150 THE FROG CHAP. 



which nutrient matters and oxygen are aDsorbed and 

 waste products got rid of are carried on in certain 

 definite portions of the circulatory system. 



In the walls of the enteric canal (Fig. 23, Ent. C), the 

 products of digestion pass into the blood (Cp. Ent. C), 

 or in the case of fats, first into the lacteals and ulti- 

 mately 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 appear- 

 ance 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 Malpighian capsules 

 (Mlp. Cp) withdraw water, those of the urinary tubules 

 (Ep. Ur. T) urea, and so on. Similarly, the various 

 gland-cells, such as those of the liver (Lvr. C), pancreas 

 (Pn. C), gastric and cutaneous (Cu. Gl) glands, with- 

 draw specific substances, or secretions, which are 



