CHAPTER IV 

 RESPIRATION 



RESPIRATION in its widest sense is the sum total of the processes by 

 which the ultimate elements of the body gain the oxygen they 

 require, and get rid of the carbon dioxide they produce. 



SECTION I. PRELIMINARY ANATOMICAL DATA. 



Comparative. In a unicellular organism no special mechanism of 

 respiration is needed; the oxygen diffuses in, and the carbon dioxide 

 diffuses out, through the general surface. The simple wants of such 

 multicellular animals as the ccelenterates, the group to which the sea- 

 anemone belongs, are also supplied by diffusion through the ectoderm 

 from and into the surrounding water, and through the endoderm from 

 and into the contents of the body-cavity and its ramifications. 



But in animals of more complex structure special arrangements 

 become necessary, and respiration is divided into two stages: (i) Ex- 

 ternal respiration, an interchange between the air or water and a cir- 

 culating medium or blood as it passes through richly vascular skin, 

 gills, tracheae, or lungs; and (2) internal respiration, an interchange 

 between the blood, or lymph, and the cells. 



In the lower kinds of worms respiration goes on solely through the 

 skin, under which plexuses of bloodvessels often exist, but in some 

 higher worms there are special vascular appendages that play the part 

 of gills. The Crustacea also possess gills, while in the other arthropoda 

 respiration is carried on either by the general surface of the body (in 

 some low forms), or more commonly by means of tracheae, or branched 

 tubes surrounded by blood spaces and communicating externally with 

 the air and internally by their finest twigs with the individual cells. 

 Most of the mollusca breathe by gills, but a few only by the skin. 



Among vertebrates the fishes and larval amphibians breathe by gills, 

 but most adult amphibians have lungs. The skin, too, in such animals 

 as the frog has a very important respiratory function, more of the 

 gaseous exchange taking place through it in some conditions than 

 through the lungs. 



One small group of fishes, the dipnoi, has the peculiarity of possessing 

 both gills and a kind of lungs, the swim-bladder being surrounded with 

 a plexus of bloodvessels and taking on a respiratory function. 



In all the higher vertebrates the respiration is carried on by lungs; 

 the trifling amount of gaseous interchange which can possibly take 

 place through the skin is not worth taking into account. The lungs 

 are to be regarded as developed from outgrowths of the alimentary 

 canal, beginning near the mouth. 



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