84 hlRST LESSONS IN ZOOLOGY 



leaves the water tlie trills liave disappeared and the 

 lungs are well developed. A few of the amphibians, 

 however, may retain the gills in addition throughout the 

 adult stage. Such forms live in water or in very moist 

 places. Moreover, most amphibians make use of the 

 skin, thin and moist like that of the frog, for respiration, 

 and thus have no necessity of great lung development. 

 The air-bladder of fishes has the position of a lung, but 

 is used ordinarily to regulate the weight of the body. 

 Still in a few forms, the garpike for example, it serves 

 sometimes as a lung. 



In the higher vertebrates the exterior skin surface is 

 not 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 b}' tube-like extensions into the 

 body cavity. This arrangement gives a much increased 

 respiratory surface. In birds and mammals the e.xtent 

 of surface is immensel)' 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 \'ery 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. Accompan)'ing this we find improved 

 means for passing the air over the exterior and bringing 

 the blood to the interior surface. A frog or salamander 

 breathing quietly enlarges the mouth cavity b)' lowering 

 its floor, and tlie air comes in through the nostrils; this 

 air is then squeezed by the upward pressure of the floor of 



