THE URO-GENITAL SYSTEM. 



THE uro-genital system comprises two groups of organs, the urinary and the 

 generative ; the former serves for the elaboration and removal of the chief excretory 

 fluid, the urine, and the latter provides for the formation and liberation of the prod- 

 ucts of the sexual glands. The primary relations between these sets of organs, as 

 seen in the lowest vertebrates, are so intimate that the excretory duct of the primitive 

 kidney may also transmit the sexual cells, both groups of organs being inseparably 

 united. In the higher vertebrates the primary relations are suggested by only tem- 

 porary conditions in the embryo, since with the development of a definite kidney 

 differentiation and separation take place until the urinary and generative organs con- 

 stitute independent apparatuses except at their terminal segment, where they are 

 more or less blended in the external organs of generation. After serving for a time 

 as the functionating excretory organ of the foetus, parts of the Wolffian body and its 

 duct become transformed into the ducts of the male sexual gland. In the female 

 analogous canals, represented by the oviducts, uterus, and vagina, are not derived 

 from the Wolffian duct, but from an additional tube, the Miillerian duct, which, how- 

 ever, is closely related to the primary canal of the fcetal excretory organ. 



THE URINARY ORGANS. 



These include the kidneys, the glands which secrete the urine, the iireters, the 

 canals which receive 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 (renes) are two flattened ovoid glands of peculiar form, described 

 as bean-shaped, deeply placed within the abdominal cavity against its posterior wall 

 and the diaphragm, one on either side of the lumbar spine. They are invested in a 

 distinct, although thin, smooth, fibrous caps^lle (tunica fibrosa) and lie behind the 

 peritoneum, surrounded by loose areolar tissue, which usually contains considerable 

 fat (tunica adiposa). This fat is particularly conspicuous along the convex lateral 

 margin and about the lower pole of the kidney and is least abundant around the 

 upper end and over the anterior surface. The fresh adult organ, of a brownish-red 

 color, weighs about 130 gm. (4^2 oz. ) in the male, slightly less in the female, and 

 measures about 11.5 cm. (4/^2 in.) in length, 6 cm. (2^ in.) in width, and 3.5 cm. 

 (i^ in.) in thickness. The left kidney is usually somewhat longer, narrower, and 

 thicker, and slightly heavier than the right. Individual variations, especially as to 

 length, are responsible in some cases for organs unusually long (15 cm.), in others 

 for those relatively short. 



Each kidney presents two surfaces, a convex anterior or visceral, when the 

 organ is in place directed forward and outward, and a posterior or parietal, some- 

 what flattened and looking backward and inward ; two rounded ends, or poles, of 

 which the upper is usually the blunter and bulkier ; and two margins, the external, 

 marking the convex lateral outline of the organ, and the straighter internal. The 

 latter is interrupted by a slit-like opening, the hilum (hilus renalis), bounded by 

 rounded edges, which leads into a more extended but narrow space, the sinus (sinus 

 renalis), enclosed by the surrounding renal tissue. The capsule is continued from 

 the exterior of the kidney through the hilum into the sinus, which it partly lines. 

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

 ney through the hilum, the sinus contains the expanded upper end of the renal duct 



1869 



