DELATION OF PARTS OF HEART TO WALL OF THORAX. 365 



the valves is more strongly marked at the mouth of the aorta than at the begin- 

 ning of the pulmonary artery. 



Irregularities in the arrangement of the arterial valves are - occasionally met with. The 

 commonest is the presence of an additional (fourth) flap in the pulmonary valve. Either 

 the aortic or the pulmonary valve may consist of only two segments ; and the pulmonary 

 valve has been seen formed by a single flap. An additional flap to the aortic valve is of 

 rare occurrence. 



The part of the left ventricle adjoining the root of the aorta forms a small 

 compartment, the aortic vestibule of Sibson, the walls of which are fibrous, or at one 

 part even fibro-cartilaginous, so that it remains uncollapsed, and allows space for the 

 bulging flaps of the aortic valve to descend during diastole. 



The interventricular septum is seen in fig. 311 to be of about the same thickness 

 as the wall of the left ventricle ; but it differs from the latter in being thickest below, 

 and becoming gradually thinner above (fig. 310 and 317). At its upper and fore 

 part there is a small region where muscular fibres are absent, and the septum con- 

 sists only of a little fibrous tissue between the two endocardial layers. This is known 

 as the pars memlranacea septi or undefended space, and is the part derived from the 

 lower end of the foetal aortic septum : it is placed at the inner and fore part of the 

 aortic vestibule, below the adjacent ends of the anterior and right posterior segments 

 of the aortic valve ; and on the right side it is covered by the fore part of the septal 

 flap of the tricuspid valve (below 8 in fig. 313). It is at this spot that an abnormal 

 communication between the two ventricles is most likely to occur (persistence of the 

 foetal interventricular foramen owing to defective development of the aortic septum). 

 Posteriorly the pars membranacea is continued into a narrow portion of the septum 

 which is situated below the attachment of the anterior flap of the mitral valve, but 

 above that of the septal flap of the tricuspid valve (fig. 310 ; fig. 313, to the left 

 of 5"), so that it separates the cavities of the right auricle and left ventricle, and 

 which is strengthened by a process of the fibro-cartilage of the heart (p. 368). 



POSITION OF THE PARTS OF THE HEART WITH RELATION TO THE 

 WALL OF THE THORAX. 



Fully two-thirds of the bulk of the heart lie to the left of the median plane 

 (fig. 318). The right auricle (3') lies behind the sternal ends of the third, fourth, 

 fifth and sixth costal cartilages, and the intervening portions of the intercostal 

 spaces, and is also partly covered by the right edge of the sternum. The point of 

 its auricular appendage is behind, or even slightly to the left of, the middle line on 

 a level with the third costal cartilages (3). The left auricle extends vertically from 

 the level of the lower border of the second left cartilage to the upper border of the 

 fourth (sternal end), and in breadth corresponds to the body of the seventh dorsal 

 vertebra and the heads of the adjoining ribs on the left side. The apex of its 

 appendage (4) is behind the third costal cartilage, about an inch and a quarter to 

 the left of the sternum. The right ventricle extends from above down from the third 

 to the seventh cartilages on the left side. Its middle and lower region is as a rule the 

 only part of the heart uncovered by lung ; but sometimes, especially during expira- 

 tion, a small portion of the left ventricle at the apex is also exposed. The auriculo- 

 ventricular sulcus corresponds with a line drawn obliquely upwards from the sternal 

 end of the sixth costal cartilage on the right side, to the third cartilage on the left. 

 The rounded margin formed by the left ventricle extends on the left side from a spot 

 on the third cartilage one inch and a half from the sternum to the apex of the heart 

 in the fifth space. The sharp margin formed by the right ventricle passes from the 

 sternal end of the sixth cartilage on the right nearly transversely behind the seventh 



