. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. 302 Biology of the Vertebrates ference, food passing straight to the esophagus where it continues on its way, while oxygen, dissolved in water, passes out over the gills hanging in the lateral gill slits, which like portholes pierce the sides of the pharynx. The paired nasal pits on the snout of a fish do not open into the mouth cavity, and have nothing to do with the pharynx or with breathing. In amphibians, the first land forms that possess lungs and bre
. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. 302 Biology of the Vertebrates ference, food passing straight to the esophagus where it continues on its way, while oxygen, dissolved in water, passes out over the gills hanging in the lateral gill slits, which like portholes pierce the sides of the pharynx. The paired nasal pits on the snout of a fish do not open into the mouth cavity, and have nothing to do with the pharynx or with breathing. In amphibians, the first land forms that possess lungs and breathe free air, the nasal pits deepen until they break through into the mouth cavity, thereby forming a pair of respiratory passage-ways. The openings into the mouth cavity are the internal nares, or choanae (Fig. 251b). These allow air to pass to the lungs without the necessity of opening the mouth, thereby exposing its mucous lining to disastrous drying up. Free air is taken into the mouth cavity through the choanae with the mouth closed. After valves in the external nares have closed, the floor of the mouth is raised. Thus the air is forced back into the lungs, a process which would be quite im- possible with the mouth open, as the air could then escape in the wrong direction. Nasal Pit. Internal Narls Food "~ir^^"iZr^~c7777777Zl~ • Chiasma' Glottis' A. FISH Lung B. AMPHIBIAN Fig. 251. Diagrams of the evolution of the pharyngeal chiasma. For man refer to Fig. 252. (After Wiedersheim.) Embryonically the lungs are ventral outgrowths from the floor of the pharynx, and thus, while the food takes a straight course from mouth to esophagus as in fishes, air, entering the nostrils dorsally, crosses the path of the food and is forced ventrally into the lungs. Vertebrates above the amphibians, that is, reptiles, birds, and mammals, have developed a hard palate, or secondary roof of the mouth, which forces the choanae backward so that the crossing of the ways is transferred
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