AND THEIR FUNCTIONS. 165 



the kidneys merely assisted the bladder and might even be 

 absent.* 



He gives some interesting information about both the 

 kidneys and the bladder in various animals. Eeferring 

 chiefly to the human kidneys, he shows clearly that he was 

 aware that each kidney had a hollow part and a compact 

 vascular part, that blood-vessels, now known as the renal 

 artery and renal vein, extend between the vascular part and 

 the aorta and great blood-vessel, respectively, and that a 

 strong tube, the ureter, passes from the hollow part of each 

 kidney to the bladder, t 



The human kidneys, Aristotle says, are similar to those 

 of an ox, being made up, as it were, of many kidneys and 

 not being compact bodies, like the kidneys of sheep or some 

 other animals.! The kidneys of the human foetus and, 

 occasionally, those of the adult are lobulated, but they are 

 not like the kidneys of an ox. In P. A. iii. c. 9, 6716, 

 he says, substantially correctly, that the kidneys of a seal 

 are like those of an ox. 



He believed that all oviparous animals, except some 

 tortoises, were without kidneys or bladder, although he 

 speaks of some birds having certain flat, kidney-like bodies. 

 The Emys, by which he seems to have meant the pond- 

 tortoise, had neither kidneys nor bladder, because fluid 

 could easily escape through the soft skin of the animal. Si 

 The marine tortoise, he says, is the only oviparous quadruped 

 which has its kidneys and bladder proportional in size to the 

 other parts of the animal, and its kidneys are, he adds, like 

 those of oxen; 11 the land tortoises have a very small bladder, 

 he says, and the marine tortoises, a large one.** 



Kidneys are present in birds, reptiles, amphibians, and 

 fishes, but a true urinary bladder is found in mammals only. 

 It is evident that Aristotle believed that tortoises were the 

 only oviparous animals in which a bladder could be said to 

 exist, and that in land tortoises it was very small. It is 

 well-known that a so-called urinary bladder is present in 

 many other oviparous animals, and that the bladder is 

 usually much larger in the land tortoises than in the marine 



* P. A. iii. c. 7, 670&. 

 f H. A. i. c. 14, s. 7 ; P. A. iii. c. 9, 6716. 

 I H. A. i. c. 14, P. 7 ; P. A. iii. c. 9, 6716. 

 H. A. ii. c. 12, s. 1 ; P. A. iii. c. 9, 671a. 

 I! P. A. iii. c. 9, 671a. H H. A. ii. c. 12, s. 1. 



** P. A. iii. c. 8, 67 



