vni THE EXCEETION OF UEINE 431 



Euneberg. Some months later Heidenhain attacked the same 

 problem iu Germany, and criticised the mechanical theory in a 

 communication to the medical section of the Breslau Gesellschaft 

 flir vaterlandische Kultur (December 1879). 



But it must not be concluded on the strength of this fact that 

 Ludwig's theory, by which the Malpighian tufts are regarded as 

 a minute apparatus intended to permit the filtering through of 

 a highly aqueous urine, is to be entirely rejected. If the walls 

 of the glonaerular vessels are really permeable, and if the intra- 

 glomerular pressure P is much higher than the pressure in the 

 capsule, or commencement of the uriniferous tubules p, there 

 must be filtration, and the quantity of urine filtered must vary as 

 P-p. If this be allowed, it follows that if the value p is altered by 

 inserting a cannula connected with a manometer into the central 

 canal of the ureter, the flow of urine must cease after a certain 

 time, because the ratio P-p = 0. In fact, when this operation is 

 performed on the dog, the mercury column rises slowly till it 

 reaches 50-60 mm., and then remains stationary. At the same 

 time the renal pelvis dilates, the kidney becomes oedematous, and 

 after some hours the capsule of the kidney is seen to be ecchymotic 

 (Ludwig). This result is not, however, decisive in favour of the 

 theory of glomerular filtration, because it lends itself to a twofold 

 interpretation. It can be held with Ludwig, that when the 

 column of mercury comes to rest all formation of urine ceases, 

 and it may also be maintained with Heidenhain that when the 

 rise of the manometer is arrested the secretion of urine continues, 

 but that reabsorption from the tubules into the lymph channels 

 commences. 



Another much simpler point, on which Ludwig laid great 

 stress, is that the efferent vessel of the glomerulus is always much 

 smaller than the afferent vessel. This constant fact must be 

 taken as the natural consequence of the heavy loss of water from 

 the blood during its passage through the glomerular vessels, owing 

 to the resistance there encountered, so that the quantity of water 

 that enters by the afferent vessels is larger than that which leaves 

 by the efferent. In any case, however, even on the assumption 

 that the glomerulus is a filter, it remains true that it is a filter 

 composed of highly sensitive living cells, and that its permeability 

 varies greatly with the alterations in the vital conditions of these 

 cells, as shewn by the striking effects of the temporary occlusion 

 of the renal artery. It is this physiological sensibility of the 

 glomerular walls that makes the rate of blood- flow more important 

 than the lateral pressure in the formation of urine. 



In other words, the same theory applies to the function of the 

 vessels of the Malpighian glomerulus as was formulated above for 

 the formation of lymph from the common blood capillaries. (See 

 Vol. I. p. 519 et seq.) The only difference is that while in that 



