THE CIRCULATION OF THE BLOOD. 101 



ventricle, and forms the boundary of the auricula-ventricular opening. 

 Fibrous tissue also exists at the origins of the pulmonary artery and 

 aorta. 



The muscular fibres of each auricle are in part continuous with those 

 of the other, and partly separate ; and the same remark holds true for 

 the ventricles. The fibres of the auricles are, however, quite separate 

 from those of the ventricles, the bond of connection between them be- 

 ing only the fibrous tissue of the auriculo-ventricular openings. 



FIG. 90. --The left auricle and ventricle opened and a part of their anterior and left walls re- 

 moved. %. The pulmonary artery has been divided at its commencement; the opening into the 

 left ventricle is carried a short distance into the aorta between two of the segments of the semi- 

 lunar valves; and the left part of the auricle with its appendix has been removed. The right auri- 

 cle is out of view. 1, the two right pulmonary veins cut short; their openings are seen within the 

 auricle; 1', placed within the cavity of the auricle on the left side of the septum and on the part 

 which forms the remains of the valve of the foramen ovale, of which the crescentic fold is seen 

 towards the left hand of 1'; 2, a narrow portion of the wall of the auricle and ventricle preserved 

 round the auriculo-ventrirular orifice; 3, 3', the cut surface of the walls of the ventricle, seen to be- 

 come very much thinner towards 3", at the apex; 4, a small part of the anterior wall of the left 

 ventricle which has been preserved with the principal anterior columna carnea or musculous papil- 

 laris attached to it; 5, 5, musculi papillares; .V, the left side of the septum, between the two ventri- 

 cles, within the cavity of the left ventricle; 6, 6', the mitral valve; 7, placed in the interior of the 

 aorta near its commencement and above the three segments of its semilunar valve which are hang- 

 ing loosely together; 7', the exterior of the great aortic sinus; 8, the root of the pulmonary artery 

 and its semilunar valves; 8', the separated portion of the pulmonary artery remaining attached to 

 the aorta by 9, the cord of the ductus arteriosus; 10, the arteries rising from the summit of the 

 aortic arch. (Allen Thomson.) 



