CHAPTER XVI 

 RESPIRATION 



SECTION I 



THE MECHANICS OF THE RESPIRATORY 

 MOVEMENTS 



IN unicellular animals the interchange of gases, i.e. the intake of 

 oxygen and the output of carbon dioxide, is as a rule carried out by 

 processes of diffusion occurring at the surface of the cell. With 

 increased size of the organism the surface becomes insufficient for this 

 purpose, and special organs make their appearance for presenting a 

 large extent of surface to the surrounding medium. In the multi- 

 cellular animals the actual process of tissue respiration is carried out 

 between the internal medium, lymph, blood, &c., and the individual 

 cells, and the use of the special organ of respiration is to bring the 

 circulating internal medium in intimate relation over a large area 

 with the surrounding fluid, whether air or water. In insects we 

 find a large branched system of tubes, the tracheae, which contain 

 air and are distributed to the finest tissues, renewal of the air in the 

 tubes being provided for by special respiratory movements. In 

 most water animals the respiratory organ is known as the gills, and 

 presents a large surface well supplied with circulating blood over 

 which a continual stream of the surrounding water is kept up. In 

 all these animals therefore we can distinguish two processes, viz. 

 (1) the interchange of gases between the tissue-cells and the surrounding 

 lymph, ' internal respiration ' ; (2) the interchange of gases between 

 the circulating fluid and the external medium, ' external respiration.' 

 In all land-breathing vertebrates the organs of external respiration, 

 the lungs, arise as paired diverticula of the anterior part of the ali- 

 mentary canal. The renewal of the air in the air-sacs formed from 

 these diverticula is effected by alternate increase and diminution in 

 their size caused by the movements of respiration, while a rapid circu- 

 lation of blood is carried out through a fine mesh work of capillaries 

 just underneath the surface of the sacs. In man the organs of external 

 respiration, the lungs, are built up in the following way : 



The trachea or windpipe, a wide tube about 4J inches long, divides 



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