730 COMPARATIVE ANATOMY 



and are unconnected therewith, while in the amphibians (Fig. 339) there 

 develops a pair of openings called the posterior-nares or choanae connect- 

 ing these two portions by openings in the roof of the mouth. This com- 

 munication is supposed to be "one of the changes inaugurated during the 

 transition from water to land and allows the ingress and egress ot air 

 to the pharynx and then to the lungs without opening the mouth, since 

 this action, although harmless for an animal immersed in water, would 

 soon cause the drying up of a mucous membrane lining a mouth cavity 

 if resorted to in air with anywhere near the same frequency. In the case 

 of the nasal cavities this is prevented in part by the small size of the 

 external openings, but still more by the -formation of slime glands capable 

 of producing an abundant secretion. The waste lacrimal fluid diverted 

 from the eyes to the nose is undoubtedly also of assistance in this re- 

 spect." 



There is a tendency in the pharynx to form diverticula in the median 

 line (Fig. 426), that is, there is here an expansion into large sacs or res- 

 ervoirs which may or may not remain in communication with the pharynx 

 itself. The air-bladders in fish are examples (Fig. 441). While this air- 

 bladder is usually a closed sac filled by gases extracted from the blood, 

 in a few animals one finds a rather small air-duct coming from these air- 

 bladders to the pharynx. In fishes, where this occurs, the animal comes to 

 the surface of the water and makes a snapping or swallowing movement. 

 In the higher forms of animals this develops into the pulmonary system, 

 the lateral sacs being the lungs and bronchia and the median duct, the 

 trachea. The opening of the trachea into the pharynx is called the glottis 

 (Fig. 428), which together with the various cartilages and muscles de- 

 rived from the visceral system, forms the larynx. While there are always 

 two lungs in lung-breathing animals there is only one air-bladder in 

 those forms of vertebrates which are not lung breathing. Then, too, 

 the air-sac or air-bladder is almost always dorsal to the pharynx, while 

 the lungs lie ventral to it. 



The flat plate of bone forming the roof of the mouth and thus sepa- 

 rating the nasal cavities from it, is called the hard palate. The soft cover 

 of this bony plate which extends backward beyond the palate is known 

 as the soft palate or velum palati. 



Just as one can easily see the sulcus ( ) in the 



median line immediately under the nose, so, in looking into the mouth, 

 a ridge will be seen which is formed right through the center of its roof. 

 As the human being, as well as all of the higher forms of animals, is 

 bilaterally symmetrical, and as different portions of the jaw begin their 

 growth from distinctly separate centers, which then grow toward the 

 midline and unite, one can readily understand not only why the ridge is 

 formed on the roof of the mouth but also why there is a sulcus immedi- 

 ately below the nose on the outer upper lip. If, due to a mechanical or 

 chemical obstruction of some kind, the two lateral portions of palate or 



