XIII 



PHYLUM CHORDATA 



373 



into the pneumatic cavities of the bones. Thus, while in other 

 animals a certain amount of unchanged or residual air is always 

 left in the lungs after each expiration, in Birds the residual air is 

 confined to the air-sacs and to the smaller branches of the bronchi, 

 every respiratory movement drawing a current of fresh or tidal air 

 through the lungs. As a result of this the aeration of the blood is 

 very complete and its temperature correspondingly high. It is 

 worthy of notice that Birds agree with Insects, the only other 

 typically aerial class, in having the inspired air distributed all 

 over the body so that the aeration of the blood is not confined 

 to the limited area of an ordinary respiratory organ. 



Circulatory Organs. The heart (Fig. 981, ht.) is of great 

 proportional size, and, like that of the Crocodile, consists of four 



T.VJI, 



FIG. 984. 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 ; I. au. left auricle ; I. p. a. left 

 pulmonary artery ; 1. rn. left ventricle ; pc. v. left pre-caval ; ptc. post-caval ; p. v. pulmonary 

 veins ; r. au. r. au'. right auricle ; r. p. a. right pulmonary artery ; r. prc. right pre-caval ; 

 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-veiitricular valve. (A, from Parker's 

 Zootomy ; B, from Headley's Birds. 



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 (Fig. 984, A, r. au.). The right ventricle 

 (Fig. 984, B) partly encircles the left, the former having a crescentic, 

 the latter a circular cavity in transverse sections. The left 

 auriculo-ventricular valve has the usual membranous structure, 

 consisting of two flaps connected with the wall of the ventricle by 

 tendons, but the corresponding valve of the right side (R. V.) is a 

 large muscular fold, very characteristic of the class. 



The right auricle receives the right and left pre-cavals (r.prc., 



