INTERIOR OF THE HEART. 207 



and very soon divides into the right and left pulmonary 

 arteries, Pd and Ps, which break up into smaller branches 

 and enter the corresponding lungs. Opening into the 

 right auricle are two great veins (see also Fig. 78), cs and 

 ci, known respectively as the upper and lower vence cavce, 

 or "hollow" veins; so called by the older anatomists be- 

 cause they are frequently found empty after death. Into 

 the back of the right auricle opens also another vein, 

 Vc, called the coronary vein or sinus, which brings back 

 blood that has circulated in the walls of the heart it- 

 self. Springing from the left ventricle, and appearing 

 irom beneath the pulmonary artery when the heart is 

 looked at from the ventral side, is a great artery, the 

 aorta, Aa. It forms an arch over the base of the heart 

 and then runs down behind it at the back of the chest. 

 From the convexity of the arch of the aorta several great 

 branches are given off, Ssi, Cs, Ab; but before that, close 

 to the heart, the aorta gives off two coronary arteries, 

 branches of which are seen at crd and crs lying in the 

 groove over the partition between the ventricles, and which 

 carry to the substance of the organ that blood which comes 

 back through the coronary sinus. Into the left auricle 

 open two right and two left pulmonary veins, ps and pel, 

 which are formed by the union of smaller veins proceeding 

 from the lungs. 



In the diagram Fig. 78 from which the branches of the 

 great vessels near the heart have been omitted for the sake 

 of clearness, the connection of the various vessels with the 

 chambers of the heart can be better seen. Opening into 

 the right auricle are the superior and inferior venae cavae 

 (cs and ci) and proceeding from the right ventricle the 

 pulmonary artery, P. Opening into the left auricle are 

 the right and left pulmonary veins (pd and ps) and spring- 

 ing from the left ventricle the aorta, A. 



The Interior of the Heart. The communication of 

 each auricle with its ventricle is also represented diagram - 

 matically in Fig. 78, and the valves which are present at 

 those points and at the origin of the pulmonary artery and 

 tli at of the aorta. Internallv the auricles are for the most 



