MAMMALIA. 705 



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

 these organs being composed of uriniferous tubules of extreme te- 

 nuity 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, be- 

 coming exceedingly flexuous, are inextricably intervolved among 

 each other, so that the entire cortex is composed of their gyra- 

 tions. At last all the uriniferous vessels terminate in blind ex- 

 tremities, and according to M tiller* have no immediate communi- 

 cation with the vascular system. 



In form the kidneys of Mammals more or less resemble the 

 human ; but there is one important circumstance, observable in many 

 tribes, which is well calculated to show that these organs, even when 

 they appear most simple, are in reality formed by the coalescence 

 of several distinct glands. In the human fetus the kidneys pre- 

 sent a lobulated appearance ; that is to say, they are evidently 

 composed of numerous divisions, each having the same structure : 

 but in the adult the lines of demarcation between these lobes be- 

 come entirely obliterated. In many genera, however, this division 

 into lobes remains permanent during the whole lifetime of the crea- 

 ture ; such, for example, is remarkably the case in amphibious CAR- 

 NIVORA, as the Otter and the Seal tribes, and still more strikingly 

 in the CETACEANS, where the kidneys are not inaptly comparable 

 to large bunches of grapes. But whatever the form of the organ, 

 or the number of lobules entering into its composition, the urine 

 secreted by each kidney is received into a common excretory duct 

 called the ureter, and is thus conveyed into the bladder prepared 

 for its reception. 



The urinary bladder exists in all the Mammalia, and receives 

 the ureters by valvular orifices in precisely the same manner as in 

 the human subject. In the male its excretory duct, the urethra, 

 is common to the urinary and generative systems, and terminates 

 at the extremity of the penis ; but in the female the urethral canal 

 is of much simpler structure, opening by a distinct orifice into the 

 vulva.-)- 



(832.) We have preferred laying before the reader the above gene- 



De Gland. Structura, p. 102. 



t The Lemurs and the Mole form remarkable exceptions, for in these creatures the 

 female urethra traverses the clitoris precisely as in the other sex. 



2 z 



