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 multicellular animals the actual process of tissue respira- 

 tion 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 trachea?, 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 

 air-breathing vertebrates the organs of external respiration, the lungs, 

 arise as paired diverticula of the anterior part of the alimentary 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 circulation of blood is carried out 

 through a fine meshwork 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 

 below into two main branches bronchi; and these subdivide again and 

 again, becoming gradually smaller. The terminal ramifications or bron- 

 chioles open into rather wider parts the infundibula, the walls of which are 



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