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 (4o), 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 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. Hach auricle com- 
municates with the ventricle of the same side; the con- 
necting orifices are furnished with valves. The right 
auriculo-ventricular valye is muscular in all birds, while 
the left is membranous. ; 
The uro-genital organs lie dorsally in the hinder part of 
the body-cavity. The dark reddish brown kidneys (A7v) 
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 testicies (Ze) 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 
