148 THE FROG CHAP. 



by drop into the cloaca, whence it is either expelled at 

 once, or stored for a time in the bladder. 



Note that the formation of urine is a process of secre- 

 tion 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, consist- 

 ing 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. 



Pulmonary and Cutaneous Excretion. The lungs, 

 besides being organs of respiration, take their share in 

 excretion, since they get rid of the important waste pro- 

 duct, 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 takes 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 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 Nutrition. We are 



