530 ZOOLOGY. 



cava superior (Fs), 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 ofE the 

 arterial trunks. Of the ventricles, the left ( Yen. 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 (-lo), 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 oil arteries to the intercostal and lumbar regions 

 and to the viscera, and terminates in acrural 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, while 

 the left is membranous. 



The nro-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 ( f>) 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 ( Yd) 

 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 



