170 THE CIRCULATION OF THE BLOOD 



The right ventricle forms the right margin of the heart. It takes no 

 part in the formation of the apex. On section its cavity is semilunar or 

 crescentic, figure 135. Into it are two openings, the venous orifice at the 

 base, and the arterial orifice of the pulmonary artery, also at the base but 

 more to the left. The part of the ventricle leading to the pulmonary artery 

 is called the conus arteriosus. Both orifices are guarded by valves, the 

 former called the tricuspid and the latter the semilunar. In this ventricle 

 are also the projections of the muscular tissue called the trabecula carnece. 



The left auricle is situated at the left and posterior part of the base of 

 the heart. The left auricle is only slightly thicker than the right and its 

 form and structure are the same as in the right. The left venous orifices 

 are oval and a little smaller than those on the right side of the heart. There 

 is a slight vestige of the foramen on the septum between the auricles. 



FIG. 136. Cross-section of a Completely Contracted Human Heart, at the Level of the 

 Lower and Middle Thirds. (According to Krehl.) 



The left ventricle occupies the posterior and apical portion of the heart, 

 and is connected directly with the great aorta. It is separated from the 

 auricle by the bicuspid or mitral valve, and the opening into the great aorta 

 is guarded by the semilunar valves. The walls of the left ventricle are two 

 or three times as heavy as those of the right, and may be as much as half an 

 inch in total thickness. 



The left ventricle is capable of containing 90 to 120 c.c. of blood. The 

 capacity of the auricles is considerably less after death owing to their con- 

 tracted condition. The whole heart is about 12 cm. long by 8 cm. at its 

 greatest width, and 6 cm. in thickness. The average weight in the adult is 

 about 300 grams. 



The walls of the heart are constructed almost entirely of layers of muscu- 

 lar fibers; but a ring of connective tissue, to which some of the muscular 



