THE URINARY ORGANS. 



THESE organs include the kidneys, the glands which secrete the urine, 

 the ureters, the canals which collect the urine and convey it from the kidneys 

 to the bladder, the receptacle in which the urine is temporarily stored, and 

 the urethra, the passage through which the urine is discharged. 



THE KIDNEYS. 



The kidneys are two flattened ovoid glands, of peculiar bean-shaped 

 form, deeply placed within the abdominal cavity against its posterior walls, 

 one on each side of the lumbar spine. They are invested by a thin fibrous 

 capsule, which is distinct from the renal tissue that it covers and is exposed only 

 after its removal. The mesial border of each kidney is interrupted by a slit- 

 like opening, the hilum, which leads into a more extended but flattened 

 space, the sinus, enclosed by the surrounding substance of the kidney. In 

 addition to the blood-vessels, lymphatics, and nerves passing to and from 

 the kidney through the hilum, the sinus contains the upper expanded end 

 of the ureter, which also emerges at the hilum. The interspaces between 

 these structures are filled with loose fatty areolar tissue. 



Architecture of the Kidney. Before describing its histological 

 details, it will be of advantage to consider the general plan upon which the 

 kidney is built its architecture as contrasted with its structure. The entire 

 organ a conspicuous example of a compound tubular gland is made up of 

 a number of divisions which in the mature condition are 

 so closely blended as to give little evidence of the striking 

 subdivision or lobulation of the foetal kidney. The exter- 

 nal surface of the latter ('Fig. 253) is broken up by 

 furrows into a number of polygonal areas, each of which 

 represents the base of a pyramidal mass of renal sub- 

 stance, the kidney lobe, separated from its neighbors 

 by connective tissue. It includes the entire thickness 

 of the organ, between the exterior and the sinus, and 

 ends internally in a conical apical projection, the renal 

 papilla. Shortly after birth, the lobulation gradually 

 disappears on the surface, which becomes smooth, the ing areas on surface 



11 . - vi-' *.\, iM corresponding to pn- 



mterlobar connective tissue septa within the organ likewise mar y lobes, 

 disappearing, while the papillae alone remain as indications 

 of the original subdivisions. Although the outlines of the lobes occasionally 

 persist on the surface of the adult human kidney, in many of the lower 

 animals (reptiles, birds, ruminants, cetaceans and certain carnivora) the 

 subdivisions are normally retained. In some mammals (rodents and insec- 

 tivora) the entire kidney corresponds to a single papilla, while in others 

 (elephant and horse) no distinct papillae exist. 



On examining the cut surface of the kidney, opened by a longitudinal 

 section passing from the convex border through the sinus (Fig. 254), the 

 papillae are seen to form the free apices of conical areas, the renal pyramids, 

 whose bases lie embedded within the surrounding kidney-substance compos- 

 ing the outer third of the organ. This peripheral zone, which in the fresh 

 kidney appears darker and granular when compared with the lighter and 



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