluted form, plus 

 serum albumin, pass into the tubules. But on its way down the 

 long and circuitous route through the tubules the albumin with 

 much water is reabsorbed by the capillaries of the convoluted 

 tubes. The first step in this case is a mechanical filtration ; the 

 second is a vital process of reabsorption of a solution of serum 

 albumin carried on by the gland cells in the tubules, aided by 

 the low pressure in the peritubular capillary plexus. This view 

 seems supported by pathological experience, which teaches that 

 the removal of the epithelium of the tubes (the glomeruli remain- 

 ing perfect), is followed by the appearance of albumin in the 

 urine, and cysts formed by the destruction of the epithelium and 

 occlusion of the tubules commonly contain a fluid somewhat like 

 plasma. 



Doubtless much remains to be found out as to the exact method 

 of secretion of the urine, and possibly future research may show 

 us that all the views here enumerated have some truth in them. 

 That a filtration, not mere osmosis, takes place, is made certain 

 by the special vascular mechanism of the glomerulus. Why 

 simply water and salts without albumin should pass through 

 the capillaries of the glomerulus, and not through any other 

 capillaries, is not sufficiently explained to make it sure that this 

 filtration differs from others. That the glandular epithelium does 

 take an active part in the elimination of the urea is rendered 

 almost indisputable from the researches of Heidenhain. And 

 yet there remain other parts, e. g., the loops of Henle, which are 

 constantly found in the kidney, and have a special vascular 

 mechanism and to which none of the foregoing theories assign 

 any special or peculiar function. 



From the foregoing evidence we may fairly suppose that most 



