The neck of the uriniferous tubule as it emerges from Bowman’s capsule is short and narrow and is lined with a few cuboidal cells. Toward the glomerular end the cells are of a transitional form gradually merging into the flat squamous type peculiar to Bowman’s capsule. The first convoluted tubule is lined with irregular cuboidal or pyramidal epithelial cells. The descending limb of Henle’s loop is narrow and is lined with a simple layer of flat epithelial cells. In the loop the epithelium changes from the flat type in the descending limb to the cuboidal in the ascending limb. The ascending limb of Henle’s loop again becomes broader and is lined with low cuboidal cells. The second convoluted, or tortuous, tubule is lined with low cuboidal cells as are also the collecting tubules. The collecting tubules originate from every part of the internal substance of the lobules, and extending to the gyrations, uniting in the pinniform structure and traversing to the margin of the lobules, following along the uneven surface, infero-laterally and toward the median body line, finally empty into the ureter. There are many arterial branches given off from the arteria renalis. The arteries which supply the kidneys are given off from the posterior aorta and ischiadic artery. As soon as these arteries enter the kidney they break up into two systems. One system supplies the kidney substance with nourishment in the form of nutrient blood; the second system supplies the glomerules with what may be considered functional blood (Gadow). The second system of arteries branches into the small arteria interlobularis, which pass between the lobules of the kidneys where they give off side branches which penetrate the cortical portion of the lobules and form the capillary plexus, the glomerule. The arteriole that enters into the structure of the glomerule is lined with endothelial cells and is surrounded by a few muscle fibers and a fine network of connective tissue. The vessel that carries the blood away is similarly constructed. _Function._—In the glomerules the liquid portion of the urine is filtered out of the blood, which urine flows through the uriniferous tubules and passes from the kidney through the ureter. In the cubical cells are extracted the solid portions of the urine which also pass through the tubules with the liquid. The urinary secretion, as found in the ureter, does not contain much liquid, but, on the contrary, is made up of a pasty material consisting of salts which are, for the most part, uric acid crystals and sodium urate (Fig. 52, _B_). This material becomes hard, like cement, soon after being exposed to the atmosphere. This secretion may be noted as a whitish pasty material on the outer parts of the feces, or droppings, voided by the birds.