2. _Secretion_ is referred to the columnar epithelium of the convoluted tubes. It is by the elective affinity or selective power of this epithelium that the solids of the urine are abstracted from the blood and passed into the urine. Crystals of uric acid have been found in these cells and it is supposed that the abundance of water furnished by the glomeruli, irrigating these convoluted tubes, dissolves and washes on the various solids and other products with which the epithelial cells are charged. The protoplasm of the cells becomes saturated with the urea, uric acid, hippuric acid coloring matter (indican, urochrome, etc.), and this is washed out, passing by exosmosis to the liquid of lesser density with which the tubes are filled. Nervous Control of Urinary Secretion. An electric current through the renal plexus of the sympathetic (vaso-motor) lessens, or suppresses urinary secretion (inhibition). Cutting the nerves of this plexus causes excessive vaso-dilation, renal pulsations synchronous with heart beats and arterial pulse, and great increase of urine. A similar increase comes from the application of cold to the surface, from fatigue, from heat exhaustion, from irritation of the floor of the fourth ventricle just in front of the origin of the vagus and from section of the splanchnic nerve. This last is, however, much less marked and more transient than from section of the renal nerve noted above; the latter causing dilation of the renal vessels only, and increased pressure, whereas the former causes dilation of the abdominal organs generally, diverting the blood largely to other parts than the kidney and preventing the same increase of pressure in the vessels of the latter. For the same reason transverse section of the medulla oblongata, or of the spinal cord as far back as the seventh cervical vertebra, lessens or interrupts the urinary secretion, the pressure in the kidney being reduced by the diversion of much of the blood elsewhere. This influence of the nervous system on the urinary secretion seems to be mainly or entirely one of increase or decrease of blood pressure in the kidney. For this reason a weak heart tends to lessen urinary secretion. =Excessive increase= of urine is only important when continuous and in the absence of visible cause, such as diuretics. PHYSICAL PROPERTIES OF THE URINE.