8 



THORAX 



if necessary, remove the medial part of the sixth cartilage completely. The 

 muscle posterior to the internal mammary artery is the transversus thoracis 

 (O.T. triangularis sterni). Endeavour to define its slips in the intervals 

 between the costal cartilages. 



The dissector should note, as an important practical point, that towards 

 the lower margin of the thorax the pleural sac is not prolonged downwards 

 to the lowest limit of the recess between the diaphragm and the costal 

 arches. Indeed, in the axillary line, it will be found to fall considerably 



Sterno-hyoid 

 Sterno-thyreoid 



Transversus thoracis 



Intercostal nerve 

 and artery 



Internal mammary 

 artery 



Sternal glands 



Transversus 

 thoracis 



Musculo- 



phrenic artery 



phrenic 

 artery 



FIG. 3. Dissection of the posterior surface of the Anterior Wall of the Thorax. 



short of this. Consequently the dissector will come down directly upon 

 the diaphragm, when the internal intercostal muscles are removed from 

 this portion of the chest wall. The fibres of the diaphragm correspond 

 somewhat in their direction with those of the internal intercostal muscles, 

 and it is no uncommon occurrence for the student to remove them, and 

 thus expose the peritoneum, under the impression that he has simply laid 

 bare the pleura. When the dissection is properly executed a strong fascia 

 will be seen. It passes from the surface of the diaphragm to the surface 

 of the costal pleura and holds the latter in position. Preserve this mem- 

 brane for further examination. 



