172 Anatomy of the; Rabbit. 



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 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 intercostals. 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. 182) 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, from the second 

 to the sixth, at the junctions of the bone ribs with the costal cartilages. 



The artery passing along the ventral wall between the foregoing muscle and 

 the internal intercostals is the internal mammary (p. 174). 



2. Dissection of structures in the superior thoracic aperture. • 



The nerves and bloodvessels of this region are concealed by the 

 thymus gland, a large triangular flattened structure of fatty consis- 

 tence, extending forward from the base of the heart to the anterior end 

 of the thorax. The thymus should be carefully scraped away, all vessels 

 except those of the structure itself being kept intact. 



The following structures, including the aortic arch and the arteries 

 arising from it, the superior caval veins, and the vagus, phrenic, and 



