EXCRETION OF WASTE PRODUCTS 



177 



uniting together to form excretory ducts which all open into 

 the two ureters. The ureters open into the bladder and this 

 communicates with the exterior by a single tube, the urethra. 

 Each tube in the kidney consists of a bulbous extremity, 

 Malpighian corpuscle, and a tube with various convolutions 

 in it. The bulbous extremity consists of an epithelium formed 

 by a single layer of flat cells. The wall has been invaginated 

 so that the Malpighian corpuscle consists of a tuft of blood 

 vessels covered by a layer of pa vem e nt epithelium . Surround- 

 ing this is another layer of pavement epithelium, so that any 

 liquid excreted from the blood 

 is contained between the two 

 layers of flattened epithelium. 



At the side of the Malpighian 

 corpuscle, away from the part f 

 where blood vessels enter, the 

 neck is continued into the 

 convoluted tubules. These 

 tubules are lined by cubical 

 epithelial cells except in one 

 portion, the descending loop 

 of Henle, where flat cells are 

 found. 



In those sea animals in 

 which the internal fluids are 

 of the same composition as the 

 surrounding water there is no 

 need of special excretory mech- 

 anism, but in the others in 



which the internal fluids are T, 



-. ,. rr ... FIG. 40. Diagram of kidney tubule, 



of different composition the A ., , 



,. rr . ... A.Malpighian corpuscle containing glornerulus. 



dilference in composition may B, Convoluted tubuies. 

 be maintained by selective ', c^necting^buie. 

 absorption or selective excre- E Duct of Bartoiini. 



, . (Cofad from "Elementary Physiology "Hu rlev. 



tlOn. Macmillan. 



We have previously con- 

 sidered the equilibrium conditions between cells and their 

 surroundings, and similar relations hold in regard to the 

 maintenance of the composition of internal fluids of multicellu- 

 lar animals. The kidneys possess a marked regulatory 

 function as it is due to their activity that the blood serum of 

 animals is kept of uniform composition. The salts contained 

 in the food vary widely, but the kidney excretes the excess of 

 salts present so that the blood remains of uniform composition. 

 12 



