XII 



PHYLUM CHORDATA 



475 



these branch again, sending off tubes which end blindly 

 near the surface of the lung and give off blind dilatations 

 commonly known as alveoli. In addition to these each 

 main bronchus also gives off branches which end in a series 

 of thin-walled air-sacs, which lie in the body-cavity, and are 

 in communication with the pneumatic cavities of the bones. 

 A 



br.a 



T.VJt 



FIG. 271. A, heart of the Pigeon, dorsal aspect, a. ao. arch of aorta; br.a. 

 brachial artery ; br. v. brachial vein ; c. c. common carotid ; ju. jugular ; /. au. 

 left auricle ; /. /. a. left pulmonary artery ; /. vn. left ventricle ; pc. i>. left pre- 

 caval ',.ptc. postcaval ; /. v. pulmonary veins ; r. au., r. an', right auricle ; r.p. a. 

 right pulmonary artery ; r.prc. right precaval ; r. vn. right ventricle. B, heart 

 of a Bird with the right ventricle opened. L. V. septum ventriculorum ; R. V. 

 right ventricle ; V. right auriculo^ventricular valve. (A, from Parker's Zoototny ; 

 B, from Headley's Birds.) 



The heart (Fig. 271) is of great proportional size, and, like 

 that of the Crocodile, consists of four chambers, right and 

 left auricles, and right and left ventricles. There is no 

 sinus venosus, that chamber being, as it were, absorbed into 

 the right auricle (A, r. au}. The right ventricle partly en- 

 circles the left (B), the former having a crescentic, the latter 

 a circular, cavity in transverse sections. The left auriculo- 



