1184 PHYSIOLOGY 



capillaries of the tubules the blood is collected again into veins, which leave the kidney 

 partly by the cortex and capsular vessels, partly by large venous trunks which join to 

 form the renal vein at the hilum of the kidney. The kidney is richly supplied with 

 nerves, which are chiefly distributed to the muscular walls of its blood vessels. Some 

 authors have described a fine nerve-plexus surrounding the tubules and sending branches 

 between and into the cells of the convoluted tubules themselves. 



The main points in the above description of the structure of the kidney 

 were made out by Bowman in 1840, and suggested the theories of urinary 

 secretion both of Bowman and of Ludwig (1844), theories which have 

 furnished the basis of all our subsequent investigation of the subject. Both 

 observers appreciated the great difference between the membrane covering 

 the glomerular loop and the lining membrane of the tubule, and % both drew 

 attention to the difference in the circulation in these two portions of the 

 kidney. The glomerular capillaries, supplied with blood through a short 

 wide artery and drained by an efferent vessel smaller than the afferent, 

 would represent a region of very high capillary blood pressure, whereas the 

 pressure in the capillaries surrounding the tubules must be low and similar 

 to that in other capillary regions. Bowman therefore suggested that the 

 urine consisted of two parts, namely, one part containing the water and 

 salts produced by a process of nitration through the walls of the glomerular 

 capillaries, and another part, containing the specific urinary constituents, 

 urea, uric acid, etc., secreted by the cells probably of the convoluted tubules. 

 To Ludwig, on the other hand, it seemed possible at first to account for the 

 whole process of formation of urine without the assumption of any active 

 intervention on the part of the cells of the tubules. He imagined that the 

 whole of the urinary constituents passed from the blood to the urinary 

 tubule in the glomerulus by a process of filtration. The glomerular transu- 

 date would represent therefore a very dilute urine containing the crystalloids 

 of the blood in the same concentration as in the blood and with no more 

 urea than the blood itself contained. The great difference in urea content 

 between the blood and the fully formed urine he ascribed to a process of 

 concentration taking place in the fluid in its passage through the tubules, in 

 which water and certain of the salts were reabsorbed, a process of reabsorp- 

 tion conditioned by the difference in protein content between the urine within 

 the tubules and the lymph under low pressure on the outside of the tubules. 

 We know now that in its original form the theory of Ludwig is untenable. 

 If a process of concentration occurs within the tubules, it must involve 

 the performance of work by the cells lining these tubules, and could not take 

 place as a result of mere differences of colloid content between the two fluids. 

 It was shown long ago by Hoppe-Seyler that, if urine be dialysed against 

 serum, there is a passage of water, not from urine to serum, but from 

 serum to urine, i. e. the latter is much more concentrated than the former. 

 The movement of water from one fluid to another through a colloid mem- 

 brane depends on the relative osmotic pressures of the two fluids, and this 

 in turn is determined by the molecular concentration of the two fluids. It 

 is easy to estimate the molecular concentration of any sample of serum or 

 urine. The method which is most convenient is to determine the depression 



