EXCRETION. 



141 



Instead of a solid ball held by one string, there should be a yarn ball 

 with two large strings attached to one side, one representing the artery, 

 the other the vein ; the yarn ball represents the dense cluster of capil- 

 laries. 



Another Illustration. A still better illustration of the urinary tube 

 and capsule may be made thus : Take a thistle tube (used in the chem- 

 ical laboratory), let down into its bulb a rubber balloon or bag of sheet 

 rubber or cloth, fastening the margin around the, rim of the bulb ; put 

 a little ball of red yarn in the depression of the bag hanging in the 

 bulb ; have two ends of the yarn projecting to represent the artery 

 entering and the vein leaving the capsule. The vein, soon after it 

 emerges, breaks up into another set of capillaries which extend around 

 the tube. A number of these primary tubes unite, and many of the 

 common ducts open at the apex of each of the urinary pyramids, 

 emptying their secretion into the cavity of the kidney. As the blood 

 flows through the tuft of capillaries in the capsule at the end of the 

 tube, a large amount of water, 

 together with salt and some 

 other substances, pass through 

 the thin partition into the 

 cavity of the capsule, and 

 thence down the tube. The 

 walls of the tube are thicker 

 than, and its cells are different 

 from, those of the capsule. 

 These cells take the urea and 

 some other substances 

 from the blood, and 

 pass them into the tube 

 to join the more watery 

 material from the capsule. 



Artery 



Vein 



Urinary Tube 

 Fig. 53. Urinary Cone Enlarged. (Diagram.) 



Comparison of the Skin 

 and the Kidneys. The 



kidneys, then, are not very 



different from the skin. 



Imagine a piece of skin rolled up with the outer surface of 



the skin turned inward. Its glands then would pour their 



