90 ANATOMY OF THE RABBIT. 
indicated in the adult condition bya short fibrous cord, the arterial 
ligament. The adult aortic arch represents only the left one of a 
pair, and since that of the right is only represented imperfectly 
by the mnominate artery and the base of the subclavian of that side, 
a condition of asymmetry results, which is mainly expressed by 
the sinistral position of the arch with reference to the oesophagus 
(Plate VII). By comparison with the embryonic plan, it is seen 
that the primitive features of the heart and the arterial vessels 
include the ventral position of the heart itself, the equivalence of 
the two atria and of the two ventricles—these structures being 
partitioned internally but imperfectly divided externally—the 
forward position of the first portion of the aorta, and the position 
of the aorta as a median vertebral trunk. 
The vascular system is noteworthy for several departures from 
the condition of symmetry, one of these having already been 
mentioned. In addition, it is seen that in a mammal, as in terres- 
trial vertebrates generally, the base of the pulmonary artery 
(Fig. 44) is rotated in a spiral fashion about the base of the aorta, 
so that from its beginning on the right ventricle it passes across the 
ventral surface of the base of the aorta, and divides on the dorsal 
side of the latter into its two main branches. Moreover, the 
separation of the ventricular portion of the heart into two cham- 
bers is associated with an enormous increase in the muscularity of 
the wall in the left ventricle, or, in other words, in that portion 
which is concerned with the larger, systemic circulation. The 
inferior caval vein (Plate VIII), a highly specialized vessel, is 
asymmetrical, since from its beginning in the pelvic cavity to its 
termination on the right atrium it lies wholly to the right of the 
median plane. The azygos vein of the thorax (Plate VII), a. 
vessel uniting the majority of the paired intercostal veins, and 
interesting as a remnant of the primitive circulation, is also asym- 
metrical, since the trunk lies to the right of the bodies of the verte- 
brae, and is connected at its base with the right superior caval vein. 
THE LYMPHATIC SYSTEM. 
The lymphatic system, both in its functional relation and in 
origin, is an appendage of the venous portion of the vascular system. 
The system is an important one, of which, unfortunately, little 
