256 ANATOMY OF THE RABBIT. 
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. 257). 
For the general relations of heart and lungs, see pp. 84 and 87. 
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 
consistence, 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 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 trom 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. 
(ay) The arch of the aorta (arcus aortae). Beginning at the 
base of the heart, the aorta passes at first forward, and then 
describing a curve, in the course of which it lies slightly to 
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. 260) the first branches 
are the large paired vessels arising from the anterior wall. 
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 carotid arises immediately 
to the left of this vessel or from its base. 
The subclavian artery (a. subclavia) is the first portion of 
the artery of the anterior limb. It passes from its point of 
origin laterad to the anterior margin of the first rib, where it 
is replaced by the axillary artery. Near its point of origin 
it gives off several branches, the relations of which are 
subject to considerable variation. They include: 
