ix NUTRITION 151 



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 kidneys; (3), carbon dioxide in the lungs 

 and skin; (4), urea, principally in the kidneys; and 

 (5), various substances in the glands. It gains (i), waste 

 products all over the body ; (2), nutrient matters in the 

 enteric canal; (3), liver-sugar (p. 135) in the liver; 

 (4), oxygen in the lungs and skin. It is therefore richest 

 in oxygen and poorest in carbon dioxide as it leaves the 

 lungs and skin, i.e., in the pulmonary and musculo- 

 cutaneous veins ; richest in nutriment as it leaves the 

 enteric canal, i.e., in the portal vein ; poorest in urea as 

 it leaves the kidneys, i.e., in the renal veins ; poorest in 

 water as it leaves the skin and kidneys, i.e., in the 

 cutaneous and renal veins. 



In this way a single closed system of pipes not only 

 supplies all parts of the body with everything necessary 

 for their sustenance, but serves also as a drainage 

 system to carry away their various waste products. 



Notice that we must distinguish between the nutrition, 

 respiration, and excretion of the frog as a whole, and of 

 its various parts. Every one of the thousands of cells, 

 fibres, &c., in the entire body is nourished, breathes, 

 and excretes, taking its nourishment and oxygen 

 directly from the blood, and discharging its waste 

 products into it. What are called the organs of nutrition 

 and respiration are special portions of the body set 

 apart for taking in fresh supplies of food or of oxygen 

 for the organism as a whole, such supplies being finally 

 distributed by the blood-system. Similarly, what are 

 called the organs of excretion are special portions of the 

 body by which the waste products, collected by the blood 

 from all parts of the organism, are finally discharged. 



