THE THORAX 323 



XL THE THORAX 

 1. Examination of the thoracic wall. 



For this purpose the lateral surface of the thorax may con- 

 veniently be cleared, on the side from which the anterior limb has 

 been removed, by dissecting away the attachments of the muscles 

 already examined in the previous dissections. These include the 

 origins of the pectorales, pectoscapularis, serratus anterior (thoracic 

 portion), obliquus externus, and rectus abdominis. 



The dorsal portion of the exposed surface is occupied by the 

 spinal musculature, to be examined at a later stage. On the ventral 

 portion appear the ribs, and between them, filling the intercostal 

 spaces, the intercostal muscles. The external intercostals (mm. 

 intercostales externi) arise from the posterior margins of the bone 

 ribs, the fibres passing obliquely downward and backward to be 

 inserted on the anterior margins of the next succeeding ribs. The 

 internal intercostals (mm. intercostales interni), the fibres of 

 which are disposed in the opposite direction, are concealed for the 

 most part from this surface, but appear ventrally between the 

 costal cartilages, where they are not covered by the external inter- 

 costals. They are best examined at a later stage from the interior 

 of the thorax. 



In preparation for the removal of a section of the thoracic wall, 

 the pectorales should be divided on the opposite side of the thorax, 

 close to the sternum, so that the limb may be displaced. 



The nerves and vessels of the neck must be kept intact until 

 the following dissection accounts for their thoracic connections. 



The scaleni muscles (p. 343) should be examined, since it is 

 necessary to destroy their costal insertions. 



A triangular section of the wall, including the sternum and the 

 costal cartilages, may be removed by making three incisions, one 

 on either side extending from the middle of the first rib backward 

 to the end of the ninth bone rib, or a point on this rib a little more 

 dorsal, and the third across the ventral surface close in front of the 

 diaphragm. Do not cut into the diaphragm.. 



The transversus thoracis muscle appears on the inner surface 

 of the section removed. It is a thin sheet of fibres arising from the 

 body and xiphoid process of the sternum and inserted on the ribs, 



