THE ESSENTIAL PHENOMENA OF RESPIRATION. 11 



made up by the union of various forms with cylindrical 

 tubes. 



We will finish this general account by the definition 

 of a few terms that merit special reference. The various 

 tissues which form the glandular organs, such as the 

 liver, kidneys, spleen, etc., are designated as parenchyma, 

 and the collective name viscera is applied to the different 

 organs contained within the large cavities of the body. 



Membranes are tissues in the form of sheets covering 

 the surfaces of certain organs. Mucous membranes are 

 those which line the interior of cavities in direct connec- 

 tion with the exterior, the mouth, for example, is lined 

 with the buccal mucous membrane. Serous membranes 

 line the interior of the large cavities of the body, and 

 are not in communication with the exterior ; they sepa- 

 rate the organs and facilitate their various movements. 



Having acquired the foregoing general notions, we will 

 pass directly to the study of the apparatus of vegetative 

 life and those of the life of relation. The first are those 

 of respiration, circulation, and digestion. The second 

 are those of movement (skeleton and muscles) and of 

 sensation (nervous organs and organs of sense). 



CHAPTER H. 

 The Essential Phenomena of Respiration. 



THE atmosphere is composed, to the extent of one-fifth 

 its volume, of a gas having very energetic properties, it 

 is called oxygen ; the other four-fifths are another gas, 

 nitrogen, which may be considered as taking no part in 

 the phenomena which are now to occupy our attention. 



