VERTEBRATES. 



309 



FIG. 116. Human embryo 

 (after Hertwig), with 

 the floor of mouth and 

 throat removed, to show 

 the rudimentary gill- 

 slits, g. I, lung; n, nos- 

 tril, still connected with 

 the mouth. 



apparently but a single slit externally. A little con- 

 sideration will show that there is 

 little real modification. In the anu- 

 rous Amphibia a similar fold is 

 found, but this unites again with 

 the body-wall behind the gills, thus 

 enclosing the external openings in 

 an atrium, with but a single open- 

 ing to the exterior (p. 337). In the 

 Sauropsida and mammals (fig. 116) 

 gill-pouches are formed in the em- 

 bryo, but according to recent observ- 

 ers these never break through, so 

 that no real clefts are formed. With 

 growth all but the first pair of these 

 pouches disappear, the first persisting as the Eustachian 

 tube. 



In all vertebrates above fishes, gills are supplemented 

 (Amphibia) or replaced by lungs. These are paired sacs 

 richly supplied with blood-vessels, and connected with the 

 external world by means of a tube (windpipe or trachea) 

 which opens by the glottis upon the floor of the pharynx. 

 The trachea is usually strengthened by the development 

 of cartilages in its wall, some of which may become large, 

 as in the case of the human 'Adam's apple.' The lungs 

 themselves may be simple sacs, but usually they become 

 greatly folded, thus increasing the respiratory surface. In 

 the Amphibia, which lack diaphragm and ribs, air is forced 

 into the lungs by swallowing; in the reptiles and birds it 

 is drawn in by means of the muscles (inter costal s) between 

 the ribs; in the mammals the intercostals are reinforced 

 by a transverse muscle (diaphragm) (fig. 114) which 

 crosses the body-cavity. This is dome-shaped, convex 



