I50 THE FROG chap. 



products of digestion pass into the blood {Cp. Ent. C), or in 

 the case of fats, first into the lacteals and ultimately into the 

 blood. In this way the due proportion of nitrogenous and 

 other food materials is kept up. 



In passing through the capillaries of the lungs {Cp. Lng) 

 carbon dioxide is exchanged for oxygen and a certain 

 amount of water is given off. In the capillaries of the skin 

 ( Cp. Sk), a large quantity of water and smaller proportions 

 of other waste matters are got rid of. In the kidneys 

 {Cp. Kd), a considerable quantity of water, together with the 

 bulk of the urea, are removed from the blood and finally 

 expelled from the body. 



Note that all these changes are produced by the special 

 activity of particular groups of epithelial cells, which, 

 however alike they may be in general appearance and 

 structure, have a marvellous selective faculty peculiar to 

 themselves. Like all other parts of the body, they are 

 constantly undergoing the usual processes of waste and 

 repair, withdrawing nutrient matters and oxygen from the 

 blood, and passing waste matters into it. But, in addition 

 to the ordinary processes of nutrition, each particular group 

 of cells has the power of withdrawing a specific substance 

 from the blood or of passing substances into it. Thus the 

 epithelial cells of the enteric canal {Ep. Ent) pass in 

 digested food, those of the skin {Ep. Sk) and glomeruli 

 {glm) withdraw water, those of the urinary tubules 

 {Ep. Ur. J), urea, and so on. Similarly, the various gland- 

 cells, such as those of the liver {Lvr. C), pancreas {Fn. C), 

 gastric, and cutaneous {Cu. Gt) glands withdraw specific 

 substances, or excretions, which are discharged on the free 

 surface of the epithelium and serve various purposes. 



We see that the blood loses — (i), nutrient matters and 

 oxygen all over the body ; (2), water in the skin, lungs, and 



