THE ANIMALS AND MAN 



In the higher vertebrates the exterior skin surface is not 

 at all adapted for respiration, which, together with the 

 generally greater activity of these animals, necessitates a 

 much greater development of the lungs. Thus instead of 

 the two simple lung sacs of the frog the lizard has a com- 

 plex double sac enlarged by tube-like extensions into the 

 body- cavity. This arrangement gives a much increased 

 respiratory surface. In birds and mammals the extent 

 of surface is immensely increased. It is estimated that 

 the inner surface of a man's lungs amounts to a thousand 

 square feet in area, or one hundred times the external sur- 

 face of the body. The windpipe gives off one large branch 

 to each lung; these branches divide again and again, the 

 last divisions bearing on their ends very small sacs of thin 

 membrane about which is clustered a net-work of capillary 

 blood-vessels. Through the walls of these small sacs the 

 oxygen and carbon dioxide pass. 



So far we have seen only 

 how increase of surface is 

 brought about. Accompany- 

 ing this we find improved 

 means for passing the air 

 over the exterior and bringing 

 the blood to the interior sur- 

 face. A frog or salamander 

 breathing quietly enlarges the 

 mouth-cavity by lowering its 



floor, and the air COmeS in FIG. 20. Tracheal tube, lungs, heart 



through the nostrils; this air and dia P hra S m of a mouse - 

 is then squeezed by the upward pressure of the floor 

 of the mouth, the valves in the nostrils close, and it 

 is thus pushed down into the lungs. The muscles in 

 the walls of the body now contract and squeeze upon 

 the air in the lungs, the nostril valves open, and the 

 air is forced out. This method is gradually improved 



