THE THORAX 1047 



should be preserved, as well as the perforating branches of the internal mam- 

 mary artery. In removing the serrations of the serratus magnus the lateral 

 cutaneous nerves and arteries^ which appear between them, should also be 

 preserved. 



The insertions of the scalenus anticus and scalenus medius into the first 

 rib, and of the scalenus posticus into the second rib, are to be left intact in 

 the meantime. If the sternum has not been interfered with, for the purpose 

 of injecting the subject from the ascending aorta, the dissector should make 

 himself familiar with the sternal angle at the junction of the manubrium 

 and body of the sternum. In this connection he should note that the angle 

 serves as the guide to the second rib, the cartilage of which articulates partly 

 with the manubrium, and partly with the body of the bone. 



The external intercostal muscle is to be dissected in at least two inter- 

 costal spaces. The direction of its fibres is to be carefully observed, and it 

 is to be noted that the muscle does not extend farther forwards than the 

 junction of the osseous rib with its costal cartilages ; indeed, in some cases 

 not so far as this. The anterior intercostal aponeurosis is also to be shown 

 in the intervals betsveen the costal cartilages. In one or more spaces a small 

 portion of the external intercostal muscle is to be carefully divided trans- 

 versely, and reflected upwards and do\vnwards in two pieces. This dissec- 

 tion will expose a limited portion of the internal intercostal muscle, and it 

 will enable the dissector to contrast the diflference in direction of the two 

 muscles. In another space as much of the external intercostal muscle as 

 possible should be detached from the upper border of the lower rib, and 

 turned upwards with great care, preserving at the same time the lateral 

 cutaneous nerve and artery. The anterior intercostal aponeurosis of the 

 same space should also be carefully removed, care being taken to preserve 

 the anterior cutaneous nerve and perforating artery. This dissection will 

 expose the internal intercostal muscle, the intercostal nerve, and the various 

 intercostal arteries. The direction of the fibres of the internal intercostal 

 muscle is to be observed, and it is to be noted that the muscle extends quite 

 up to the lateral border of the sternum, where it is pierced by the anterior 

 cutaneous nerve. 



The intercostal nerve will be found under cover of the lower border of the 

 upper rib, where it lies in the subcostal groove, having the aortic intercostal 

 artery above it, the intercostal vein being in turn above the artery. It is 

 well, therefore, to make slight traction upon the nerve before attempting to 

 display it, and for this purpose the lateral cutaneous nerve wiU suffice, which 

 is one reason for preserving that nerve. The first intercostal nerve, as a rule, 

 gives off no lateral cutaneous branch, and those of the succeeding nerves will 

 be found piercing the external intercostal muscles about midway between 

 the vertebral column and the sternum. Up to this point the intercostal 

 nerve will be met with between the two intercostal muscles, but subse- 

 quently it lies in the substance of the internal intercostal muscle, and finally, 

 between the costal cartilages, it is placed beneath the internal intercostal 

 muscle. 



The arteries to be dissected in each space are as follows : (i) the aortic 

 intercostal and its collateral intercostal branch, both passing forwards, the 

 former within the lower border of the upper rib, above the nerve, and the 

 latter along the upper border of the lower rib ; and (2) the anterior intercostal 

 arteries, two in each space, except the lower t^vo spaces, one lying along the 

 lower border of the upper rib, and the other along the upper border of the 

 lower rib. The internal intercostal muscle should now be carefully removed, 

 aiter which an instructive view of the parietal pleura will be obtained. This, 

 however, must be left intact. 



The dissector is next to turn his attention to the internal mammary vessels 

 and anterior intercostal or sternal glands. These structures are to be dis- 

 played in a different manner on the two sides. 



On one side the costal cartilages are not to be interfered with in the mean- 

 time, and on the other side the second, third, fourth, fifth, cind sixth costal 

 cartilages are to be cut through at their inner and outer ends, and then lifted 



