THE URINARY ORGANS. ! 95 



with the outer layer of the capsule at the stalk of the glomerulus, at 

 which point the vessels and the capsule are intimately united. 



Each uriniferous tubule begins within the labyrinth as the 

 dilated capsule of Bowman. A greatly constricted neck, situated 

 at the pole of the Malpighian body opposite the position of the 

 vascular stalk, leads into the first or proximal convoluted tubule, 

 which is characterized by its considerable size and tortuous course. 

 Leaving the labyrinth, to which it has thus far been confined, the 

 tubule enters the medullary ray and passes towards the medulla as 

 the slightly wavy spiral portion ; on reaching the medulla a marked 

 diminution in the size of the tubule takes place, the reduced tube 

 passing into the medulla as far as the papillary zone as the descend- 

 ing limb of Henle's loop, the narrowest part of the entire urinifer- 

 ous tubule. The spiral tubule is practically the beginning of the 

 descending limb of Henle's loop, and takes the place of this arm in 

 the medullary ray, into the constitution of which, strictly regarded, 

 it does not enter. 



Just before reaching the loop itself the tubule becomes slightly 

 larger, obtaining a diameter which is retained throughout the loop 

 and the ascending limb ; on again reaching the cortex, the ascend- 

 ing limb enters the medullary ray as its second constituent until it 

 once more enters the labyrinth, to become, for a short distance, the 

 conspicuous irregular tubule. The succeeding second or distal 

 convoluted portion resembles very closely the proximal part of 

 like name, possessing a similar size and tortuous course. The uri- 

 niferous tubule finally leaves the labyrinth as the arched collecting 

 tubule, to enter, for the third time, the medullary ray as the straight 

 collecting tube. In consequence of the frequent union of canals of 

 smaller size, the collecting tubes rapidly increase in diameter as they 

 traverse the medulla, until, in the papillary layer, the narrow tubules 

 have become the large excretory ducts, or tubes of Bellini, whose 

 orifices on the free surface of the papillae are recognizable by the 

 unaided eye. A certain number of tubules probably do not form 

 loops of Henle, but pass directly to become the collecting canals 

 (Rose). 



From the foregoing it will be seen that the 



Labyrinth contains 



' Malpighian bodies glomeruli and cap- 

 sules ; 



Constricted necks of tubules ; 



Proximal convoluted tubules ; 



Irregular tubules ; 



Distal convoluted tubules ; 

 ^ Arched collecting tubules. 



CXx^-i 



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