XIII 



PHYLUM CHORDATA 



389 



The aortic arch curves over the right bronchus to reach the 

 dorsal body-wall, and then passes directly backwards as the dorsal 

 aorta (d. ao.}. Owing to the immense size of the pectoral muscles, 

 the arteries supplying them are of corresponding dimensions, 

 and the right and left innominate arteries (in. a.), from which the 

 carotids (c. c.), subclavians (br. a.), and pectorals (pc. a.) arise, 

 are actually larger than the aorta itself beyond their origin. In 

 correspondence with the position of the legs, the femoral (f. a.) 

 and sciatic (sc. a.) arteries arise very far forward : the caudal 

 artery (c.) is naturally small. 



The most characteristic feature in the disposition of the 

 circulatory organs is the almost complete disappearance of the 



Ir.a 



FIG. 1054. A, heart ol' the Pigeon, dorsal aspect, a.ao. arch of aorta ; br. a. brachial 

 artery ; br. u. brachial vein ; c.c. common carotid ; ju. jugular ; /. au. left auricle ; l.p.a. left- 

 pulmonary artery ; l.vn. left ventricle ; pc.v. left precaval ; ptc. postcaval ; p. v. pul- 

 monary veins ; r.au, 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 Headley's Birds.) 



renal portal system. There are two renal portal veins (r.p.) formed 

 by the bifurcation of the caudal ; but each, instead of breaking 

 up into capillaries in the kidney, sends off only a few small 

 branches (a. r. v.) which apparently carry blood to that organ, 

 the main vein passing forwards, through the substance of the 

 kidney, and joining the femoral vein (/. v.) from the leg to form 

 the iliac vein (i. v.), which, uniting with its fellow of the opposite 

 side, forms the post-caval (pt. c.). Thus the main part, at any 

 rate, of the blood from the caudal and pelvic regions is taken 

 directly to the heart, and not through the renal capillaries as in 

 most Fishes and all Amphibians and Reptiles. 



At the point of bifurcation of the caudal veins a large coccygeo- 

 mesenteric vein (c. m. v.) comes off, and, running parallel with the 



VOL. II B B 



