THE CIRCULATORY SYSTEM 157 



hanging downward and held by .slender white tendinous 

 cords attached to little projecting columns in the walls of 

 the ventricle (papillary muscles). These cords and mus- 

 cles keep the valves from being pressed back into the 

 auricle further than is necessary to close the communica- 

 tion. Blood can thus flow from the auricle into the ven- 

 tricle, but not from the ventricle to the auricle. The 

 passage into the pulmonary artery is guarded by three 

 folds, or pockets, in the lining membrane, called semilunar 

 valves, which have their free edges turned upward, so that 

 when the blood is forced upward into the artery, the valves 

 lie flat against its walls, while if the blood should begin to 

 flow backward, the little pockets would at once be filled, 

 their edges crowded together in the center of the tube, 

 and the opening closed. 



216. The Left Ventricle occupies the chief part of the 

 hinder surface of the heart, and includes the apex (Figs. 

 87 and 88). It opens at its upper side from the left auricle 

 and into the aorta, the great artery whose branches bear 

 the blood to the general system. The walls are much 

 thicker than in any other part of the heart because greater 

 force is required here to send the blood to the most dis- 

 tant parts of the body. 



The mitral or bicuspid valves guard the opening into 

 the left auricle. They are similar to the tricuspids, 

 except that they have only two main divisions instead of 

 three. Tendinous cords and papillary muscles hold their 

 edges in place as in the case of the tricuspids. A strong 

 fibrous ring surrounds the end of the aorta, and within its 

 mouth are three semilunar valves, thicker and stronger 

 than those of the pulmonary artery. Their action is like 

 that of the other semilunar valves. 



217. All the cavities of the heart are lined with a 



