no THORAX 



distinction to the pulmonary valve, one of the cusps is placed 

 anteriorly and the other two posteriorly. The cusps of the 

 aortic valve are stronger than the cusps of the pulmonary 

 valve described on p. 102, but correspond .with them in all 

 details of structure. 



Before terminating his examination of the left ventricle 

 the dissector should note that the muscular wall of the 

 cavity is thickest a short distance from the atrio-ventricular 

 orifice and thinnest at the apex ; and he should examine the 

 interventricular septum. 



Septum Ventriculorum. The interventricular septum is 

 a musculo-membranous partition which separates the left 

 ventricle not only from the right ventricle, but also from the 

 lower part of the right atrium. In the greater part of its extent 

 the septum is thick and muscular septum musculare ventri- 

 culorum but its upper and posterior part which is connected 

 with the fibrous rings round the atrio-ventricular orifices, and 

 the orifices of the pulmonary artery and the aorta, is mem- 

 branous septum membranaceum ventriculorum (Fig. 55). The 

 muscular part of the septum is thickest below and anteriorly, 

 where it springs from the lower border of the heart immediately 

 to the right of the apex and opposite the cardiac notch, but 

 it becomes gradually thinner as it passes upwards and back- 

 wards to its union with the membranous part. The mem- 

 branous portion is the thinnest part of the septum. Occasionally 

 it is deficient in whole or in part, and in such cases a 

 communication exists between the two ventricles, and, in 

 some rare cases, between the left ventricle and the right 

 atrium. 



The membranous part of the septum was exposed, from 

 the right side, when the anterior part of the medial cusp of 

 the tricuspid valve was removed if the dissection of the atrio- 

 ventricular bundle was attempted (see p. 101). 



Finally, the dissector should note that the interven- 

 tricular septum is placed obliquely, so that its anterior 

 border lies to the left and its inferior border to the right; 

 and that its right lateral surface, which looks forwards and 

 to the right, bulges towards the cavity of the right ventricle 



(Fig. 5). 



The Aorta. The aorta is the great arterial trunk of the 

 body. It commences from the upper, posterior and right 

 portion of the left ventricle, at the level of the third inter- 



