THE CIRCULATORY OR VASCULAR SYSTEM 89 



The right auricle is a little larger than the left, its walls 

 are somewhat thinner, and its cavity is capable of contain- 

 ing about 2 ounces. The following openings are found in 

 the right auricle: the superior vena cava, inferior vena 

 cava, coronary sinus, foramina Thebesii. 



The right ventricle is pyramidal in form, and extends 

 from the right auricle to near the apex of the heart. 

 Its anterior or upper surface is rounded and convex, and 

 forms the larger part of the front of the heart. The right 

 ventricle is provided with the opening of the pulmonary 

 artery, and contains upon its interior the following valves : 

 the tricuspid and semilunar, the latter guarding the 

 pulmonary artery and the former the opening between the 

 auricle and ventricle. 



The left auricle resembles the right in having a principle 

 cavity and an appendix, but its walls are thicker, measur- 

 ing about one and one-half lines. It contains openings for 

 the pulmonary veins, four in number, two emptying into 

 the right and two into the left side of the auricle ; frequently 

 the latter terminate by one common opening. 



The opening between the ventricle and auricle is pro- 

 vided with the bicuspid or mitral valves. 



The left ventricle is longer than the right, and more 

 conical in shape. It forms a small part of the left side of 

 the anterior surface. It forms the apex of the heart from 

 its projection beyond the right ventricle. Its walls are 

 much thicker than those of the right side, the proportion 

 being 3 to i . It is thinner at the apex, and gradually gets 

 thinner at the base. It is provided with the mitral valves 

 and semilunar valves, the former making a division be- 

 tween the auricle and ventricle during contraction of the 

 ventricle, and the latter guarding the aortic orifice. The 

 aortic opening is to the right side. 



The Valves. On the right side, the valve which con- 

 trols the opening from the right auricle to the right ven- 



