374 ZOOLOGY SECT. 



pc. v.} and the post-caval (ptc.), the left, four large pulmonary veins 

 (p. v.). The left ventricle (Fig. 985, /. m), 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 (rp.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. 



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 (/. 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 

 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 Amphibia and Reptiles. 



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

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

 rectum, from which it receives tributaries, joins the portal vein. 

 The abdominal vein of Amphibia and Reptiles appears to be 

 represented, in part at least, by the epigastric vein (epg.), which 

 returns the blood, not from the ventral body wall, but from the 

 f/ ret ft omentum, a fold of peritoneum, loaded with fat, lying ventral 

 to the intestines and gizzard : the epigastric discharges into the 

 hepatic vein. 



The red blood corpuscles are oval and nucleated. The tempera- 

 ture of the blood is unusually high over 38 C C. (100 F.) 



Nervous System.- -The brain (Fig. 986) completely fills the 

 cranial cavity, and is remarkable for its short, broad, rounded form. 



