610 CHORD ATA. 



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. 631), the pul- 

 monary artery and the right aortic arch, arising from the left ven- 

 tricle, are retained, the left venous arch being lost. The septum 

 between the ventricles is complete. The striking features of the 

 alimentary canal (fig. 00) are the crop (not always present), a 

 glandular stomach or proventriculus {<;), and a muscular chewing 

 stomach or gizzard (d), as well as two long, rarely rudimentary, 

 caeca {k) at the junction of small and large intestine. Liver and 

 gall bladder {e, /), pancreas {g), and spleen are present. A bhnd 

 sac (the bursa Fabricii), the paired ureters (/«), and the sexual 

 ducts («) open into the cloaca. The latter show the peculiarity 

 that the right oviduct and ovary are degenerate, while those of the 

 left side are correspondingly larger. Since copulation occurs the 

 large eggs (the ' yolk ') are fertilized in the oviduct (fig. 99). As 

 they pass slowly through the duct, tliey become enveloped first 

 with a thick layer of albumen, 'white' (w), then with a double 

 egg membrane {mn, sm,) the two parts being separate and enclos- 

 ing an air chamber at the larger end of the egg. Lastly comes the 

 shell. All of these accessory structures are secreted by the gland- 

 ular walls of the enlarged oviducts. During the passage down the 

 oviduct the first phenomena of development (segmentation, gastru- 

 lation) occur, and after ovijoosition the development stops and again 

 starts when tlie necessary warmth is supplied. 



The care for the young, the sexual life connected with copula- 

 tion, and the comjilicated conditions of ex- 



jlt- Zol istence connected with flight have resulted in 



/^ \. an intelligence far superior to that of the 



/ m Ky reptiles, which finds its expression in the bet- 



a;'- f'^'^Miiasm "^ ^^^ development of sense organs and brain. 



Pj^^^H ^TZ? In the brain (fig. (341) the cerebellum, which 



"^^^m-'-ffff" is the central organ for the coordination of the 



^^^. ,r^ action of parts, is strikingly developed. Cor- 



\]7 n. respondingly large are the cerebral hemi- 



FiG. 641. -Brain of pig- Spheres, the frontal lobes of which begin to 

 h%m.rTn.^Vei^mis'. cover the olfactory lobes, the temporal lobes 

 cCTebeUum''^'''}''''oife?- i^ ^^^^ manner extendine; back over the "twixt 

 to??'iobeirMi'^.ptS 'Tain and optic lobes. ^Corresponding to the 

 spSSiTmcerc': ™t"^' apparatus, the ear is highly organized, 

 ijrum ; z, pi'neaiis. the Ligcna of tjie labyrintli being greatly en- 

 larged and the sound-conducting apjiaratus (columella, tympanum. 



