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 ata 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. 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, 
