TJKINAKY APPAKATTJS. 817 



grooved, but imperforate penis, even where that organ was most fully 

 developed, was sufficient for the purpose of impregnation. 



(2409.) Widely different, however, is the arrangement of the male 

 genito-urinary system in the class we are now considering. The cloacal 

 cavity is no longer met with, the terminations of the rectum and of the 

 sexual ducts being now remotely separated ; the penis is traversed by a 

 complete urethral canal, through which the seminal fluid is forcibly 

 ejaculated ; and moreover, subsidiary glands not met with in any of 

 the preceding classes add their secretions to that of the testes, and thus 

 facilitate the intromission of the fecundating fluid. A urinary bladder 

 is now superadded to the renal apparatus, wherein the urine is per- 

 mitted to accumulate in considerable quantities, prior to its expulsion 

 through the urethra the excretory duct common to both the urinary 

 and generative organs. 



(2410.) Not less remarkable are the corresponding changes observable 

 in the disposition of the female reproductive organs. The Mammifers 

 are appointed to bring forth living young; a uterine receptacle is 

 therefore necessarily provided for the reception of the foetus, and mam- 

 mary glands are given to support the tender offspring during the earlier 

 portion of its existence. But the history of these organs cannot be laid 

 before the reader at a glance, and we must therefore patiently trace out 

 their development step by step, and gradually ascend from the Ovipa- 

 rous type up to the most complete forms of the genito-urinary system. 



(2411.) Commencing with the urinary apparatus, the first parts that 

 offer themselves to our notice are the kidneys, the ureters, and the 

 bladder, in describing which the same remarks will be found applicable 

 to both sexes. 



(2412.) The kidneys in all the Mammiferous orders occupy a similar 

 position, being situated in the loins, on each side of the aorta, from 

 whence they receive a copious supply of arterial blood by the renal 

 arteries, which, after having supplied the urinary secretion, is returned 

 to the circulation by the emulgent veins that empty themselves into the 

 inferior cava. 



(2413.) As relates to their intimate structure, the kidneys of all 

 quadrupeds are essentially similar to those of our own species, each of 

 these organs being composed of uriniferous tubules of extreme tenuity 

 that terminate in central papillae from which the urine flows. These 

 tubules, as they advance into the medullary substance of the kidney, 

 bifurcate again and again, until they arrive at the cortical or external 

 portion, where they spread out on all sides, and, becoming exceedingly 

 flexuous, are inextricably intervolved among each other, so that the 

 entire cortex is composed of their gyrations. At last all the uriniferous 

 vessels terminate in blind extremities, and, according to Muller*, have 

 no immediate communication with the vascular system. 

 * De Glandularum Structure,, p. 102. 



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