148 THE FROG chap. 



Note that the formation of urine is a process of secretion 

 of a similar nature to the secretion of gastric juice, bile, &c. 

 The fluid secreted, is, however, of no further use to the 

 animal, and would, in fact, act as a virulent poison if retained 

 in the system. It is therefore got rid of as soon as possible. 

 Secretions of this kind, consisting not of useful but of 

 harmful or waste matters, are distinguished as excretions. 



Bile is also in part an excretion as it contains pigments due to the 

 disintegration of haemoglobin, and thus by its means the effete colouring 

 matters of the blood are passed into the intestine and got rid of, 



Fnlmonary and cutaneous Excretion. — The lungs, be- 

 sides being organs of respiration, take their share in excretion, 

 since they get rid of the important w^aste product, carbon 

 dioxide, together with a considerable quantity of water. 

 Similar functions are discharged by the skin, which is also an 

 organ both of respiration and of excretion. Interchange of 

 gases take place between the outer air and the blood in the 

 capillaries of the derm : the carbon dioxide of the non-aerated 

 blood brought to the skin by the cutaneous artery (p. 93 

 and Fig. 23) is exchanged for oxygen, and the blood, in the 

 the aerated condition, is returned by the musculo-cutaneous 

 vein to the heart. Moreover the cells of the cutaneous 

 glands separate water and other constituents from the blood, 

 and the fluid thus formed is poured out on the surface of 

 the body. Here it serves to keep the skin moist, and is 

 finally lost, either by evaporation or by mingling with the 

 water in which the frog is immersed. The cutaneous 

 secretion has also poisonous properties, and so probably 

 serves as a defence against some of the animal's enemies. 



Summary of the processes of STutrition. — We are now 

 in a position to understand the general features of the 

 whole complicated series of processes which have to do with 

 the nutrition of the frog, which are collectively spoken of 



