PHYLUM CHORDATA 



401 



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. 1040, lit.) 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 (Fig. 1043, A, r. aw.). The right ventricle 

 (Fig. 1043,B) partly encircles the left, the former having a crescentic, 

 the_ latter a circular cavity in tranverse sections. The left 

 auriculo-ventricular valve has the usual membranous structure, 

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



T.vn, 



Fig. 1043. — A, heart of the Pigeon, dorsal aspect, a. ao. arch of aorta ; hr. a. brachial 

 artery ; 6r. v, brachial vein ; c. c. common carotid ; ju. jugular ; l. au. left auricle ; l.'p. a. left 

 pulmonary artery ; I. vn. left ventricle ; pc. v. left pre-caval ; ptc. post-caval ; p. v. pulmonary 

 veins ; r. aw, r. au'. right auricle ; r. p. a. right pulmonary artery ; r. pr. c. 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-ventricular valve. (A, from Parker's 

 Zootomy ; B, from Headlcy's Birda.) 



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, 

 pc. V.) and the post-caval {ptc.) ; the left four large pulmonary veins 

 (p. v.). The left ventricle (Fig. 1044, 1, vn.), as in the Crocodile, gives 

 origin to the right aortic arch (a. ao.), but the right ventricle (r. vn.) 

 gives off only one trunk, the pulmonary artery, which soon divides 

 into two (r.p.a., l.p.a.). The left aortic arch is absent in the adult, 

 and it is the right alone which is continued into the dorsal aorta. 

 The result of this is that the systemic arteries receive pure arterial 

 blood from the left side of the heart, and the only mingling 

 of aerated and non-aerated blood is in the capillaries. This is 

 perhaps the most important physiological advance made by Birds 

 over Reptiles. 



