anatomy of renal tissue; from 1862 onwards, he described in a series of publications the presence of personal thoughts about this controversy: tubular loops running perpendicular to the kidney surface, and penetrating at a variable depth in the ‘We have not to pay attention to those theoretical views which medulla. The descending portion of these loops had a consider urine formation as a vital process: they do not oVer small outer diameter (thin limb) as compared to that any hope for critical analysis; but we shall examine the of the ascending portion located in the outer medulla relevance of mechanical theories of circulation; they are prob(thick limb). Figure 1 reproduces illustrations pub- ably erroneous theories, since they neglect one aspect of the lished by Henle in 1866 [1]: note the remarkable question, but they are clearly formulated and easy to understand’.