530 ZOOLOGY. 



cava superior ( Vs), just above which opens the vena cava 

 inferior ( Vi). The two larger and posterior chambers, the 

 ventricles, form the apex of the heart, and give off the 

 arterial trunks. Of the ventricles, the left ( Ven. s) is the 

 largest, has the thickest walls, and alone extends to the apex 

 of the heart ; it gives off the aorta, a short trunk which 

 divides into a right and left branch, from which spring the 

 carotid arteries for the head and neck, and which continue 

 as the subclavian or auxiliary arteries A and A' for the 

 wings. From the base of the right branch A arises the 

 large aorta (Ao), which turns around the bronchus of the 

 .same side, and runs to the front and right of the vertebral 

 column through the abdomen, forming the descending aorta 

 which gives off arteries to the intercostal and lumbar regions 

 and to the viscera, and terminates in a crural branch to each 

 leg. The right ventricle ( Ven. d) has much thinner walls 

 than the left ; from it arises the pulmonary aorta (Pa) 

 which soon branches to each side. 



Birds are distinguished from reptiles by having a four- 

 chambered heart and a single permanent aortic trunk ; 

 from mammals by the persistence of the right instead of 

 the left aortic arch to form the aorta. Each auricle com- 

 municates with the ventricle of the same side ; the con- 

 necting orifices are furnished with valves. The right 

 auriculo-ventricular valve is muscular in all birds, Avhile 

 the left is membranous. 



The uro-genital organs lie dorsally in the hinder part of 

 the body-cavity. The dark reddish brown kidneys (Ki) 

 consist, as in most birds, each of three lobes, the posterior 

 being the largest ; they lie immediately behind the lungs. 

 The ureters ( Ur) are slightly curved, whitish tubes, which 

 pass back from the kidneys and open into the dorsal side of 

 the cloaca. The testicles (Te) are two large oval whitish 

 bodies, each situated immediately behind the lung and be- 

 low the kidney of the same side. The vasa deferentia ( Vd) 

 arise from the anterior and inner surfaces of the testicles, 

 have a flexuous course, and, after forming terminal enlarge- 

 ments, open separately into the cloaca, in front of the 



