IV. VERTEBRATA: AVES 



539 



The circulation in the birds has arisen from that of the reptiles by 

 complete separation of systemic and pulmonary systems. Of the three 

 great arterial trunks present there (fig. 574), the pulmonary artery and 

 the right aortic arch, arising from the left ventricle, are retained, the left 

 venous arch being lost. The septum betvi^een the ventricles is complete. 

 The striking features of the alimentary canal (fig. 61) are the crop (not 

 always present), a glandular stomach or proventriculus (c), and a mus- 

 cular chewing stomach or gizzard (d), as well as two long, rarely rudimen- 

 tary, ca;ca {k) at the junction of smaU and large intestine. Liver and 



IV -+ 





Fig. 590. Fig. 591. 



Fig. 590. — Right lung of hen, somewhat diagrammatic. A window shows a meso- 

 bronchus with its branches. A, artery; bm-, mesobronclius, arising from the vestibule; 

 br, bronchus swelling to vestibule; eb, ectobronchus; I, lung pipes; I-IV, entobronchi; 

 V, vein; 1-5, ducts of lung sacs. 



Fig. 591. — Cross-section of a lung pipe with parts of an adjacent pipe; between the 

 two a blood-vessel; the air space injected (after Fischer). 



gall bladder {e, f), pancreas (g), and spleen are present. A blind sac 

 (the bursa Fabricii), the paired ureters {m), and the sexual ducts {n) open 

 into the cloaca. The sexual ducts are peculiar in that the right oviduct 

 and ovary are degenerate. Since copulation occurs the large eggs (the 

 'yolk') are fertilized in the oviduct (fig. 102). As they pass slowly 

 through the duct, they become enveloped first with a thick layer of albu- 

 men ('white,' w), then with a double egg membrane {ism, sm), the two 

 parts being separate and enclosing an air chamber at the larger end of the 

 egg. Lastly comes the shell. During the passage down the oviduct the 

 first phenomena of development (segmentation, gastrulation) occur; after 

 oviposition the development stops and again starts when the necessary 

 warmth is supplied. 



The care for the young, the sexual life connected with reproduction, 

 and the complicated conditions of existence connected with flight have 



