THORACIC CAVITY 



in 



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

 It terminates at the level of the lower part 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- 



Opening of coronary sinus Interatrial septum 



Right atrium ; ,' 

 Musculi pectinati ^-^ass=t^ J ,' Part of right sinus of the aorta 



Left atrium 



Central 

 fibrous mass 

 Fibrous ring of . 

 tricuspid orifice T 

 Inferior cusp of > 

 tricuspid valve f 

 Medial cusp of t- 

 tricuspid valve | 



A trabecula \ 

 carnea"" 



Interventricular 

 septum 



Lower left 

 pulmonary vein 



Wall of left 

 atrium 



Central 

 fibrous mass 



Pars mem- 

 branacea septi 



Great cardiac vein 

 Fibrous ring of 

 mitral orifice 



Posterior cusp 



of mitral valve 



Anterior cusp 



of mitral valve 



- Wall of left ventricle 



FIG. 55. Section of the Heart showing the Interventricular and Inter a tria 

 Septa and the Fibrous Rings round the Orifices. 



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

 descending part is divided into (a) thoracic and (K) 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 

 (Figs. 20, 28, 53, 56) commences at the aortic orifice of 

 the left ventricle and runs upwards, to the right and 

 slightly forwards, posterior to the first piece of the body of 

 the sternum, to the level of the sternal end of the second 

 right costal cartilage, where it becomes the arch of the 

 aorta. It lies in the middle mediastinum, is enclosed in 



