132 



THE HEART. 



Fig. 114. 



The right ventricle is a triangular cavity, 

 with thick walls, and containing more than 

 any other cavity of the heart. Its muscu- 

 lar structure is in the form of large fleshy 

 fibres, called columnce carnecB, from which 

 proceed thin, white cords, called chordcB 

 tendinecE^ which are attached to the edge of 

 the tricusjnd valve. The tricuspid valve is 

 circular, and attached to the tendinous line 

 of the ostium venosum ; its lower edge has 

 three spear-pointed or leaf-like processes, 

 whence its name ; since it opens downwards, 

 the contraction closes the ostium venosum, 

 and prevents the blood from returning into 

 the auricle, and therefore it passes out by 

 the 'pulmonary artery. The orifice of the 

 'pulmonary artery is protected by three valves opening outwards, 

 which are called semilunar valves, formed by three crescentic folds 

 of the lining membrane ; in their loose edges is a small tubercle, 

 called corpus arantii: the use of these valves is to prevent the 

 blood returning from the artery to the ventricle, when it dilates. 

 Behind each valve is a pouch or dilatation, called the &inus of Val- 

 salva^ into which the blood flows by its reflux tendency, upon the 

 dilatation of the ventricle, and thus these valves are closed. The pul- 

 monary artery is of the same diameter as the aorta, but its walls 

 are thinner; after its origin it curves upwards and backwards, and 

 divides into two branches, the right of which is both longer and 

 larger than the left,'' and passes under the arch of the aorta. 



The left auricle^ is more concealed in its natural position than the 

 right. Into it enter four pulmonary veins, which give it a quad- 

 rangular shape. Its walls are muscular, and somewhat thicker than 

 those of the right auricle. In its ear-like appendage the muscular fibres 

 are arranged as 7nusculi pectin ati ; the opening, by means of which 

 it communicates with the right ventricle, is called ostium venosum, 

 and surrounded by a white tendinous ring. The septum between the 

 auricles is not always perfect in adult life. 



The left ventricle.'^ — This cavity forms the apex of the heart; it 

 is conoidal in its shape. Its walls are thick, and its columnce car nece 

 numerous, strong, and projecting ; the chordce tendinece are well de- 

 veloped, and attached to the bicuspid or mitral valve. This valve 

 consists of but two leaflets, one of which is much larger than the 

 other; its base is attached to the ring surrounding the ostium veno- 

 sum, and its edge opens downwards into the left ventricle, hence the 

 contraction of the ventricle closes this opening, and the blood passes 

 out by the aorta^ at whose orifice there are three semilunar or sig- 

 TTwid valves, each having a corpuscle of Arantius in its edge, and 



