ORGANS OF EXCRETION. 9 



full supply of oxygen and water, which is necessary for 

 dissolving out and carrying away hurtful products. 



The skin, like the lungs, gives off carbonic acid, although in 

 very much smaller quantities ; and it absorbs a minute quan- 

 tity of oxygen from the air. In man, the lungs expire about 

 200 times the amount of carbonic acid given off by the skin. 



RECIPROCAL ACTION OF THE ORGANS OF EXCRETION. 



In the lowest forms of animal life, the functions of absorp- 

 tion and excretion have no special organs, but are performed 

 by the entire surface. Insects breathe through openings 

 (stigmata] which are distributed all over the body. In frogs, 

 according to Bischof, the function of breathing is about 

 equally divided between the lungs and skin. As we ascend 

 the scale, we find that the processes connected with absorption 

 and excretion become more and more localised. Thus, in the 

 horse as in man, the absorption of oxygen from the air 

 is practically confined to the lungs ; and that of food and 

 water, to the stomach and intestines. In excretion, the lungs 

 carry away by far the greater part of the carbonic acid ; the 

 kidneys remove waste nitrogenous matter (urea, hippuric acid, 

 etc.), along with the urine ; the kidneys, lungs, and skin get 

 rid of the used up water ; and the bowels expel the un- 

 absorbed portions of food. 



This tendency to the localisation of the functions of ab- 

 sorption and excretion are in no case complete. Thus, 

 although the lungs are the special organs for the excretion 

 of carbonic acid, the skin, as already mentioned, exhales it to 

 a slight extent. Bouley was of opinion that his experiments 

 proved that this transpiration from the skin (cutaneous expira- 

 tion} was essential to the life of the lower animals. Of two 

 horses, the skins of which he had shaved and covered with 

 tar, one died in nine days, the other in ten days, and 

 a third horse, which had been treated in the same way, 



