THORACIC CAVITY 83 



cardiac notch. The muscular part becomes gradually thinner 

 as it passes upwards and posteriorly and, a short distance 

 from the atrio-ventricular orifices, it terminates in a fibrous 

 membrane, the pars membranacea septi, which connects the 

 muscular part of the septum with the fibrous rings which 

 surround the atrio-ventricular orifices and the orifices of the 

 pulmonary artery and the aorta. The pars membranacea 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 auricle. 



The pars membranacea was exposed from the right side 

 when the anterior part of the medial cusp of the tricuspid 

 valve was removed during the dissection of the atrio-ventri- 

 cular bundle (see p. 74). 



Finally the dissector should note that the inter-ventricular 

 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 anteriorly and to the right, 

 bulges towards the cavity of the right ventricle (Fig. 37). 



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

 body. It commences from the upper, anterior and right 

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

 costal spaces and posterior to the left margin of the sternum. 

 It terminates at the level of the lower border of the fourth 

 lumbar vertebra, to the left of the median plane, where it 

 divides into the right and left common iliac arteries. It is 

 described as consisting of three main parts: (i) the ascend- 

 ing part, (2) the arch, and (3) the descending part. The 

 descending part is divided into (a) thoracic and (8) abdominal 

 portions. The first two parts and the thoracic portion of the 

 third part are met with in the dissection of the thorax. 



The Ascending Part of the Aorta. The ascending aorta 

 commences at the aortic orifice of the left ventricle and runs 

 upwards to the right and slightly anteriorly, posterior to the 

 first piece of the body of the sternum, to the level of the sternal 

 end of the right second costal cartilage, where it becomes the 

 arch of the aorta. It lies in the middle mediastinum, is 

 enclosed in the fibrous sac of the pericardium, and is en- 

 sheathed by a covering of the serous sac which is common 

 to it and the stem of the pulmonary artery. The lumen of 

 this portion of the aorta is not of uniform diameter ; on the 

 ii 6 a 



