IX NUTRITION 151 



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 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 aud 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. 



Evolution of Heat. — The oxidation of the tissues, like 

 that of coal or wood in a fire, is accompanied by a rise in 

 temperature. But in the frog, as in other cold-blooded 

 animals, the evolution of heat is never sufficient to raise the 



