ANGEIOLOGY. 79 



nati, which leave between them deep interstices, in which the external and 

 internal membranes of the heart come in contact. 



The right ventricle. This forms the greater part of the anterior surface 

 of the heart, and is separated posteriorly from the left ventricle by a thick 

 septum. The internal surface of the cavity of the right ventricle is covered 

 by muscular fasciculi, known as the columnce carnece, some of them passing 

 from one side to the other, and others contributing to the valvular arrange- 

 ment between the right auricle and ventricle. These, called chordce te7idinece, 

 pass to the edge of the tricuspid valve, which arises from around the margins 

 of the ostium, and projects into the cavit}^ of the ventricle. 



The opening for the pulmonary artery is situated above the ostium veno- 

 sum. It is round, and about an inch in diameter, and furnished with three 

 valves, called semilunar and sigmoid, which are capable of completely 

 closing this orifice, should the blood flow back from the pulmonary artery, 

 but which lie close pressed to the sides when the current is in its normal 

 direction. Through the pulmonary artery venous blood is carried to the 

 lungs. 



The blood, after circulating through the lungs, is returned by the pulmo- 

 nary veins into the left auricle, at the anterior inferior side of which is seen 

 the left auriculo-ventricular opening, or the communication with the left 

 ventricle, about an inch in diameter. This, like the right auricle, is consti- 

 tuted by a sinus venosus and auricular appendage. 



The left ventricle constitutes the principal bulk of the heart, and its walls 

 are nearly three times as thick as those of the right ventricle. Its internal 

 surface is roughened by the same columnse carnese as the right. The ostium 

 venosum, or opening between the left ventricle and auricle, is on the side 

 also fortified by a valvular arrangement, the mitral valve, which prevents 

 the reflux of blood. It is retained in the left ventricular cavity by chordae 

 tendineae. Close to the ostium is the opening for the aorta, guarded by 

 three semilunar valves, very similar in arrangement to those of the pulmo- 

 nary artery, being, however, stronger. 



Passage of blood through the heart. By the alternate contraction and 

 dilation of the different chambers of the heart, the blood is caused to circu- 

 late through it and the bloodvessels proceeding from it. The right and left 

 ventricles contract, while the two auricles expand, and vice versa ; contrac- 

 tion of the ventricles being known as the systole, and their dilation the 

 diastole. Commencing with the right side of the heart, we find that when 

 the auricle contracts, the blood is forced towards the auriculo ventricular 

 opening, and backwards into the venae cavae. The simultaneous expansion 

 of the right ventricle, however, affords a free passage for the blood from 

 the right auricle, which accordingly rushes through the ostium. The ven- 

 tricle now contracting, forces the blood towards the ostium, and towards 

 the pulmonary artery. Through the former it cannot pass, by reason of 

 the tricuspid valve; and as the semilunar valves at the entrance of the pul- 

 monary artery afford no impediment, the blood is drawn into the lungs. 

 Simultaneously with this contraction of the ventricles, the right auricle 

 dilates and becomes filled with fresh venous blood from the venae cavae. 



ICONOGRAPHIC ENCYCLOP/KDIA. — VOL. II. 50 ^85 



