i3 2 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 innominate, 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 



