THORACIC WALL 13 



The anterior intercostal arteries are two in number for each 

 space, except the last two. At their origins they lie under 

 cover of the internal intercostal muscles, and they run later- 

 ally in relation to the upper and lower margins of the ribs 

 bounding the spaces. After a short course they pierce the 

 internal intercostal muscles, and end by anastomosing with 

 branches of the aortic and superior intercostal arteries. 



The anterior intercostal veins accompany the corresponding 

 arteries ; the lower ones end in the musculo-phrenic vein, and 

 the upper in the venae comites of the internal mammary 

 artery. 



The veins which accompany the aortic intercostal arteries 

 and the branches of the superior intercostal artery will be 

 traced to their terminations after the thorax has been 

 opened (see p. 33). 



Dissection. The dissector should next proceed to remove 

 the intercostal muscles and membranes from all the intercostal 

 spaces. This dissection must be done with great care, for 

 immediately subjacent to the internal inter costals and the ribs 

 is the delicate pleural membrane which lines the inner surface 

 of the chest wall. The membrane must not be injured or 

 detached from the deep surfaces of the ribs during this stage of 

 the dissection. As the internal intercostal muscles are removed, 

 the anterior perforating branches of the internal mammary and 

 musculo-phrenic arteries, and the anterior cutaneous nerves, 

 must be preserved. 



When the muscles are removed the internal mammary artery, 

 with its two accompanying veins, will be seen behind the costal 

 cartilages, about half an inch from the side of the sternum 

 (Figs. 20, 27). Clean those vessels in the intervals between 

 the cartilages, and note the small sternal lymph glands which lie 

 beside them. Each internal mammary artery ends by dividing 

 into superior epigastric and musculo-phrenic terminal branches 

 in the interval between the sixth and seventh rib cartilages. 

 Most likely that space will be so narrow that a view of the bifur- 

 cation cannot be obtained. If that is the case, pare away the 

 edges of the cartilages or, if necessary, remove a portion of the 

 sixth cartilage completely. The muscle posterior to the internal 

 mammary artery is the transversus thpracis (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 and anterior parts 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 mid-axillary line, and along the costal arch on each side, 

 it will be found to fall considerably short of that limit. Con- 

 sequently, when the internal intercostal muscles are removed 

 from the anterior parts of the lower intercostal spaces, the 

 dissector will come down directly upon the diaphragm (Fig. 9). 



