The Thorax. 255 



XI. THE THORAX. 



1. Examination of the thoracic wall. For this purpose the 

 lateral surface of the thorax may be conveniently 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, pecto- 

 scapularis, 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 lor 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. 268) 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 of the sternum, extending from the middle of the first 

 rib backward to the end of the ninth bone rib, and the third across 

 the ventral surface close in front of 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, 



