132 ELEMENTS OF MAMMALIAN ANATOMY. 
of the auricle into which the above veins open is the sinus 
venosus. At the dorsal part of the septum which divides 
this auricle from the adjoining one is an oval depression, 
the fossa ovalis, where, in the embryo, the two auricles com- 
municated. ‘The left auricle is in contact with the right 
dorsally, and receives three pulmonary veins (Fig. 71). 
From the right ventricle arises the pulmonary artery 
which carries the impure blood to both lungs. Its mouth 
is guarded by three semilunar valves which prevent the re- 
turn of the blood. The left ventricle gives origin to only 
one important vessel, the aorta, which arches dorsally 
around the left auricle, and at the apex of the arch gives 
off two branches, the imnominate, and the left subclavian. 
Three semilunar valves also guard the mouth of the aorta. 
The arteries of the heart may easily be distinguished from 
the veins by the fact that the walls of the former are much 
thicker. 
THE ARTERIES OF THE TRUNK. 
The blood-vessels form two systems of circulation; the 
one known as the pulmonary circulation includes the arteries 
carrying blood from the right ventricle to the lungs, and the 
veins returning the blood from the lungs to the left auricle; 
the other, known as the systemic circulation, includes the 
arteries conveying the blood from the left ventricle to all 
parts of the body, and the veins returning the blood to the 
right auricle. 
The main artery of the systemic circulation is the aorta, 
which, beginning in the left ventricle, arches dorsad to the 
heart and extends along the spinal column to the tail. That 
portion in the thoracic cavity is termed the thoracic aorta, 
while that portion in the abdominal cavity is the abdominal 
aorta. Three semilunar valves guard the mouth of the 
aorta at its opening from the ventricle, in order that the 
