524 



ORGAN SYSTEMS OF MAN 



and the genital system are thus distinctly * 

 separated. Perhaps because this system of 

 getting rid of the germ cells was so fortui- 

 tous, it became necessary to provide tubes 

 for this purpose. By chance, or perhaps by 

 proximity, the urinary ducts took over that 

 task, and an intimate relationship became 

 established between the excretory and re- 

 productive systems. This was done very 

 gradually through several groups of ani- 

 mals over a period of many millions of 

 years. 



Among the fishes and amphibia, the 

 testes became connected with the upper 

 end of the kidney by means of tiny tubules, 

 the vasa eflFerentia. Sperms then made their 

 way out of the body through the same tubes 

 as the urine, namely, the urogenital duct 

 ( Fig. 20-2 ) . New ducts, the oviducts, were 

 formed for conveying eggs out of the body. 

 The lower ends of these ducts became large 

 and sac-like to form the uteri (singular— 

 uterus) in order to accommodate the great 

 numbers of eggs that accumulated before 

 deposition. The uteri, too, opened into the 

 cloaca. This seemed to be a very satisfactory 

 arrangement, and not until mammalian evo- 

 lution was well underway did further radi- 

 cal changes take place. 



In a mammal, such as a cat, we find that 

 the kidneys have become "kidney-shaped" 

 and much more compact than the long thin 

 organs of the fishes or even the thicker 

 structures of the amphibians. As in the 

 cyclostomes, the tubes which convey urine 

 away from the kidneys have no affiliation 

 with the genital system. Indeed, these 

 ducts, the ureters, are new tubes which 

 formed very late in evolution. They connect 

 with a bladder (urinary), thence through 

 a tube, the urethra, to the outside. The old 

 urogenital ducts of the frog have lost their 

 urinary function and have been taken over 

 completely by the genital system. Their sole 

 function in the mammal is to carry sperm 

 cells. These tubes, the vasa deferentia, con- 

 nect with the urethra, which is urogenital 

 throughout the rest of its course to the out- 



side of the body. The terminal portion is 

 modified into a copulatory organ, the penis. 



The path of the eggs in mammals is not 

 greatly modified from that of the frog. They 

 pass into the oviducts from the ovaries and 

 then into the uteri, which may be paired as 

 in the cat or fused as in man. The eggs of 

 mammals are much smaller, of course, but 

 they follow essentially the same path as in 

 the amphibians and fishes. A new struc- 

 ture, the vagina, has been added which re- 

 ceives the penis of the male in sperm trans- 

 fer, an essential for land animals. 



The embiyological development of the 

 urogenital system of mammals follows 

 basically the same course as its evolution. 

 That is to say, at one time the kidney re- 

 sembles that of a cyclostome, and a little 

 later, that of a frog. Finally, some time be- 

 fore birth, the true mammalian kidney and 

 associated organs are formed. It must be 

 remembered that although the excretory 

 and reproductive systems are anatomically 

 intimately related, they bear no relationship 

 to one another functionally. The job of re- 

 production and excretion are two separate 

 and distinct functions. 



The human kidney 



The two kidneys in man are about 4 

 inches long and are located near the mid- 

 dorsal line just below the stomach. The 

 ureter and blood vessels emerge from a 

 depression on the medial side. The kidneys 

 lie in a capsule of peritoneum which ex- 

 cludes them from the coelom. If a kidney 

 is sliced lengthwise, it will be seen to con- 

 sist of an outside layer, the cortex, and an 

 inner capsule, the medulla (Fig. 20-3), 

 both of which are visible to the naked eye. 

 At the point where the ureter leaves, there 

 is a large cavity, the pelvis ( not to be con- 

 fused with the pelvis of the skeleton), 

 which is a depository for the urine as it 

 comes from the millions of tiny tubules of 

 the kidney. The entire internal kidney is 

 tied together with connective tissue and 

 interlaced with blood vessels, and it is a 



