Animal life in the sea and on the land . 248 ANIMAL LIFE IN THE SEA AND ON THE LAND. pebbles which birds swallow arenecessary to help in this grinding,and without them the gizzardcould not crush the food properly. 12. The digestive organs of abird, as seen in Fig. 163, may beeasily studied in the common long oesophagus (3) is enlargedto form a crop (4). Here the foodis moistened with digestive juices,and then passed on to the gizzard(7) in small quantities. 13. A Perfect Four-celledHeart.—After noticing thegradual modifications inthe hearts of fishes andreptiles, we have now thesatisf
Animal life in the sea and on the land . 248 ANIMAL LIFE IN THE SEA AND ON THE LAND. pebbles which birds swallow arenecessary to help in this grinding,and without them the gizzardcould not crush the food properly. 12. The digestive organs of abird, as seen in Fig. 163, may beeasily studied in the common long oesophagus (3) is enlargedto form a crop (4). Here the foodis moistened with digestive juices,and then passed on to the gizzard(7) in small quantities. 13. A Perfect Four-celledHeart.—After noticing thegradual modifications inthe hearts of fishes andreptiles, we have now thesatisfaction of finding inbirds the first instance ofa perfect - working four - celledheart, which succeeds admira-bly in keeping the pureblood and the impureblood from becom-ing mixed. 14. The right sideof the heart, as shownin the ideal plan inFig. 164, is set apartto receive the impureblood and no other, and to send it to the lungs, while the left side receivesthe pure blood that is returned from the lungs and. Fig. 163.—Digestive Organs op a Fowl. 3, oesophagus ; 4, crop; 7, gizzard. CHARACTERISTICS OF BIRDS. 2-W pumps it into the arteries, which carry it to all parts ofthe body. 15. The High Temperature of Birds.—This perfect circu-lation is found in all the higher animals, giving them ahigh temperature. Still, the blood of birds is hotter thanthat of other animals, owing partly totheir perfect circulation, to the abun-dant supply of air in their bodies, andalso to their covering of down andfeathers, which keeps the heat from es-caping. The temperature of birds is104° ; that of the human body is 98°. 16. The Music of Birds.—The ex-quisite music of the birds is producedby an arrangement of bones and car-tilages at the lower end of the wind-pipe, or trachea, as it is called. Cur-rents of air passing through this partof the trachea give a quivering motionto the membrane stretched across it,and produce all the varied notes sopleasing to our ears. 17. Birds evidently sing s
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1887
