324 ANATOMY OF THE RABBIT 



from the second to the sixth, at the junctions of the bone ribs with 

 the costal cartilages. 



A very thin layer of muscle fibres passing in the same direction as those of 

 the internal intercostal muscles has been described as lying immediately internal to 

 them in the lateral wall of the thorax and is named intracostal (more dorsal) and 

 subcostal (more ventral) muscles. These, however, are not present as a distinct 

 layer in the rabbit. They and the transversus thoracis muscle together represent 

 a thoracic continuation of the transversus abdominis. The main branches of the 

 intercostal nerves run between these and the internal intercostal muscles. 



The artery passing along the ventral wall between the transverse thoracic 

 muscle and the internal intercostals is the internal mammary (p. 326). 



The importance of the above described muscles in the act of breathing is in- 

 dicated on p. 108. For the general relations of heart and lungs, see pp. 106 and 109. 



2. Dissection of structures in the superior thoracic aperture. 



The nerves and blood-vessels of this region are concealed by 

 the thymus gland, a large triangular flattened structure of fatty 

 consistence, lying partly ventral to the heart and extending forward 

 from its base to the anterior end of the thorax (cf. p. 133). The 

 thymus should be carefully raised from behind and dissected away, 

 all vessels except those of the gland 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 sympathetic nerves, cannot be dissected exactly in 

 the order given below, but must be separated from one another 

 and identified as they appear. The left superior caval vein is 

 superficial, crossing the ventral surface of the aortic arch. Care 

 should be taken not to injure the nerves (c-f) in exposing the 

 branches of the subclavian artery. 



(a) The arch of the aorta (arcus aortae). Beginning at the 

 anterior end of the heart, the aorta at first passes forward, 

 and then describing a curve, in the course of which it lies 

 slightly to the left of the median plane, turns backward 

 along the ventral surfaces of the bodies of the thoracic 

 vertebrae. With the exception of the coronary arteries 

 (p. 330) the first branches are the large vessels arising from 

 the convex surface of the arch. They comprise the common 

 carotid and subclavian arteries. On the right side the 

 carotid and subclavian arise from a short common trunk, 

 the innominate artery (a. anonyma). The left common 



