SECRETION AND EXCRETION 333 



The gland also serves to form sugar (glycogen) from 

 part of the digested food, and may well be called a 

 chemical workshop for the body. In animals of slow 

 respiration, as crustaceans, mollusks, fishes, and reptiles, 

 fat accumulates in the liver. "Cod-liver oil" is an 

 example. 



The Great Excreting Organs are the lungs, the kidneys, 

 and the skin ; and the substances which they remove from 

 the system carbonic acid, water, and urea are the 

 products of decomposition, or organic matter on its way 

 back to the mineral kingdom. 124 Different as these 

 organs appear, they are constructed upon the same 

 principle : each consisting of a very thin sheet of tissue 

 separating the blood to be purified from the atmosphere, 

 and straining out, as it were, the noxious matters. All, 

 moreover, excrete the same substances, but in very dif- 

 ferent proportions : the lungs exhale carbon dioxide and 

 water, with a trace of urea ; the kidneys expel water, 

 urea', and a little carbon dioxide ; while the skin par- 

 takes of the nature of both, for it is not only respiratory, 

 especially among the lower animals, but it performs part 

 of the work of the kidneys in case they are diseased. 



1. The lungs (and likewise gills) are mainly excretory 

 organs. The oxygen they impart sweeps with the blood 

 through every part of the body, and unites with the tis- 

 sues and with some elements of the blood. Thus are 

 produced heat and work, whether muscular, nervous, 

 secretory, etc. As a result of this oxidation, carbon 

 dioxide, water, and urea, or a similar substance, are 

 poured into the blood. The carbon dioxide and part of 

 the water are passed off from the respiratory organs. 

 This process is more immediately necessary to life than 

 any other ; the arrest of respiration is fatal. 



2. While the lungs (and skin also, to a slight degree) 

 are sources of gain as well as loss to the blood, the kid- 



