. Elements of comparative zoology. Zoology. 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 mammal
. Elements of comparative zoology. Zoology. 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 (intercostals) 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. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Kingsley,
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectzoology, bookyear1904
