ORGANS OF RESPIRATION. 521 



CHAPTER FOURTH. 



ORGANS OF RESPIRATION. 



FIRST SECTION. 



General observations on the Respiratory Organs. 



As the function of respiration is that by which the fluids of 

 living bodies are oxygenated and decarbonized, and which 

 renovates their vital properties, and prepares them for afford- 

 ing the materials of their various secretions, it is one of the 

 most influential and most general in both kingdoms of organic 

 nature. Every living plant has the power of decomposing 

 the atmosphere, to effect its respiration, and every animal, 

 from the monad to man, has the power of renewing the stra- 

 tum of the surrounding element in contact with its exterior 

 or its interior surface, to aerate the fluids of its body. The 

 monad excites currents in the water around it, to aerate its 

 surface, by the rapid vibration of its cilia, and man produces 

 similar currents in the air, for the same purpose, by the 

 alternate motions of his ribs and diaphragm, and by the 

 myriads of vibratile cilia which line the mucous membrane 

 of the passages and cells of his lungs. The absorption of 

 oxygen from the atmosphere into the system, and the secre- 

 tion of carbon from the living fluids, being always effected 

 through delicate membranes constituting a respiratory surface, 

 it is obvious that this chemical function may alike be per- 

 formed by the general surface of the skin, or by the mucous 

 lining of the alimentary canal, or by any external or internal 

 organ especially appropriated to it. The lowest animals 

 respire by their general cutaneous mucous surface, without a 

 special organ for the convenient exposure of their fluids to 

 atmospheric influence, and the external skin, or its internal 

 mucous prolongation, is the origin of most forms of respira- 

 tory organs met with in higher tribes. The gills of aquatic 

 animals, and the pulmonic cavities of the invertebrated tribes, 



