The Inner (Alimentary) Tube and Its Respiratory Derivatives oQ 



small vessels lying closely adjacent to the very thin lining (pulmonary 

 epithelium) of the lung. 



( ii lis are the characteristic respiratory organs of fishlike aquatic 

 vertebrates. Lungs are characteristic of land-dwellers. But some fish- 

 like vertebrates have both gills and lungs, and whales have only lungs. 

 Both gills and lungs are derived from the pharyngeal region of the 

 digestive tube. 



PHARYNGEAL CLEFTS 



One of the most unique features of vertebrate structure is found 

 in the pharyngeal region. Paired right and left lateral apertures in the 

 wall of the pharynx lead into passages or chambers which usually 

 penetrate to the external surface of the body and open there as a longi- 

 tudinal series of slits or clefts (Figs. 5, 56, 58). These pharyngeal 

 chambers occur in the embryos of all vertebrates. They vary in num- 

 ber from 14 pairs in some cyclostome eels (Myxine) to a comparatively 

 small number in land vertebrates. In fishes the respiratory gill-struc- 

 tures develop in the walls of these passages, which then persist through- 

 out life. In land vertebrates the pharyngeal chambers or pouches occur 

 only temporarily in the early embryo and may not completely per- 

 forate to the exterior. They are later obliterated, except that those of 

 the first pair persist in a highly modified way in connection with the 

 development of the accessory mechanism of the ear. 



STRUCTURE OF LUNGS 



Lungs develop as a mid ventral outgrowth from the wall of the 

 pharynx (Fig. 59). The outgrowth bifurcates to form the right and 

 left lungs. Their common connection with the pharynx is the trachea 

 ("windpipe"), whose median pharyngeal aperture is the glottis. Pos- 

 teriorly the trachea divides into right and left bronchi, each bronchus 

 passing into the corresponding lung. The region of the trachea adjacent 

 to the glottis may be elaborated into a vocal apparatus, the larynx 

 (Figs. 60, 61). 



Such lungs as occur in fishes and amphibians are relatively simple 

 sacs whose thin walls are, in some cases, quite devoid of complication 

 (Fig. 62). In the other vertebrates, however, the lumen of the lung 

 is, in one way or another, more or less elaborately and minutely di- 

 vided into air-spaces, all communicating with the bronchus (Fig. 63). 

 The area of the pulmonary surface is thus increased. Respiratory 

 efficiency depends on the amount of surface available for exposure of 

 blood to oxygenation by the air. 



