104 THE STOCK owner's ADVISER. 



ventricular opening- and the ])iilmonarv artery. The auriculo- 

 ventricnlar opening is guarded by the tricuspid valves, con- 

 sisting of three triangular cusps or segments, which, connected 

 at their bases, surround the opening. The entrance of the pul- 

 monary artery is guarded by three semilunar or sigmoid valves, 

 which consist of semi-circular folds of the lining membrane. 

 When blood passes from the ventricle to the pulmonary artery 

 the valves are laid against the sides of the vessel; when the cur- 

 rent is checked a portion of it falls back towards the ventricle, 

 and the valves are now thrown inwards; they become distended, 

 overlap and completely close the tube. The blood, after being 

 purified in the lungs re-enters the heart at the left auricle, 

 which is smaller than the right, but its walls are thicker. It is 

 situated at the left postero-superior part of the heart. It re- 

 ceives two pulmonary veins on each side, and opens into the left 

 ventricle through the mitral valve. The interior of the left 

 auricle is smooth like the right, its appendix presenting musculi 

 pectinati. The remaining opening in the left auricle is the 

 auriculo-ventricular opening, which occupies the floor and com- 

 municates with the left ventricle. The blood leaves the left 

 auricle, enters the left ventricle, which is conical, and occupies 

 the posterior left region of the heart. Two openings present 

 themselves — the auriculo-ventricular, guarded by the mitral, 

 and the aortic, guarded by the semilunar valves. The venous 

 blood is carried into the right auricle by the anterior and pos- 

 terior vense cavse. It then passes through the right auriculo- 

 ventricular opening into the right ventricle; thence through 

 the pulmonary artery to the lungs. It returns by the pulmonary 

 veins to the auricle; tlience to the left ventricle, which propels 

 it through the aorta and its branches into the system generally, 

 the veins returning it again to the heart. The circulation is, 

 therefore, double, the pulmonary or lesser being performed by 

 the right, and the somatic or greater circulation 1)y the left side. 

 The arteries are elastic and contractile tubes, which convey 

 the blood from the heart to the capillaries. Each artery has 



