The bladder is situated in the pelvic cavity or in the lowest part of the abdomen. When full, the bladder is pear-shaped; when empty, it is collapsed and lies low in the pelvis. The functions of the bladder are to collect and retain the urine, which has reached it drop by drop from the kidneys through the ureters, until a certain quantity accumulates, and then to expel it from the body. Illustration: Fig. 109.—Vertical Section of the Back. (Showing kidneys _in situ_ and the relative position of adjacent organs and vessels.) [Posterior view.] A, 12th dorsal vertebra; B, diaphragm; C, receptaculum chyli; D, small intestines In the kidneys, as elsewhere, the vaso-motor nerves are distributed to the walls of the blood-vessels, and modify the quantity and the pressure of blood in these organs. Thus, some strong emotion, like fear or undue anxiety, increases the blood-pressure, drives more blood to the kidneys, and causes a larger flow of watery secretion. When the atmosphere is hot, there is a relaxation of the vessels of the skin, with a more than ordinary flow of blood, which is thus withdrawn from the deeper organs. The blood-pressure in the kidneys is not only diminished, but the total quantity passing through them in a given time is much lessened. As a result, the secretion of the kidneys is scanty, but it contains an unusual percentage of solids. When the atmosphere is cold, the reverse is true. The cutaneous vessels contract, the blood is driven to the deeper organs with increased pressure, and there is a less amount of sweat, but an increased renal secretion, containing a smaller proportion of solids. Certain drugs have the power of increasing or diminishing the renal secretion. As the waste matters eliminated by the kidneys are being constantly produced in the tissues, the action of the renal organs is continuous, in marked contrast with the intermittent flow of most of the secretions proper, as distinguished from the excretions.